A technique for investigation of the infrared absorption of water vapor and carbon dioxide under simulated atmospheric conditions has been developed. The “total absorption” or area under the curve giving fractional absorption as a function of frequency can be determined for each spectral region in which characteristic absorption occurs. The apparatus includes a 22-m multiple-traversal absorption cell which permits controlled variation of the following parameters: (1) geometrical path length, (2) pressure of absorbing gas, (3) pressure of the nonabsorbing gases nitrogen and oxygen, and (4) the temperature of the gaseous mixture. A prism spectrometer is used to measure fractional absorption as a function of frequency under various experimental conditions. Although the observed shape of a given absorption band depends upon the effective slit widths of the spectrometer, the total absorption of a band depends, within wide limits, only on the foregoing listed parameters. For the range of temperatures encountered in the lower atmosphere, the influence of temperature variation on total absorption is so small that it can be neglected. On the basis of results obtained by the techniques described, it is possible to make accurate predictions of absorption of infrared radiation in the earth’s atmosphere.

Figure 4. The 2.7- and 3.2 -μ bands of H_{2}O, showing the absorption parameters variable with the multiple-traversal cell.

The recent development of spectroscopic techniques for extreme infrared frequencies, below 100 cm^{−1}, has enabled us to observe collision-induced absorption in non-polar gases, viz. oxygen, nitrogen, air, and helium–argon mixtures. This absorption results from the modulation of the distortion dipole moment of each pair of colliding molecules by their rotational and translational motions. It is of the kind observed previously at higher frequencies by Kiss, Gush, and Welsh using hydrogen gas. By making measurements at pressures up to 120 atmospheres, the absorption coefficients have been determined over the frequency range 30–100 cm^{−1}. Attention is drawn to the possibilities that such processes may play a role in atmospheric energy balance and might give rise to significant attenuation if beams of infrared radiation were used for communications.

Figure 2. The frequency dependence of the absorption coefficient α for oxygen (curve A) and nitrogen (curve B).

Figure 6. Comparison of observed and calculated transmittances in the 18-36 cm^{-1} region for representative samples of H_{2}O and H_{2}O+N_{2}. The observed values are represented by data points. Calculated values of Tonic (v) with the instrumental response function taken into account are represented by the solid lines, and calculated true transmittance T'(v) by the short-dashed lines, D and F. The long-dashed curves, A, E, and H represent TnIc (v) with the following modifications to the parameters: Panel I, Curve A, the contribution by the continuum was not included. Panel II, Curve E, the absorption coefficient was increased by 10% at all wavenumbers. Panel IV, Curve H, the self-broadening factor Be for the continuum was assumed to be 5 instead of 13. The sample parameters are

The shapes of the extreme wings of self-broadened CO_{2}(lines have been investigated)in three spectral regions near 7000, 3800, and 2400 cm^{-1}. Absorption measurements have been made on the high-wavenumber sides of band heads where much of the absorption by samples at a few atm is due to the extreme wings of strong lines whose centers occur below the band heads. New information has been obtained about the shapes of self-broadened CO_{2} lines as well as CO_{2} lines broadened by N_{2}, O_{2}, Ar, He, and H_{2}. Beyond a few cm^{-1} from the line centers, all of the lines absorb less than Lorentz-shaped lines having the same half-widths. The deviation from the Lorentz shape decreases with increasing wavenumber, from one of the three spectral regions to the next. The absorption by the wings of H_{2}- and He-broadened lines is particularly low, and the absorption decreases with increasing temperature at a rate faster than predicted by existing theories.

Influence of assumed line width on calculated curves of χ (v- v_{o}) and χ (ν) in the 7000 cm^{-1} region. The curve in the lowerpanel represents the experimental results for k_{s}^{0}. The + 's represent the values calculated on the basis of lines whose χ is given by the solid curve in the upper panel and whose half-widths are given by Fig. 4. The squaries represent values calculated on the basis of the same χ but with α^{0} = 0.092 cm^{-1} for all lines. Values of k_{s}^{0} based on lines with α^{0}= 0.092 cm^{-1} and x modified according to the dashed curve agree with the experimental curve to within ±42%.

The shapes of the extreme wings of self-broadened CO_{2}(lines have been investigated)in three spectral regions near 7000, 3800, and 2400 cm^{-1}. Absorption measurements have been made on the high-wavenumber sides of band heads where much of the absorption by samples at a few atm is due to the extreme wings of strong lines whose centers occur below the band heads. New information has been obtained about the shapes of self-broadened CO_{2} lines as well as CO_{2} lines broadened by N_{2}, O_{2}, Ar, He, and H_{2}. Beyond a few cm^{-1} from the line centers, all of the lines absorb less than Lorentz-shaped lines having the same half-widths. The deviation from the Lorentz shape decreases with increasing wavenumber, from one of the three spectral regions to the next. The absorption by the wings of H_{2}- and He-broadened lines is particularly low, and the absorption decreases with increasing temperature at a rate faster than predicted by existing theories.

Figure 16. Curves of χ and k_{N2}^{0} showing influence of assumed line shape on the calculated absorption coefficient in the 3800 cm^{-1} region. Curve A in the lower panel represents the experimental results for K_{N2}^{0}. The circles represent the values calculated on the basis of a line shape whose χ is given by curve A in the upper panel. Variations in the line shape given by curves B, C, D, and E in the upper panel were assumed, and the corresponding calculated curves of k_{N2}^{0} are shown in the lower panel. The six vertical lines in the lower panel indicate the wavenumbers at which experimental measurements were made.

The collision‐induced absorption of the (0–0) and (1–0) bands of the ^{1}Δ_{g} ← ^{3}Σ_{g}^{−} transition of oxygen has been studied in the gas phase from 87°–300°K. Low pressures were used and only binary collisions are important in inducing transitions. At low temperatures the absorption is enhanced because the attractive intermolecular potential causes clustering in the gas. However, no direct evidence for bound‐state (O_{2})_{2} molecules was found. The frequency maxima of the absorption of collision pairs were only slightly shifted from those calculated from monomer oxygen. This suggests that the shapes of the internuclear potentials and separation of the ^{1}Δ_{g} and ^{3}Σ_{g} electronic states are little affected by these binary collisions. A near‐Boltzmann relation of the broad band shapes, moreover, suggests that the intermolecular potentials for ^{1}Δ_{g}⋅⋅⋅^{3}Σ_{g}^{−}and^{3}Σ_{g}^{−}⋅⋅⋅^{3}Σ_{g}^{+} interactions are nearly the same. The mechanisms of the induced absorption collisions are discussed.

Figure 7. Boltmann-weighted Lorentzian curves fitted to the 2 components of the (1-0) ^{1}Δ_{g} ← ^{3}Σ_{g}^{-}- band at 87° and 300°K. The dashed curves are calculated for the individual components and the solid curve is the sum of these. The curve with dots gives the experimental data.

The collision‐induced absorption spectra have been measured at room temperature and at 87°K for bands in the ^{1}Δ_{g} + ^{1}Δ_{g} ← ^{3}Σ_{g}^{−} + ^{3}Σ_{g}^{−} and ^{1}Δ_{g} + ^{1}Σ_{g}^{+} ← ^{3}Σ_{g}^{−} + ^{3}Σ_{g}^{−} simultaneous electronic systems for oxygen. The binary absorption coefficients were found to increase with decreasing temperature for ^{1}Δ_{g} + ^{1}Δ_{g} ← ^{3}Σ_{g}^{−} + ^{3}Σ_{g}^{−}. The band shapes for this system suggest that the Hamiltonian which is responsible for intensity borrowing depends on the angular orientation of the O_{2} molecules in the collision pair since ΔK = 0,± 2,± 4 selection rules are needed to account for the Δν_{1 / 2} ∼ 200cm^{−1} bandwidth. The relative intensity of the (1–0) and (0–0) bands indicates that the interaction Hamiltonian is also strongly modulated by the vibrational coordinates of O_{2}. The frequency shift of this simultaneous transition indicates that the intermolecular distance parameter for ^{1}Δ_{g}⋅⋅⋅^{1}Δ_{g} is 3% larger than for ^{3}Σ_{g}^{−}⋅⋅⋅^{3}Σ_{g}^{−}. The unusual band shape for the ^{1}Δ_{g} + ^{1}Σ_{g}^{+} ← ^{3}Σ_{g}^{−} + ^{3}Σ_{g}^{−} band is interpreted in terms of an exiton interaction for the ^{1}Δ_{g}⋅⋅⋅^{1}Σ_{g}^{>+} combination. Although bound state (O_{2})_{2} molecules undoubtedly exist at low temperatures these data provide no unambiguous spectroscopic evidence of their presence.

Figure 1. Spectra of the (0-0) band of the ^{1}Δ_{g} + ^{1}Δ_{g }← ^{3}Σ_{g}^{-}+^{3}Σ_{g}^{- }simultaneous transition. The pathlength is 168 m. The slitwidth is 1 cm^{-1}. The oxygen desnity is 2.1 amagats at both temperatures. The tick mark indicates the band origin calculated from free-molecule spectroscopic constants.

The collision‐induced absorption spectra have been measured at room temperature and at 87°K for bands in the ^{1}Δ_{g} + ^{1}Δ_{g} ← ^{3}Σ_{g}^{−} + ^{3}Σ_{g}^{−} and ^{1}Δ_{g} + ^{1}Σ_{g}^{+} ← ^{3}Σ_{g}^{−} + ^{3}Σ_{g}^{−} simultaneous electronic systems for oxygen. The binary absorption coefficients were found to increase with decreasing temperature for ^{1}Δ_{g} + ^{1}Δ_{g} ← ^{3}Σ_{g}^{−} + ^{3}Σ_{g}^{−}. The band shapes for this system suggest that the Hamiltonian which is responsible for intensity borrowing depends on the angular orientation of the O_{2} molecules in the collision pair since ΔK = 0,± 2,± 4 selection rules are needed to account for the Δν_{1 / 2} ∼ 200cm^{−1} bandwidth. The relative intensity of the (1–0) and (0–0) bands indicates that the interaction Hamiltonian is also strongly modulated by the vibrational coordinates of O_{2}. The frequency shift of this simultaneous transition indicates that the intermolecular distance parameter for ^{1}Δ_{g}⋅⋅⋅^{1}Δ_{g} is 3% larger than for ^{3}Σ_{g}^{−}⋅⋅⋅^{3}Σ_{g}^{−}. The unusual band shape for the ^{1}Δ_{g} + ^{1}Σ_{g}^{+} ← ^{3}Σ_{g}^{−} + ^{3}Σ_{g}^{−} band is interpreted in terms of an exiton interaction for the ^{1}Δ_{g}⋅⋅⋅^{1}Σ_{g}^{>+} combination. Although bound state (O_{2})_{2} molecules undoubtedly exist at low temperatures these data provide no unambiguous spectroscopic evidence of their presence.

Figure 2. Spectra of the (1-0) band of the ^{1}Δ_{g} + ^{1}Δ_{g }← ^{3}Σ_{g}^{-}+^{3}Σ_{g}^{-}- a simultaneous path length tranSItion. of 122 m The and a upper slitwidth spectrum of about was 1.2 recorded cm^{-1}. with The lower spectrum was recorded with a path length of 168 m and a slitwldth of about 2.5 cm^{-1}. The oxygen density was 2.1 amagats at both temperatures. The tick mark indicates the band origin calculated from the spectroscopic constants of the free molecule.

The two components of the (ν_{1}, 2ν_{2}) Fermi doublet of gaseous CO_{2} in an absorption path length of 56 m at 192 °K show a complex structure. When the allowed C^{16}O^{18}O absorption and the pressure-induced CO_{2} absorption are removed by computational procedures, the residual spectrum consists of two similar symmetric patterns of five maxima. These are interpreted in terms of the rotation and vibration of (CO_{2})_{2} dimers held by quadrupole–quadrupole interaction in the locked T position at an intermolecular distance of 4.1 Å.

Figure 1. The (v_{1}, 2v_{2}) infrared band of CO_{2}. (a) Ob- served spectrum (I = 56 m, ρ = 0.92 amagat, T = 192^{0}K). (b) Profile with the (v_{1}, 2v_{2}) band of C^{16}O^{18}O removed. (c) Calculated pressure-induced profile for free-free collisions. (d) (= (b) - (c)) Spectrum ascribed to the (CO_{2})_{2} dimer. (e) The v_{1}^{d} band with assignments of the maxima.

By means of a photoelectrical technique used in conjunction with a 3 m vacuum monochromator, the absorption cross section of the O_{2} continuum in the region 2350–1814 Å and the absorption cross section of CO_{2} in the region 2160–1718 Å have been measured. The cross section of the O_{2} continuum is 3.8 × 10^{−24} cm^{2} at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 × 10^{−24} cm^{2} at about 1980 Å, then increases very rapidly and reaches 7.1 × 10^{−22} cm^{2} at about 1814 Å. In the case of CO_{2}, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross section of the CO_{2} continuum is about 2 × 10^{−24} cm^{2} at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 × 10^{−24} cm^{2} at 2000 Å. The continuum rises rapidly at 2000 Å and its value is 1.19 × 10^{−20} cm^{2} at 1718 Å. Origins of the continua of both O_{2} and CO_{2} are briefly discussed.

Table 2. Absorption coefficients and cross sections of the CO_{2} continuum in the region 2160-1718 A.

By means of a photoelectrical technique used in conjunction with a 3 m vacuum monochromator, the absorption cross section of the O_{2} continuum in the region 2350–1814 Å and the absorption cross section of CO_{2} in the region 2160–1718 Å have been measured. The cross section of the O_{2} continuum is 3.8 × 10^{−24} cm^{2} at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 × 10^{−24} cm^{2} at about 1980 Å, then increases very rapidly and reaches 7.1 × 10^{−22} cm^{2} at about 1814 Å. In the case of CO_{2}, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross section of the CO_{2} continuum is about 2 × 10^{−24} cm^{2} at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 × 10^{−24} cm^{2} at 2000 Å. The continuum rises rapidly at 2000 Å and its value is 1.19 × 10^{−20} cm^{2} at 1718 Å. Origins of the continua of both O_{2} and CO_{2} are briefly discussed.

Figure 5. Absorption coefficients of CO_{2} in the region 2160-1880 A. The right and left curves correspond to the right and left scales, respectively.

By means of a photoelectrical technique used in conjunction with a 3 m vacuum monochromator, the absorption cross section of the O_{2} continuum in the region 2350–1814 Å and the absorption cross section of CO_{2} in the region 2160–1718 Å have been measured. The cross section of the O_{2} continuum is 3.8 × 10^{−24} cm^{2} at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 × 10^{−24} cm^{2} at about 1980 Å, then increases very rapidly and reaches 7.1 × 10^{−22} cm^{2} at about 1814 Å. In the case of CO_{2}, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross section of the CO_{2} continuum is about 2 × 10^{−24} cm^{2} at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 × 10^{−24} cm^{2} at 2000 Å. The continuum rises rapidly at 2000 Å and its value is 1.19 × 10^{−20} cm^{2} at 1718 Å. Origins of the continua of both O_{2} and CO_{2} are briefly discussed.

Figure 6. Absorption coefficients of CO_{2} in the region 1885-1800 A.

By means of a photoelectrical technique used in conjunction with a 3 m vacuum monochromator, the absorption cross section of the O_{2} continuum in the region 2350–1814 Å and the absorption cross section of CO_{2} in the region 2160–1718 Å have been measured. The cross section of the O_{2} continuum is 3.8 × 10^{−24} cm^{2} at 2350 Å; it slowly increases towards shorter wavelengths and reaches 10.7 × 10^{−24} cm^{2} at about 1980 Å, then increases very rapidly and reaches 7.1 × 10^{−22} cm^{2} at about 1814 Å. In the case of CO_{2}, numerous discrete bands were found overlapping a weak continuum in the wavelength region below 1980 Å. The absorption cross section of the CO_{2} continuum is about 2 × 10^{−24} cm^{2} at 2100 Å; it gradually increases toward the shorter wavelength side, and reaches about 4 × 10^{−24} cm^{2} at 2000 Å. The continuum rises rapidly at 2000 Å and its value is 1.19 × 10^{−20} cm^{2} at 1718 Å. Origins of the continua of both O_{2} and CO_{2} are briefly discussed.

Figure 7. Absorption coefficients of CO_{2} in the region 1805-1720 A.

The infrared fundamental band and the five strongest near-infrared and visible electronic bands of gaseous oxygen were studied from 90 to 115 K with path lengths up to 140 m in two low-temperature multiple-traversal absorption cells. The profile of the fundamental band is in good agreement with the theory of quadrupole-induced absorption except for a low-intensity residual in the Q-branch region. Although the electronic bands are less amenable to complete analysis, the general validity of a Boltzmann relation in their intensity distributions confirms their collision-induced nature. The temperature variation of the integrated band intensities is indicative of quadrupole induction for the fundamental and of overlap induction for the electronic bands; a somewhat too sharp rise at low temperatures may be due to the neglect of the quadrupole–quadrupole coupling in evaluating the pair distribution function.

Figure. 3. The 1.06 and 1.26 μ bands of O_{2} at 90 and 112 K. The densities were 2.91 and 4.98 amagat, respectively, and the absorption path was 137 m.

The infrared fundamental band and the five strongest near-infrared and visible electronic bands of gaseous oxygen were studied from 90 to 115 K with path lengths up to 140 m in two low-temperature multiple-traversal absorption cells. The profile of the fundamental band is in good agreement with the theory of quadrupole-induced absorption except for a low-intensity residual in the Q-branch region. Although the electronic bands are less amenable to complete analysis, the general validity of a Boltzmann relation in their intensity distributions confirms their collision-induced nature. The temperature variation of the integrated band intensities is indicative of quadrupole induction for the fundamental and of overlap induction for the electronic bands; a somewhat too sharp rise at low temperatures may be due to the neglect of the quadrupole–quadrupole coupling in evaluating the pair distribution function.

Figure 4. The 5770 and 6290 A bands of O_{2} at 90, 113, and 295 K. The densities were 2.66, 5.61, and 4.42 amagat, respectively, and the absorption path was 137 m at the low temperatures and 165 m at room temperature.

Collision induced microwave absorption is reported in pure CO_{2}, and CO_{2}-Ar, CO_{2}-CH_{4} mixtures in the 70 G H z (2.3 cm^{-1}) region at a temperature of 22°C, using a sensitive cavity technique previously described. The results in pure CO_{2} in the very low density region from 5 to 30 amagat accurately establish the dependence of the loss on the square and cube of the density, and the relaxation times are calculated. The experimental results agree well with previously reported lower frequency data at 0.3-0.8 cm^{-1} which establishes the linear dependence on frequency of the absorption up to 2.3 cm^{-1}. There is also good agreement with an extrapolation of higher frequency infrared results of Ho et al. The relaxation times associated with the two and three body collisions are shown to be nearly equal at room temperatures with T~ = 0.84 x 10^{-12} s and T~ = 1.0 x 10^{-12} S. Higher order dependence on the density is observed for the CO_{2}-Ar and CO_{2}-CH_{4} mixtures. The results are compared with earlier low frequency measurements at 0.8 cm^{-1} and with the theory of Maryott and Kryder, taking account of correction terms in the dielectric virial coefficient according to Bose and Cole.

Figure 6. A plot of the absorption coefficient a(v) divided by v^{2} vs. v, showing a solid line through the experimental results for a_{2}(v)/v^{2} of Ho et al. (1971), Frenkel and Woods (1966). and the present results, and a dotted line through the results of Birnbaum et al. (1971) and the present result for a_{3}(v)/v^{2}.

Figure 1. Theoretical water vapor absorption (water vapor density 18 g/m^{3}, temperature 296°K) based on the Gross line-shape formula with laser frequencies in the gaps shown by arrows.

Figure 1. Theoretical water vapor absorption (water vapor density 18 g/m^{3}, temperature 296°K) based on the Gross line-shape formula with laser frequencies in the gaps shown by arrows.

The continuum absorption by H_{2}O has several characteristics that are common throughout the windows in the infrared and millimeter-wave regions. Values of the continuum absorption coefficient calculated on the basis of simple, widely used line shapes may differ greatly from observed values in the windows between strong absorption lines. The temperature dependence of this absorption is also not predictable from present day understanding of line shapes or of dieters, which may also contribute. The shapes of self-broadened H_{2}O lines are quite different from those of N_{2}-broadened lines, and the difference increases with increasing distance from the centers of the lines. Data obtained from laboratory samples and from atmospheric paths are presented to compare the various windows in the infrared and millimeter regions.

Figure 13. Comparison of the spectral curves from 0 to 3100 cm^{-1} of the empirical continuum for self broadening (C), the absorption coefficient of liquid water (L), and the average intensities of the H_{2}O vapor lines (V). Note scale change at 50 cm^{-1}.

Absorptions of the ρ_{στ}, 0.8 μm and a bands of water vapor were measured using a long pathlength, multiple-traversal absorption cell. The band intensities were found to be 842.3, 53.2, and 50.12 cm^{-1}/g-cm^{-2} at 334 K for the ρ_{στ}, 0.8 μm and a bands, respectively. The measured equivalent widths gradually exceed calculated values for the Lorentz line profile and the self-broadening coefficients of 5 with increasing absorber amounts. Possible causes of this excess absorption are discussed.

Figure 1. Predicted spectra for fogs comprising molecular absorption and particle loss computed from standard models; corresponding to visibilities of 50(…), 10(---) and 150(___) m. Spectra observed in the atmosphere corresponding to the models. The standard deviation in the measured spectra is 3 dB km^{-1}. In such fogs the liquid drops are all small compared with the wavelengths and show Rayleigh region behavior in which attenuation varies monotonically. Nether this component nor equilibrium dimers can accounr for the observed additional structure between monomer lines.

Absorption of 9.6-µm CO_{2} laser radiation by CO_{2} at temperatures between 296 and 625 K has been measured at a pressure of 200 Torr. Experimental results for the R10—R26 and P10—P28 transitions have been obtained and compared with computed values of absorption. The relative optical broadening coefficients due to He and N_{2} have been measured on the R16—R22 and P16—P22 transitions over the same temperature range.

Figure 1. Experimental and calculated values of absorption coefficientk at T = 620°K over the R and P branches of the 9.6-μm CO_{2} transition.

Using a tunable source of monochromatic radiation (BWO) and a pneumatic detector, the field and laboratory investigations have been performed to obtain spectral distribution of atmospheric water vapor absorption coefficient in transparent windows centered at the wavelengths of 0.88 and 0.73 mm.

The measurements have not found any spectral features mentioned repeatedly in the literature, besides the usually observed excess of the measured absorption above the calculated one for water vapor monomers.

Figure 1. Results of our laboratory measurements in comparison with laboratory investigations of (30) and theoretical data (ρ=19 g.m^{-3}, T=25.5^{o}C, P =730 Torr). Our total absorption (A) and excess one (B) are shown by circles. Theoretical values for water vapor monomers (chain-dot curve), excess absorption spectrum (30) (solid curve) and theoretical dimer absorption (7) (dotted curve) are shown as well.

[7]. A.A.Viktorova, S.A.Zhevakin,DAN USSR, v.194, 540 (1970).
[30]. R.J. Emery, P .Moffat, R.A. Bohlander, H. A. Gebbie, J. Atm.Terr.Phys. v.7,587 (1975).

Using a tunable source of monochromatic radiation (BWO) and a pneumatic detector, the field and laboratory investigations have been performed to obtain spectral distribution of atmospheric water vapor absorption coefficient in transparent windows centered at the wavelengths of 0.88 and 0.73 mm.

The measurements have not found any spectral features mentioned repeatedly in the literature, besides the usually observed excess of the measured absorption above the calculated one for water vapor monomers.

Figure 2. Our results of field measurements as compared with field investigation (30) and the theory (r =8.5 g.m^{-3}, T=8.5°C, P= 727 Torr). Our total absorption (A) and excess one (B) are shown by circles. All the data are shown as in Figure 1. (A) and excess one (B) are shown by circles. Theoretical values for water vapor monomers (chain-dot curve),excess absorption spectrum (30) (solid curve) and theoretical dimer absorption (7) (dotted curve) are shown as well.

[7]. A.A. Viktorova, S.A. Zhevakin, Поглощение микрорадиоволн димерами водяного пара атмосферы, DAN USSR, v.194, 540 (1970).
[30]. R.J. Emery, P. Moffat, R.A. Bohlander, H.A. Gebbie, Measurements of anomalous atmospheric absorption in the wavenumber range 4-15 per cm, J. Atm. Terr. Phys. v.7,587 (1975).

In this paper we present the first accurate determination, from laboratory measurements, of the temperature dependence of CO_{2} continuous absorption beyond the v_{3} band head, with N_{2} and O_{2} as perturbers. The χ form factors previously published have been tested and optimized, and new form factors are proposed. It has been clearly demonstrated for N_{2}, as well as for O_{2} broadening, that a χ factor independent of T in the range 193 ≤ T ≤ 300 K gives calculated values of the absorption coefficient in fair agreement (±20% level) with observed values in the spectral region 2400 < σ < 2500 cm^{-1}.

Figure 2. Normalized absorption coefficient B_{0}(σ,T) in cm^{-1}amagat^{-2} for CO_{2} broadened by N_{2}: (a) wave number dependence of B_{0}(σ,T) for two temperatures 296° and 193°K; (b) temperature dependence at various wave numbers (cm^{-1}).

In this paper we present the first accurate determination, from laboratory measurements, of the temperature dependence of CO_{2} continuous absorption beyond the v_{3} band head, with N_{2} and O_{2} as perturbers. The χ form factors previously published have been tested and optimized, and new form factors are proposed. It has been clearly demonstrated for N_{2}, as well as for O_{2} broadening, that a χ factor independent of T in the range 193 ≤ T ≤ 300 K gives calculated values of the absorption coefficient in fair agreement (±20% level) with observed values in the spectral region 2400 < σ < 2500 cm^{-1}.

Figure 3. Normalized absorption coefficient B_{0}(σ,T), in cm^{-1} amagat^{-2} for CO_{2} broadened by O_{2}: (a) wave number dependent of B_{0}(σ,T) for two temperatures 296° and 193°K; (b) temperature dependence at various wave numbers (cm^{-1}).

The collision-induced absorption (CIA) spectrum for nitrogen has been measured in the spectral region below 360 cm^{−1} at 126, 149, 179, and 212 K. The measurements have been obtained using Fourier transform infrared (FTIR) techniques, a far infrared (FIR) laser system operating at 84.2 and 15.1 cm^{−1}, and microwave cavity techniques. The experimental line shapes have been compared with the theoretical predictions of Joslin, based on Mori theory, and of Joslin and Gray, based on information theory alone. The data have been used to determine the quadrupole moment employing various intermolecular potentials. One Lennard–Jones potential has resulted in a quadrupole moment of 1.51 B, the value that was used in generating the theoretical line shapes. These results, when combined with our forthcoming measurements on nitrogen mixed with methane and argon, may be helpful in determining the role of CIA in calculating the opacity of some planetary atmospheres.

Figure 3. A plot of A(v)/ρ^{2} versus the frequency (cm^{-1}) measured using an FIR interferometer and indicated by the solid curve. The results are displayed for the four different temperatures used: (a) 212 K, (b) 179 K, (c) 149 K, (d) 126 K. In each case the laser measurements at 84.2 and 15.1 cm^{-1} are shown. Also shown are the theoretical line shapes generated using Mori theory (dashed line) as discussed in the text. In (d) the results of Buontempo et al. (10) are given for comparison.

A practical atmospheric Millimeter‐Wave Propagation Model (MPM) is formulated that predicts attenuation, delay, and noise properties of moist air for frequencies up to 1000 GHz. Input variables are height distributions (0–30 km) of pressure, temperature, humidity, and suspended droplet concentration along an anticipated radio path. Spectroscopic data consist of more than 450 parameters describing local O_{2} and H_{2}O absorption lines complemented by continuum spectra for dry air, water vapor, and hydrosols. For a model (MPM^{*}) limited to frequencies below 300 GHz, the number of spectroscopic parameters can be reduced to less than 200. Recent laboratory measurements by us at 138 GHz of absolute attenuation rates for simulated air with water vapor pressures up to saturation allow the formulation of an improved, though empirical water vapor continuum. Model predictions are compared with selected (2.5–430 GHz) data from both laboratory and field experiments. In general, good agreement is obtained.

Figure 8. Water vapor attenuation rates α(v) across atmospheric window range W4 at two temperatures, 5^{o} and -10^{o} C; pluses, measured data [Fedoseev and Koukin, 1984]; lines, MRM.

The temperature dependence of the high frequency far wings of the self-broadened CO_{2} lines has been investigated in the 2400–2600-cm^{-1} spectral region. The temperature dependence of the corrective shape factor X(σ,T) is demonstrated for the first time.

Figure 5. Wave number dependence of the normalized absorption coefficient A_{0}(σ,T), in cm^{-1} amagat^{-2}, for two temperatures: 296° and 193°K.

The temperature dependence of the high frequency far wings of the self-broadened CO_{2} lines has been investigated in the 2400–2600-cm^{-1} spectral region. The temperature dependence of the corrective shape factor X(σ,T) is demonstrated for the first time.

Figure 9. Comparison of experimental and calculated spectrum (A_{0} in cm^{-1} amagat^{-2}) (a) T=296°K, (b) T = 218°K: *, observed, ----, Lorentz absorption, -, best fit obtained with the two-parameter lineshape factor of Birnbaum [see Eq. (5)]. The optimized values of the parameters are given in Table IV.

Figure.1. Comparison of measured (Ref.5) (dots) and theoretical (curves) roto-translational spectra of H_{2}-CH_{4} pairs at five temperatures from 140 to 296 K and frequency range from 150 to 850 cm^{-1}.

P. Codastefano, P. Dore, L. Nencini, Temperature dependence of the far-infrared absorption spectrum of gaseous methane, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 35, Issue 4, April 1986, Pages 255-263, https://doi.org/10.1016/0022-4073(86)90079-8.

Existing laboratory measurements of the far-infrared collision-induced spectra of gaseous nitrogen at temperatures from 124 to 300 K are analyzed on the basis of quantum line shapes computed from a suitable, advanced isotropic potential and multipole-induced dipole functions. The input is chosen to represent most closely the measurements at all temperatures and over the full range of frequencies. Simple analytical expressions are specified which represent the spectral profiles closely. It is thus possible to reproduce the collision-induced absorption spectra of nitrogen effortlessly in seconds at temperatures from 50 to 300 K on small computers, even in the far wings which never have been modeled from a quantum formalism before. The work thus gives new and reliable spectral intensities and their temperature dependence for a detailed analysis of the Voyager IRIS spectra of Titan's atmosphere.

Figure 2. Far-infrared absorption spectrum of N_{2}+N_{2} at (a) 126, 149, and 179 K and (b) 228.3° and 300°K. Open circles, experimental data at (a) 124°K, and (b) 300°K of Buentempo et.al. (1975). Filled circles, squaries and triangles denote new results (Stone et al. 1984; Dagg et al. 1985). Lines, fitted spectrum, with the Ling and Rigby isotropic potential (see Appendix) and the dipole parameters given in Table 3. In (a), the scale is shifted by a factor of 2 at each temperature.

U. Buontempo, S. Cunsolo, G. Jacucci, and J. J. Weis, The far infrared absorption spectrum of N_{2} in the gas and liquid phases, The Journal of Chemical Physics, 1975, Volume 63, Pages 2570, DOI: 10.1063/1.431648, https://doi.org/10.1063/1.431648
Dagg, I.R., and Gray C.G., 1985, in Phenomenoa induced by intermolecular interaction, ed. G.Birnbaum (New York; Plenum), p.109.
Dagg, I.R., Anderson, A., Yan, S., Smith, W. and Read, L.A.A., Collision-induced absorption in nitrogen at low temperatures, Canadian Journal of Physics, 1985, Volume 63, Issue 5, Pages 625-631, DOI: 10.1139/p85-096, https://doi.org/10.1139/p85-096.
Stone, N. W. B., Read, L. A. A., Anderson, A., Dagg, I. R., & Smith, W., Temperature dependent collision-induced absorption in nitrogen, Canadian journal of physics, 62(4), 338-347. (1984).

The absolute intensities and air-broadening coefficients, together with their temperature dependence have been determined for a series of very high-J R lines in the ν_{3} band of CO_{2} located in the spectral region of some Advanced Moisture and Temperature Sounder (AMTS) channels. These channels are located in the transmission microwindows limited by these lines. The temperature dependence of the absorption coefficient in these microwindows has been measured for N_{2} and O_{2} broadening. It has been clearly demonstrated that a correcting shape factor χ(σ) independent of T cannot provide a good agreement between observed and calculated absorption in the range 193 K ≤ T ≤ 300 K for the spectral region of the AMTS channels.

Figure 1. Frequency dependence of the normalized absorption coefficient B^{N}_{2}_{0} (σ,T) (in cm^{-1} amagat^{-2}) in the centers of the R-branch troughs for N_{2} broadening: 296°K; 238°K; 193°K.

The absolute intensities and air-broadening coefficients, together with their temperature dependence have been determined for a series of very high-J R lines in the ν_{3} band of CO_{2} located in the spectral region of some Advanced Moisture and Temperature Sounder (AMTS) channels. These channels are located in the transmission microwindows limited by these lines. The temperature dependence of the absorption coefficient in these microwindows has been measured for N_{2} and O_{2} broadening. It has been clearly demonstrated that a correcting shape factor χ(σ) independent of T cannot provide a good agreement between observed and calculated absorption in the range 193 K ≤ T ≤ 300 K for the spectral region of the AMTS channels.

Figure 2. Frequency dependence of the normalized absorption coefficient B^{O}_{2}_{0} (σ,T) (in cm^{-1} amagat^{-2}) in the centers of the R-branch troughs for O_{2} broadening: 296°K; 238°K; 193°K.

Figure 1. Comparison of measured (dots) and theoretical (curves) roto-translational spectra of H_{2}+N_{2} pairs at five temperatures from 91° to 298°K.

The collision-induced absorption spectra of nitrogen–methane gas mixtures have been measured in the spectral region below 400 cm^{−1} at four temperatures, namely, 212, 179, 149, and 126 K. The measurements have involved the use of Fourier-transform infrared and microwave techniques as well as a far-infrared laser operating at 84.2 and at 15.1 cm^{−1}. These are compared with a theoretical line shape obtained from a convolution of free rotational spectra and a translational component as determined from information theory. The calculated spectra show good agreement with the experimental results only in the lower frequency region. An important feature of the theoretical development is that no adjustable parameters need be introduced.

Figure 4. Plots of the absorption coefficient per product density (a_{mix}) as a function of the frequency v at the four temperatures (a) 212°, (b) 179°, (c) 149°, and (d) 126°K. The experimental results are shown as the solid curves in the figures, and our theoretically derived line shapes are represented as the dotted curves. In (d), the theoretical contributions due to the various induction mechanisms are labelled Q, 0, and H for quadrupole, octopole, and hexadecapole induction, respectively.

The rototranslational absorption spectrum of gaseous methane has been measured at seven different temperatures from 296 to 140 K. We have analyzed both the spectral moments and the experimental absorption shapes, assuming that only octupolar and hexadecapolar induction mechanisms contribute to the absorption. This assumption allows us to parameterize the temperature dependence of both the intensity and the shape of the absorption band. The results obtained indicate that other contributions to absorption are not negligible.

Experimentally determined absorption band at 163°K and the best-fit curve obtained by using the MLEW model to describe the single line profiles. (a) octupolar contribution; (b) hexadecapolar contribution.

The rototranslational absorption spectrum of the CH_{4}-H_{2} gaseous mixture has been measured at five different temperatures from 296 to 140 K, in the frequency range 150–850 cm^{-1}. The absorption spectra due to the CH_{4}-H_{2} interactions have been analyzed considering only the quadrupolar induction of H_{2} on CH_{4} and the octupolar and hexadecapolar inductions of CH_{4} on H_{2}. Under this assumption, the experimental data have been well accounted for, thereby providing a reliable way of predicting the CH_{4}-H_{2} spectrum in a wide temperature range.

Figure 3. Absorption coefficient A_{HM}(v ) at 296° and 195°K. The vertical bars indicate the typical uncertainties in the experimental points. The broken line is the CH_{4}+H_{2 }absorption coefficient as obtained in previous measurements

Existing rototranslational collision-induced absorption (CIA) spectra of methane pairs are analyzed with the help of spectral profiles computed from quantum mechanics. Dipoles induced by octopolar and hexadecapolar fields, hexadecapolar overlap, and the gradient of the octopolar field are considered. The spectral contributions of both bound and free pairs of molecules are accounted for. The analysis which suggests a centrifugal distortion of rotating methane molecules permits one to reproduce from theory the measured CIA spectra at all temperatures (126–300 K) and over the full range of frequencies (> 700 cm^{−1} at high temperatures) with rms deviations that are smaller than the experimental uncertainties. The values of the octopole and hexadecapole moments of (nonrotating) CH_{4} molecules needed for that purpose are consistent with state-of-the-art ab initio computations for the first time in such work.

Figure 1. Absorption coefficients α(ω) of CH_{4}-CH_{4} fitted at different temperatures. Dots: experimental data at temperatures from 140° and 295°K from (20); and at 126°K from (22). J-dependent multipole moments according to Eqs. (9) and (10) are assumed (Table III). For all temperatures above 126°K the displayed spectra are shifted up by one decade; the maximum of the absorption coefficient at each temperature is near 10^{-5} cm^{-1} amagat^{-2} at all temperatures. As an example, the contributions due to the octopole and hexadecapole induction are shown for one temperature.

20. P. Codastefano, P. Dore, and L. Nencini, J . Quant. Spectrosc. Radiat. Transfer 35, 255-263 (1986).
22. I. R. Dagg, A. Anderson, S. Yan, W. Smith, C. G. Joslin, and L. A. A. Read, Canad. J. Phys.1986, in press.

In previous work (Borysow and Frommhold, 1986) rigorous quantum computations of the rototranslational absorption spectra of CH4-CH4 pairs have been communicated which closely reproduce existing laboratory measurements at temperatures from 124 to 300 K and over a range of frequencies from 0 to 750 per cm. Since the computations are complex, this work shows how the spectra can be reproduced from simple, analytical functions that closely model the quantum profiles and, thus, the laboratory measurements. For temperatures from 50 to 300 K, the rototranslational, collision-induced spectra of methane pairs can thus be obtained accurately on small computers in seconds. No measurements exist for comparison with the computational results at the lower temperatures (less than 124 K) but these data are believed to constitute the most dependable theoretical prediction that can presently be made.

Figure 1. CIA spectra of CH_{4}-CH_{4} at the temperatures of 50°, 110°, and 300°K, computed from a quantum mechanical formalism described elsewhere (Borysow and Frommhold 1987). The dimer bands are convoluted with a triangular slit function of 4.3 cm^{-1} resolution. The octopole-induced (dashed curve) and hexadecapole-induced (dot-dashed curve) contributions are shown, together with the total (solid curve).

The rototranslational absorption spectrum of gaseous N_{2} is analyzed, considering quadrupolar and hexadecapolar induction mechanisms. The available experimental data are accounted for by using a line-shape analysis in which empirical profiles describe the single-line translational profiles. We thus derive the simple procedure that allows one to predict the N_{2} spectrum at any temperature. On the basis of the results obtained for the pure gas, we also propose a procedure to compute the far-infrared spectrum of the N_{2}–Ar gaseous mixture. The good agreement between computed and experimental N_{2}–Ar data indicates that it is possible to predict the far-infrared absorption induced by N_{2} on the isotropic polarizability of any interacting partner.

Figure 1. Pure N_{2} absorption spectrum at 297 and 149 K: ... , experimental data from ref. 7; ----. best fit; - - -, hexadecapolar component. Vertical bars indicate typical uncertainties for the experimental points. At 297K, the best-fit parameters are S^{Φ}_{nn} = 1690 K-Å^{6}, δ^{0}_{nn} = 19.6 cm^{-1}, S^{0}_{nn} = 49 K-A", and δ^{Φ}_{nn} = 34.5 cm^{-1}. At 149 K, s:, = 1915 K-Å^{6}, δ^{0}_{nn} = 15 cm^{-1}: S^{Φ}_{nn} = 47 K-Å^{6}, and δ^{Φ}_{nn} = 23.5 cm^{-1}.

The rototranslational absorption spectrum of gaseous N_{2} is analyzed, considering quadrupolar and hexadecapolar induction mechanisms. The available experimental data are accounted for by using a line-shape analysis in which empirical profiles describe the single-line translational profiles. We thus derive the simple procedure that allows one to predict the N_{2} spectrum at any temperature. On the basis of the results obtained for the pure gas, we also propose a procedure to compute the far-infrared spectrum of the N_{2}–Ar gaseous mixture. The good agreement between computed and experimental N_{2}–Ar data indicates that it is possible to predict the far-infrared absorption induced by N_{2} on the isotropic polarizability of any interacting partner.

Figure 2. Pure N_{2} absorption spectra at 140 and 93 K: ^{... }, experimental data from ref. 9; ----, computed spectra multiplied by a normalizing factor F (F = 0.93 at 140 K and 1.12 at 93 K); ---, computed 90-K spectrum from ref. 5 (multiplied by a factor of 2 because of a different definition of the absorption coefficient).

A spectrum of the carbon dioxide trimer van der Waals species has been recorded near 3614 cm^{−}^{1} at sub‐Doppler resolution using an optothermal (bolometer‐detected) molecular‐beam color‐center laser spectrometer. A planar, cyclic structure with C_{3h} symmetry has been determined for the complex with a carbon–carbon separation of 4.0382(3) Å. The observed perpendicular band, corresponding to an in‐plane E^{′}‐symmetry vibration of the trimer, has been attributed to a localized excitation of the 2ν^{0}_{2} +ν_{3} combination mode of a CO_{2} subunit by virtue of its small blue shift (∼0.98 cm^{−}^{1}) from that of the isolated monomer.

Figure 1. Observed and calculated spectra of the CO_{2} trimer in the region of the 2ν^{0}_{2} + ν_{3} combination band of the monomer indicated by the negative-going R (0) line. Observed spectra for 3% CO_{2}:He mixture at 2, 3, and 5 atm driving pressure. Asterisks indicate transitions attributed to the CO_{2} dimer. Calculated transitions are simulated with Gaussian profiles of 20 MHz FWHM and an effective rotational temperature of 1.3 K

This is a comprehensive study of water-vapor line and continuum absorption in the 8–14μm atmospheric window by laser-photoacoustic spectroscopy. The characteristics of laser-photoacoustic spectroscopy and detectors are discussed with results on continuum and line absorption at selected CO_{2}-laser wavelengths.

We have assigned four weak absorption lines which occur at the CO_{2}-laser emissions 10P(40), 10R(20), 9P(38) and 9R(36) to pure rotational transitions of H_{2}O, and have determined the dependence of the continuum water-vapor absorption over the temperature range +70°C and −20°C. The measured negative temperature coefficient of the continuum is consistent with both monomer and dimer models, yet not with predictions of larger water clusters.

Experiments with supersaturated water vapor indicate that for S ⩾ 1 collision broadening of distant strong lines as well as water dimer absorption contribute to the continuum. However, the dimer absorption is an order of magnitude too small to cause a significant contribution at ambient atmospheric conditions.

We have investigated the effect of UV-radiation on the 8–14μm absorption of water vapor, buffered either with N_{2} or synthetic air. The observed changes are explained by UV-photodissociation of H_{2}O molecules and by ozone production. There is no evidence in favor of a cluster model.

Finally, we compared our measured spectra with LOWTRAN 6 and HITRAN models. The LOWTRAN yields a stronger negative temperature dependence than observed while the HITRAN does not predict the observed continuum absorption.

Figure 19. Measured temperature dependence of the pressure quadratic coefficients C_{s} of the continuum absorption at the (a) 10P(20), (b) 10P(24) and (c) 10P(30) CO_{2}-laser emissions. Dashed lines correspond to least square fits based on equation (25). Solid lines correspond to best fits on the basis of equation (29).

Dimer model: Suck S.H., Kassner J.L., Jr., Jamaguchi J. Water clusters interpretation of IR absorption spectra in the 8-14 μm wavelength region Appl.Opt. 18, No.15, 2609-2617 (1979).

This is a comprehensive study of water-vapor line and continuum absorption in the 8–14μm atmospheric window by laser-photoacoustic spectroscopy. The characteristics of laser-photoacoustic spectroscopy and detectors are discussed with results on continuum and line absorption at selected CO_{2}-laser wavelengths.

We have assigned four weak absorption lines which occur at the CO_{2}-laser emissions 10P(40), 10R(20), 9P(38) and 9R(36) to pure rotational transitions of H_{2}O, and have determined the dependence of the continuum water-vapor absorption over the temperature range +70°C and −20°C. The measured negative temperature coefficient of the continuum is consistent with both monomer and dimer models, yet not with predictions of larger water clusters.

Experiments with supersaturated water vapor indicate that for S ⩾ 1 collision broadening of distant strong lines as well as water dimer absorption contribute to the continuum. However, the dimer absorption is an order of magnitude too small to cause a significant contribution at ambient atmospheric conditions.

We have investigated the effect of UV-radiation on the 8–14μm absorption of water vapor, buffered either with N_{2} or synthetic air. The observed changes are explained by UV-photodissociation of H_{2}O molecules and by ozone production. There is no evidence in favor of a cluster model.

Finally, we compared our measured spectra with LOWTRAN 6 and HITRAN models. The LOWTRAN yields a stronger negative temperature dependence than observed while the HITRAN does not predict the observed continuum absorption.

Figure 20. Measured temperature dependence of the pressure quadratic coefficients C_{s} of the continuum absorption at the (a) 10P(20), (b) 10P(24), (c) 10P(30) and (d) 10P(38) CO_{2}-laser emissions. Solid lines represent best fits on the basis of equation (29).

This is a comprehensive study of water-vapor line and continuum absorption in the 8–14μm atmospheric window by laser-photoacoustic spectroscopy. The characteristics of laser-photoacoustic spectroscopy and detectors are discussed with results on continuum and line absorption at selected CO_{2}-laser wavelengths.

We have assigned four weak absorption lines which occur at the CO_{2}-laser emissions 10P(40), 10R(20), 9P(38) and 9R(36) to pure rotational transitions of H_{2}O, and have determined the dependence of the continuum water-vapor absorption over the temperature range +70°C and −20°C. The measured negative temperature coefficient of the continuum is consistent with both monomer and dimer models, yet not with predictions of larger water clusters.

Experiments with supersaturated water vapor indicate that for S ⩾ 1 collision broadening of distant strong lines as well as water dimer absorption contribute to the continuum. However, the dimer absorption is an order of magnitude too small to cause a significant contribution at ambient atmospheric conditions.

We have investigated the effect of UV-radiation on the 8–14μm absorption of water vapor, buffered either with N_{2} or synthetic air. The observed changes are explained by UV-photodissociation of H_{2}O molecules and by ozone production. There is no evidence in favor of a cluster model.

Finally, we compared our measured spectra with LOWTRAN 6 and HITRAN models. The LOWTRAN yields a stronger negative temperature dependence than observed while the HITRAN does not predict the observed continuum absorption.

Figure 24. Temperature dependence of the pressure quadratic coefficients C_{s} of the continuum absorption for the (a) 10P(20) and (b) 10P(24) CO_{2}-laser emissions. Solid lines represent best fits on the basis of equations (26) and (29).

An F‐center laser–molecular beam spectrometer has been used to obtain a sub‐Doppler resolution infrared spectrum of the carbon dioxide dimer. The vibrational mode investigated in this study corresponds to the ν_{1}+ν_{3} combination mode of the monomer located at 3716 cm^{−}^{1}. A qualitative assignment of the spectrum shows unambiguously that the equilibrium structure of the dimer is the slipped parallel, rather than the T‐shaped, geometry. The observed spectrum cannot be fit to within experimental error using conventional asymmetric rotor formalism. This may be due to a number of factors such as Fermi resonance between the upper state levels of the band and nearby levels of the dimer, such as seen in the monomer, or it could arise from tunneling effects arising from the two large amplitude motions in the dimer.

Figure 3. A comparison between a calculated and experimental carbon dioxide dimer spectrum. The calculated spectrum was obtained using the rotational constants given in Table II. Although most of the features in the spectrum are properly accounted for there are some extra transitions in the observed spectrum, most likely associated with the parallel band. A very accurate fit to the spectrum was not possible due to what appears to be a staggering of the K band origins due to tunneling.

For the purpose of calculating the far-infrared thermal radiation from Titan's atmosphere, the pressure-induced absorption in pure N_{2}, CH_{4} and in N_{2}-Ar, N_{2}-CH_{4}, and N_{2}-H_{2} mixtures has been modeled using recent laboratory measurements. The results are tabulated as a function of wavenumber and temperature, and gaseous absorption spectra are calculated for the composition and temperatures known to be typical of the troposphere of Titan.

Figure 2. The collision-induced spectrum of pure N_{2} + Ar at 126, 149, 179, 212, and 298°K. The measurements are from Dagget al. (1986). Points, FTIR data: Δ. microwave data at 15.1 cm-1; O, laser data at 84.2 cm^{-1}). The solid lines were calculated with the semi-empirical model described in the text. The spectra for different temperatures are vertically offset.

Dagg, I.R., Anderson, A., Yan, S., Smith, W. and Read, L.A.A., Collision-induced absorption in nitrogen at low temperatures. Canadian journal of physics, 63(5), pp.625-631. 1985, DOI: 10.1139/p86-002.

For the purpose of calculating the far-infrared thermal radiation from Titan's atmosphere, the pressure-induced absorption in pure N_{2}, CH_{4} and in N_{2}-Ar, N_{2}-CH_{4}, and N_{2}-H_{2} mixtures has been modeled using recent laboratory measurements. The results are tabulated as a function of wavenumber and temperature, and gaseous absorption spectra are calculated for the composition and temperatures known to be typical of the troposphere of Titan.

Figure 5. The collision-induced spectrum of N_{2 }+ H_{2} mixture at 91, 141, 165, 195 and 298°K. The measurements are from Dore et al. (1986).

Dore, P., A. Borysow, and L. Frommhold, Roto-translational far-infrared absorption spectra of H_{2}-N_{2} pairs, J. Chem. Phys. 84, 5211 (1986).

For the purpose of calculating the far-infrared thermal radiation from Titan's atmosphere, the pressure-induced absorption in pure N_{2}, CH_{4} and in N_{2}-Ar, N_{2}-CH_{4}, and N_{2}-H_{2} mixtures has been modeled using recent laboratory measurements. The results are tabulated as a function of wavenumber and temperature, and gaseous absorption spectra are calculated for the composition and temperatures known to be typical of the troposphere of Titan.

Figure 3. The collision-induced spectrum of pure CH_{4} at 140°, 163°, 195°, and 295°K. The measurements are from Codastefano et al. (1985, 1986).

Codastefano, P., P. Dore, and L. Nencini 1985. Far-infrared absorption spectra in gaseous methane from 138 to 296 K. In Phenomena Induced by Intermolecular Interactions (G. Birnbaum, Ed.), pp. 119-128. Plenum, New York.
Codastefano, P., P. Dore, and L. Nencini. 1986. Temperature dependence of the far-infrared absorption spectrum of gaseous methane. J. Quant. Spectrosc. Radiat. Transfer 35, 255-263.

For the purpose of calculating the far-infrared thermal radiation from Titan's atmosphere, the pressure-induced absorption in pure N_{2}, CH_{4} and in N_{2}-Ar, N_{2}-CH_{4}, and N_{2}-H_{2} mixtures has been modeled using recent laboratory measurements. The results are tabulated as a function of wavenumber and temperature, and gaseous absorption spectra are calculated for the composition and temperatures known to be typical of the troposphere of Titan.

Figure 4. The collision-induced spectrum of N_{2} + CH_{4} mixture at 126°, 149°, 179°, and 212°K. The measurements are from Dagg et al. (1986).

Dagg, I. R., A. Anderson, S. Yan, W. Smith, G. G. Joslin, and L. A. A. Read, 1986. Collision-induced absorption in gaseous mixtures of nitrogen and methane. Canad. J. Phys. 64, 1467-1474.

Existing measurements of the collision-induced rototranslational absorption spectrum of gaseous mixtures of helium and methane are analyzed using quantum profiles computed from recent interaction potentials and various assumptions concerning the interaction-induced dipole components. The computed profiles are expressed in terms of simple functions that reproduce closely both the computed spectra at temperatures from 40 to 350 K and the existing measurements at temperatures from 150 to 353 K. Such a simple analytical representation of the HeCH_{4} rototranslational spectra facilitates accurate modeling of planetary atmospheres in the far-infrared region of the spectrum.

Figure 1. Density normalized absorption coefficients α(ω)/n_{1}n_{2} of He+CH_{4} at three temperatures. Dots: Experimental data at 150°, 293°, and 353°K by Afanas’ev et al. (7). Solid line: Calculated spectra determined by fitting the range and amplitude of only the isotropic (A = 0) overlap contribution to the dipole induction. For all the temperatures above 150°K the displayed spectra are shitkl up by one decade; the maximum of the absorption coefficient at each temperature is near 10^{-6} cm^{-1}/amagat^{2}.

7. A. D. Afana'sev, M. 0. Bulanin, and M. V. Tonkov, Sov. Technol. Phys. Lett. 6, 1444-1446 (1980). [in Russian]

The absorption by pure CO_{2} beyond the ν_{3} bandhead has been measured with a grating spectrometer. Experiments have been made in the 0–60-bar and 291–751-K pressure and temperature ranges. Our room temperature determinations are in good agreement with previous ones and the measured temperature dependence above room temperature is consistent with recent determinations below 300 K. Lorentzian calculations, modified by the introduction of a line shape corrective factor x, are presented. Good agreement between the observed and calculated spectra is obtained when a temperature independent x factor, determined by Cousin et al. at 296 K, is used.

Figure 5. Wavenumber dependence of the pure CO_{2} normalized absorption coefficient at (a) 291°K; (b) 534°K; (c) 751°K; *, experimental; calculated with the Lorentzian model, Lorentzian model with the χ (296°K) factor of Refs. 4 and 5. 4. R. Le Doucen, C. Cousin, C. Boulet, and A. Henry, "Temperature Dependence of the Absorption in the Region Beyond the 4.3-μm Band Head of CO_{2}.1: Pure CO_{2} Case," Appl. Opt. 24,897-906 (1985).
5. C. Cousin-Lucasseau, "Absorption I.R. du CO2 dans la fenetre atmospherique autour de 4.2 μm - Determination de la dependance en temperature du coefficient d'absorption.- Influencedes interferences spectrales sur le profil observe," Thesis,Rennes (1987).

We present experimental results on the absorption by CO_{2}-N_{2} mixtures in the 4.3 μm region. Measurements have been made in the 300–800 K and 0–60 bar ranges; these are in good agreement with previous determinations below 296 K. Frequency- and temperature-dependent factors χ are introduced in order to account for the subLorentzian behavior of the CO_{2} far line-wings. Their dependences on temperature are deduced from experimental results beyond the 4.3μm band-head in the 193–773 K range for both CO_{2}-CO_{2} and CO_{2}-N_{2} and fitted by simple analytical functions. Comparisons are presented between experimental and theoretical spectra on both the low- and high-frequency sides of the band (2100–2600 cm^{-1} range). It is shown that calculations using χ factors are inaccurate near line-centers at elevated pressures.

Figure 4. Transmission spectra for pure CO_{2} (in a 4.4 cm long cell) for (a) 291°K and 50 bar and (b) 627°K and 32 bar (), Experimental data; ( ..... ), Eq. (4) with B_{i} parameters for the given temperature; ( .... ), Eqs. (3) and (4) with the parameters α_{i}, β_{i}, ε_{i} of Table 3.

We present a new analysis of the roto-translational absorption spectrum of gaseous methane for which experimental data are available at different temperatures. We consider components of the induced dipole associated at long range with induction by the octopole and hexadecapole multipole moments together with components associated with the gradient of both the octopolar and hexadecapolar fields. A satisfactory description of experimental data is obtained only if short range anisotropic overlap is included with the octopolar and hexadecapolar induction. A better description of the experimental absorption bands is obtained if, in addition, the intensity of the double transitions is slightly increased with increasing temperature. This increase is attributed to anisotropic overlap effects on the double transition spectrum, which have not been explicitly taken into account.

Figure 1. Methane absorption coefficient at 296°K. A. Comparison between experimental and computed spectrum. The computed spectrum is given by the sum of l= 3 (a), l = 4 (b) and double transition (c) components. B. The two components (3, 3) and (3, 4) of the double transition spectrum are shown. The parameters used in the computations are reported in the table (case a).

An attempt is made to describe the observed band shape deformations of a number of CO_{2} vibro-rotational absorption bands (1.4, 1.6 and 2.0 pm) making use of the ideas on the associative equilibrium in compressed gas. It is shown that the model of monomer-dimer equilibrium permits representation of the IR absorption band shapes within an experimental accuracy up to 5.0 MPa. The comparison of the found dimeric band shape with the calculated synthetic spectra of P- and T-isomers is also discussed.

Figure 3. Comparison of calculated and observed spectra for the 2.0 μm (a) and 1.6 μm (b) CO_{2} bands. - Measured spectra; o - present work profiles; --- sub-Lorentzian profile.

Spectra of the weakly bound van der Waals complexes H_{2}–N_{2} and H_{2}–CO have been studied in the mid‐ and far‐infrared regions corresponding to H_{2} vibrational and pure rotational frequencies, respectively. The experiments were done using a Fourier transform infrared spectrometer to study equilibrium gas samples at low temperature (77 K) with a long absorption path. The spectra of the complexes appear as fine structure located near the peaks of the lines in the collision‐induced spectrum of hydrogen. Compared to earlier studies of these species, the present results have more complete coverage of the various possible transitions, better resolution, and better signal to noise. New structure is reported which corresponds to hindered rotational transitions of the N_{2} component of an H_{2}–N_{2} complex and is reminiscent of patterns observed for the N_{2}–Ar and (N_{2})_{2} complexes. The relation of these results to far‐infrared measurements of Saturn’s satellite Titan, made by the Voyager spacecraft, is discussed.

Figure 1. Observed spectra of the enhancement by N_{2} of the collision-induced fundamental band of H_{2} at 77°K, recorded with a total gas density of about 1.3 amagats and a path length of 154 m. The upper curve (a) is for para-H_{2} and the lower curve (b) for normal H_{2}. The broad underlying absorption is due to collision-induced absorption in H_{2}-N_{2} collision pairs. Of interest here is the sharper structure, concentrated near the peak of each collision-induced line, which is due to bound H_{2}-N_{2} complexes.

Figure 8. The 4-μm continuum region at T = 296°K. (a) C_{s} vs wavenumber [27] and (b) the absorption coefficient vs wavelength.[37,38].

[27] D. E. Burch and R. L. Alt, AFGL-TR-84-0128, Ford Aerospace and Communications Corporation, Aeronutronic Division (1984).
[37] K. O. White, W. K. Watkins, C. W. Bruce, R. E. Meredith and F. G. Smith, Appl. Opr. 17, 2711 (1978)
[38] F. S. Mills. Dissertation. The Ohio State University (1975).

Thermal emission from the deep atmosphere of Venus can be detected on the nightside around 2.3 μm. The analysis of this radiation requires a reliable knowledge of the absorption in the far wings of the nearby allowed CO2 bands and of the absorption due to collision‐induced bands. We measured absorption coefficients for pure CO2 at pressures varying from 30 to 60 bars in the frequency range 3910–4570 cm−1 at 297.5 K. Values between 1.0 and 1.6 × 10−7 cm−1 amagat−2 are found in the 4100–4500 cm−1 interval where emission from the Venus nightside occurs. The comparison of experimental results with synthetic spectra calculated from a line by line code demonstrates that the Lorentzian line shape strongly overestimates the observed absorption, whereas the use of a χ factor extrapolated from the 3800–4000 cm−1 region does not provide enough opacity.

Figure 2. Binary absorptions coefficients α_{11}(ν) for pure CO_{2} at 297.5°K in semi logarithmic scale: pluses, experimental results. The upper curve in Figure 2b represents the Lorentzian calculation (i.e., χ=1 in equation (1)). The solid curve in Figure 2a and the lower curve in Figure 2b have been calculated with the Burch et al. [1969] χ-factor. The calculations do not include the very weak contributions from the allowed bands located in the 3900-4700 cm^{-1} region.
Burch, D.E., D.A. Gryvnak, R. R. Patty, and C. E. Bartky, Absorption of infrared radiant energy by CO_{2} and H_{2}O. IV-Shapes of collision-broadened CO_{2} lines, J. Opt. Soc. Am., 59, 267-280, 1969.

The absorption in the 2400-cm^{−1} region is dominated by continuum absorptions from carbon dioxide and nitrogen, and it is important to be able to describe the temperature dependence of these two continua. A series of measurements of atmospheric transmission over a 5.7-km range were carried out during the summer and winter seasons of 1988. Following a brief description of the experiments a selected number of spectra, covering the temperature range from −21.4 to 30.3°C, are presented. These measurements are compared with predictions by the atmospheric transmission model fascod2 and modified versions using models of the carbon dioxide and the nitrogen continua derived from experimental laboratory measurements. Finally, an improved temperature-dependent model for the nitrogen continuum is derived from atmospheric transmission measurements. The model covers the range of temperatures found in the troposphere and differs significantly from laboratory-based measurements.

Relative attenuation contributions at temperature extremes:(a) T = +30.3^{0}C, humidity = 14.9 g m^{- 3}; (b) T =-21.4^{o}C, humidity = 0.77 g m^{- 3}

We present a simple model which allows for the computation of the rototranslational band of the collision induced absorption spectra of N_{2}-CH_{4} pairs at temperatures between 70 and 300 K and at frequencies of up to ~550 cm^{-1}. The agreement with current experimental data has been obtained by adjusting the classical, multipole induced dipolar components by adding semiempirical quantum corrections. We have included a Q_{6}-induction term, never considered before, which we believe is essential for the agreement we have obtained. With the set of temperature-independent parameters derived from fitting the experimental data, our model reproduces recent laboratory measurements within 124-300 K temperature and the 10-600 cm^{-1} frequency range, within few percent root mean square. The method provides reliable temperature dependence of the absorption coefficient as a function of frequency at temperatures as low as 70 K, which are much lower than those at which laboratory measurements are taken. The work is of significance for modeling the infrared opacity of Titan's atmosphere.

Figure 2. Rototranslational collision-induced spectra of N_{2}-CH_{4} pairs. Markers denote experimental data by (Dagg et al. 1986) at temperatures of 126°, 149°, 179°, and 212°K. Solid lines show our theoretical results based on fitted parameters μ obtained at this work.

I. R. Dagg, , A. Anderson, , S. Yan, , W. Smith, , C. G. Joslin, and , L. A. A. Read. Collision-induced absorption in gaseous mixtures of nitrogen and methane. Canadian Journal of Physics, 1986, 64(11): 1467-1474, https://doi.org/10.1139/p86-260.

We present a simple model which allows for the computation of the rototranslational band of the collision induced absorption spectra of N_{2}-CH_{4} pairs at temperatures between 70 and 300 K and at frequencies of up to ~550 cm^{-1}. The agreement with current experimental data has been obtained by adjusting the classical, multipole induced dipolar components by adding semiempirical quantum corrections. We have included a Q_{6}-induction term, never considered before, which we believe is essential for the agreement we have obtained. With the set of temperature-independent parameters derived from fitting the experimental data, our model reproduces recent laboratory measurements within 124-300 K temperature and the 10-600 cm^{-1} frequency range, within few percent root mean square. The method provides reliable temperature dependence of the absorption coefficient as a function of frequency at temperatures as low as 70 K, which are much lower than those at which laboratory measurements are taken. The work is of significance for modeling the infrared opacity of Titan's atmosphere.

Figure 3. Rototranslational collision-induced spectra of N_{2}-CH_{4} pairs. Markers denote experimental data by (Birnbaum et al. 1993) at temperatures of 162°K, 195°K, and 297°K. Solid thick lines show our theoretical results, based on fitted parameters m obtained in this work. Separate dipolar contributions are marked with thin lines and denote induction by: dots, N_{2} Θ, dash-dot-dot, N_{2} Φ; dashes, CH_{4} Ω; dash-dot, CH_{4} Φ; dot-dash-dash, CH_{4} Q_{6} ; and long dashes, double transitions.

G. Birnbaum, A. Borysow, A. Buechele, Collision‐induced absorption in mixtures of symmetrical linear and tetrahedral molecules: Methane–nitrogen, J. Chem. Phys. 99, 3234 (1993); https://doi.org/10.1063/1.465132.

We present a simple model which allows for the computation of the rototranslational band of the collision induced absorption spectra of N_{2}-CH_{4} pairs at temperatures between 70 and 300 K and at frequencies of up to ~550 cm^{-1}. The agreement with current experimental data has been obtained by adjusting the classical, multipole induced dipolar components by adding semiempirical quantum corrections. We have included a Q_{6}-induction term, never considered before, which we believe is essential for the agreement we have obtained. With the set of temperature-independent parameters derived from fitting the experimental data, our model reproduces recent laboratory measurements within 124-300 K temperature and the 10-600 cm^{-1} frequency range, within few percent root mean square. The method provides reliable temperature dependence of the absorption coefficient as a function of frequency at temperatures as low as 70 K, which are much lower than those at which laboratory measurements are taken. The work is of significance for modeling the infrared opacity of Titan's atmosphere.

Figure 4. Comparison of the theoretical estimations based on the current (solid line) and the previous (Courtin 1988) (dashed line) models. The absorption spectra are shown at temperatures of 70°, 120°, and 170°K and frequencies up to 600 cm^{-1}.

Régis Courtin, Pressure-induced absorption coefficients for radiative transfer calculations in Titan's atmosphere, Icarus, Volume 75, Issue 2, August 1988, Pages 245-254, https://doi.org/10.1016/0019-1035(88)90004-8

We present experimental and theoretical studies of medium infrared absorption by pure water vapor. Measurements have been made in the 1900–2600 cm^{-1} and 3900–4600 cm^{-1} regions, for temperatures and pressures in the 500–900 K and 0–70 atm ranges, respectively. They are consistent with available data and enable the determination of continuum absorption parameters. It is shown that calculations with line shapes derived from the impact approximation are very inaccurate. Models accounting for the finite durations of collisions and line-mixing through wave-number dependent effective broadening parameters are introduced. The latter have been determined using two different approaches, which are (i) empirical determinations from fits of experimental data and (ii) direct predictions from first principles using a statistical approach. Effective broadening parameters obtained using these two different approaches are in satisfactory agreement for both the temperature and wavenumber dependencies. These data are tested by calculations of continua in various spectral regions and the agreement with measured values is satisfactory. The remaining discrepancies probably result from the influence of the internal structures of the absorption bands considered and thus from the influence of line-mixing. Nevertheless, accurate predictions are obtained in wide temperature and spectral ranges when the total absorption at elevated density is considered. This agreement, which is due to the relatively weak continuum absorption and large contributions of nearby lines, makes the present models suitable for most practical applications involving elevated densities.

Figure 4. Experimental pure H_{2}O transmission spectra for 575 K.

We present experimental and theoretical studies of medium infrared absorption by pure water vapor. Measurements have been made in the 1900–2600 cm^{-1} and 3900–4600 cm^{-1} regions, for temperatures and pressures in the 500–900 K and 0–70 atm ranges, respectively. They are consistent with available data and enable the determination of continuum absorption parameters. It is shown that calculations with line shapes derived from the impact approximation are very inaccurate. Models accounting for the finite durations of collisions and line-mixing through wave-number dependent effective broadening parameters are introduced. The latter have been determined using two different approaches, which are (i) empirical determinations from fits of experimental data and (ii) direct predictions from first principles using a statistical approach. Effective broadening parameters obtained using these two different approaches are in satisfactory agreement for both the temperature and wavenumber dependencies. These data are tested by calculations of continua in various spectral regions and the agreement with measured values is satisfactory. The remaining discrepancies probably result from the influence of the internal structures of the absorption bands considered and thus from the influence of line-mixing. Nevertheless, accurate predictions are obtained in wide temperature and spectral ranges when the total absorption at elevated density is considered. This agreement, which is due to the relatively weak continuum absorption and large contributions of nearby lines, makes the present models suitable for most practical applications involving elevated densities.

Figure 12. Pure H_{2}O transmissivities in the wing of the ν_{2} band (2.5 cm^{-1} resolution): - experimental data; --- calculated from Eqs. (15) and (17) and Table 4. Fig. (12) 575°K and the densities: 10.5 Am; 21.3 Am; and 38.2 Am. Fig. (112) 775°K and the densities: 8.30 Am; 14.2 Am; and 25.6 Am.

The shape of the 00^{0}3–00^{0}0 CO_{2} band in helium has been^{ }investigated at room temperature over an extended range of perturber^{ }pressures (0–140 atm). Various and strong deviations from an additive^{ }superposition of Lorentzian lines have been observed, due to important^{ }line mixing effects enhanced by the specific structure of the^{ }R branch in this band.

Observed absorption coefficient in the region just before the bandhead for various experimental conditions

A simple approach is developed in order to model the influence of collisions on the shape of infrared absorption by linear molecules. It accounts for line‐mixing effects within, as well as between, the different branches (P, Q, R) of the band. It is based on use of the strong collision model, of a classical representation of rotational levels, and of the rigid rotor approximation. The absorption coefficient then has a very simple analytical expression; its wave number and pressure dependencies are computed by using eight parameters which depend on the considered vibrational transition, the temperature, and the nature of the perturber only. These quantities are band‐averaged values of the detailed spectroscopic and collisional parameters of the molecular system. Tests of the model are presented in the ν_{3} and 3ν_{3} bands of CO_{2} perturbed by He and Ar at elevated pressures. They demonstrate the accuracy of our approach in accounting for the effects of collisions on the spectral shape in a wide density range; indeed, the superposition of Lorentzian individual lines at low pressure, as well as the collapse (narrowing) of the band at very high pressure are satisfactory predicted.

Figure 1. Absorption spectra in the 3ν_{3} CO_{2} band perturbed by He at room temperature 297 K. • Experimental values (Ref. 6); — calculated values (Eqs. (22), (24), and (25) and fitted parameters of Table II); --- calculated from the Lorentzian model and HITRAN database (Ref. 8).
[6] L. Ozanne, Nguyen-Van-Thanh, C. Brodbeck, J. P. Bouanich, J. M. Hartmann, and C. Boulet, J. Chem. Phys. 102, 7306 (1995).
[8] L. S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. C. Benner, V. Malathy Devi, J. M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, and R. A. Toth, J. Quant. Spectrosc. Radiat. Trans. 48, 469 (1992).

A simple approach is developed in order to model the influence of collisions on the shape of infrared absorption by linear molecules. It accounts for line‐mixing effects within, as well as between, the different branches (P, Q, R) of the band. It is based on use of the strong collision model, of a classical representation of rotational levels, and of the rigid rotor approximation. The absorption coefficient then has a very simple analytical expression; its wave number and pressure dependencies are computed by using eight parameters which depend on the considered vibrational transition, the temperature, and the nature of the perturber only. These quantities are band‐averaged values of the detailed spectroscopic and collisional parameters of the molecular system. Tests of the model are presented in the ν_{3} and 3ν_{3} bands of CO_{2} perturbed by He and Ar at elevated pressures. They demonstrate the accuracy of our approach in accounting for the effects of collisions on the spectral shape in a wide density range; indeed, the superposition of Lorentzian individual lines at low pressure, as well as the collapse (narrowing) of the band at very high pressure are satisfactory predicted.

Figure 2. Absorption spectra in the 3ν_{3} CO_{2} band perturbed by He at room temperature 297 K. n_{CO2}=4.61 Am, n_{He}=409 Am. • Experimental values (Ref. 6); — calculated values (Eqs. (22), (24), and (25) and fitted parameters of Table II); --- calculated from the Lorentzian model and HITRAN database (Ref. 8).
[6] L.Ozanne, Nguyen-Van-Thanh, C. Brodbeck, J. P. Bouanich, J. M. Hartmann, and C. Boulet, J. Chem. Phys. 102, 7306 (1995).
[8] L.S. Rothman, R. R. Gamache, R. H. Tipping, C. P. Rinsland, M. A. H. Smith, D. C. Benner, V. Malathy Devi, J. M. Flaud, C. Camy-Peyret, A. Perrin, A. Goldman, S. T. Massie, L. R. Brown, and R. A. Toth, J. Quant. Spectrosc. Radiat. Trans. 48, 469 (1992).

Figure 4. Portion of observed (CO_{2})_{2} +(CO_{2})_{3}. Note 3:1 spacing of (CO_{2})_{3} Q-branches, and sharp appearance of the (CO_{2})_{3}^{r}Q_{0}-branch due to l-type doubling (conditions: 1.8% CO_{2} in He, P_{0}=1 bar).

Figure 3. Portion of observed (CO_{2})_{2}+(CO_{2})_{3} infrared spectrum including central a -type Q-branch of (CO_{2})_{2}. Note resolved P-branch transitions for (CO_{2})_{3}. The P-branch transitions are labeled only for subbands where K"≥15 (conditions: 1.8% CO_{2} in He, P_{0}=1.0 bar).

Figure 10. Unidentified Q-branch like absorptions in the observed (CO_{2})_{2}+(CO_{2})_{3 } spectrum. The features labeled Q_{A} and Q_{B} clearly do not belong to the dimer or cyclic trimer. Moreover, they are within 0.4 cm^{-1} of the predicted vibrational origin of the trimer asymmetric top isomer from model potentials. Overlap of (CO_{2})_{3} transitions with the head of both QA and QB is coincidental (conditions: 1.8% CO_{2} in He,P_{0}=1 bar)

High resolution infrared spectra of a previously unidentified noncyclic isomer of (CO2)3 have been obtained via direct absorption of a 4.3 μm diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled νI and νIII) are observed, corresponding to the two most infrared active linear combinations of the three constituent CO2 monomer asymmetric stretches: νI is redshifted −5.85 cm−1 from the monomer vibrational origin and is predominately a c‐type band of an asymmetric top, while νIII is blueshifted +3.58 cm−1 and is predominately an a‐type band. Transitions with Ka+Kc=odd (even) in the ground (excited) state are explicitly absent from the spectra due to the zero nuclear spin of CO2; this rigorously establishes that the noncyclic isomer has a C2 symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipole interaction model between the high‐frequency stretches of the CO2 monomers. Rotational constants are determined by fits of transition frequencies to an asymmetric top Hamiltonian. These results are used to determine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C2 symmetry about the b inertial axis. The structural parameters are then used to test several trial CO2–CO2interaction potentials.

Figure 4. Portion of noncyclic (CO_{2})_{3} isomer nI band spectrum. The observed spectrum corresponds to a c-type band of an asymmetric top, with a hybrid character of <0.10. The model spectrum (Gaussian width 575 MHz, 0.87 c type, 0.13 a type) correctly reproduces the observed features, including the strong Q branch at 2343.3 cm^{-1}. The gaps in the observed spectrum are due to CO_{2} monomer transitions ~including hot band and ^{16}O^{12}C^{17}O transitions! that have been excised for visual clarity. Conditions: 2% CO_{2} seeded in He at 0.8 bar, 20 passes of probe through expansion.

High resolution infrared spectra of a previously unidentified noncyclic isomer of (CO2)3 have been obtained via direct absorption of a 4.3 μm diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled νI and νIII) are observed, corresponding to the two most infrared active linear combinations of the three constituent CO2 monomer asymmetric stretches: νI is redshifted −5.85 cm−1 from the monomer vibrational origin and is predominately a c‐type band of an asymmetric top, while νIII is blueshifted +3.58 cm−1 and is predominately an a‐type band. Transitions with Ka+Kc=odd (even) in the ground (excited) state are explicitly absent from the spectra due to the zero nuclear spin of CO2; this rigorously establishes that the noncyclic isomer has a C2 symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipole interaction model between the high‐frequency stretches of the CO2 monomers. Rotational constants are determined by fits of transition frequencies to an asymmetric top Hamiltonian. These results are used to determine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C2 symmetry about the b inertial axis. The structural parameters are then used to test several trial CO2–CO2interaction potentials.

Figure 6. Portion of ν_{III} band of the noncyclic (CO_{2})_{3} isomer. Some of the main P-branch transitions of the noncyclic trimer are labeled in the model spectrum, and indicated by arrows in the observed spectrum. Also note the strong Q branch marked with an asterisk in the observed spectrum. This corresponds to none of the predicted features, and probably belongs to a cluster larger than the trimer. Conditions: 0.6% CO_{2} in He, P_{0}=2.1 bar, 2 passes. Model spectrum: T_{rot}=6.5 K, linewidth is 50 MHz. Hybrid character: 0.78 a type, 0.22 c type.

The ν_{2} and 3ν_{3} bands of CO_{2} in helium baths at 193 K have studied with a Fourier transform interferometer. The behavior of the band shapes has been explored at moderate densities. The energy corrected sudden (ECS) approximation is used to model the relaxation matrix in order to account for line mixing effects. The basis cross-sections were calculated with the simple power law (P). Computed spectra are in good agreement with the observed ones. Measured broadening coefficients are also comparable with the ones derived from the ECS-P model.

Figure 2. Comparison between experimental, Lorentzian and ECS absorption coefficients in the 3ν_{3} band region. (a) P[CO_{2}]= 46 Torr., P[He] = 2.5 atm., L = 56 m; (b) P[CO_{2}] = 159 Torr, P[He] = 5 atm., L = 96 m.

We present an experimental study of the self- and N_{2}-broadened H_{2} O continuum in microwindows within the ν_{2} fundamental centered at ~1600 cm^{−1}. The continuum is derived from transmission spectra recorded at room temperature with a BOMEM Fourier transform spectrometer at a resolution of ~0.040 cm^{−1}. Although we find general agreement with previous studies, our results suggest that there is significant near-wing super-Lorentzian behavior that produces a highly wave-number-dependent structure in the continuum as it is currently defined.

Figure 5. Self-broadened continuum coefficients from this research, Burch, CKD models, Ma and Tipping, and impact theory.

[5] R. H. Tipping and Q. Ma, Theory of the water vapor continuum and validations, Atmos. Res. 36, 69–94 (1995), and references therein.
[8] S. A. Clough, in The water vapor continuum and its role in remote sensing, in Optical Remote Sensing of the Atmosphere, Vol. 2 of 1995 OSA Technical Digest Series Optical Society of America, Washington, D.C., 1995, pp. 76–78.
[14] D. E. Burch, Continuum absorption by H_{2}O, Ford Aerontronic Rep. AFGL-TR-81-0300, U.S. Air Force Geophysics Laboratory, Hanscom Air Force Base, Mass., 1982.

The shapes of Q-branches in CO_{2} spectra in the 580–850 cm^{−1} region for pure gas and mixtures CO_{2}---He and CO_{2}---Ar have been studied at resolutions up to 0.002cm^{−1}. The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11^{1}02-02^{2}01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q-branches. The observed Q-branch shape transformation is explained by line mixing effects.

Figure 5. Comparison between experimental and Lorentzian profiles for the Q-branch at 720 cm^{-1}. The insert gives the ratio of experimental absorption coefficient to that expected from a sum of Lorentzian lines.

The shapes of Q-branches in CO_{2} spectra in the 580–850 cm^{−1} region for pure gas and mixtures CO_{2}---He and CO_{2}---Ar have been studied at resolutions up to 0.002cm^{−1}. The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11^{1}02-02^{2}01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q-branches. The observed Q-branch shape transformation is explained by line mixing effects.

Figure 6. Comparison between experimental and Lorentzian profiles for the Q-branch at 720 cm^{-1}. The insert gives the ratio of experimental absorption coefficient to that expected from a sum of Lorentzian lines.

The shapes of Q-branches in CO_{2} spectra in the 580–850 cm^{−1} region for pure gas and mixtures CO_{2}---He and CO_{2}---Ar have been studied at resolutions up to 0.002cm^{−1}. The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11^{1}02-02^{2}01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q-branches. The observed Q-branch shape transformation is explained by line mixing effects.

Figure 7. Comparison between experimental and Lorentzian profiles for the Q-branch at 618 cm^{-1}. The insert gives the ratio of experimental absorption coefficient to that expected from a sum of Lorentzian lines.

The shapes of Q-branches in CO_{2} spectra in the 580–850 cm^{−1} region for pure gas and mixtures CO_{2}---He and CO_{2}---Ar have been studied at resolutions up to 0.002cm^{−1}. The branch broadening coefficients were measured in the pressure range from a few torr to 50 atm. Those for 11^{1}02-02^{2}01 Π-Δ transition are different for pressures below and above 5 atm. At higher pressures the branch broadening coefficients are similar for all Q-branches. The observed Q-branch shape transformation is explained by line mixing effects.

Figure 10. Comparison between experimental and Lorentzian profiles in the wings of the ν_{2} Q-branch. The insert gives the ratio of experimental absorption coefficient to that expected from a sum of Lorentzian lines. The ratio is only given in the region where the experimental profile is not saturated.

Measurements of pure CO_{2} absorption in the 2.3-μm region are presented. The 3800–4700-cm^{−1} range has been investigated at room temperature for pressures in the 10–50-atm range by using long optical paths. Phenomena that contribute to absorption are listed and analyzed, including the contribution of far line wings as well as those of the central region of both allowed and collision-induced absorption bands. The presence of simultaneous transitions is also discussed. Simple and practical approaches are proposed for the modeling of absorption, which include a line-shape correction factor χ that extends to approximately 600 cm^{−1} from line centers.

Figure 7. Room temperature pure CO_{2} absorption coefficients for a density of 20 amagats: o, experimental values; solid curve, computed values accounting for allowed and induced transitions with the optimized x factor of Table 2 and the HITEMP database; dashed curve, computed contribution of the far wings of allowed bands centered outside the considered spectral region. The lower plot gives the relative difference between observed and computed spectra.

Measurements of absorption coefficients in the 3v_{3} band of CO_{2} at 1.44 μm perturbed by Ar up to 146 bar have been analyzed by using two line-mixing theoretical calculations within the impact approximation. In the first approach, the relaxation operator is treated semi-classically with adiabatic corrections. In the second, the relaxation operator is modelled with the Energy Corrected Sudden (ECS) approximation associated with a power fitting law providing the basic rotational state-to-state rates. Although the line-coupling spectroscopic cross-sections of the two models are significantly different, they both lead to satisfactory agreement with bandshapes at moderate densities (< 100 Amagat). Significant deviations between experimental and calculated spectra appear at higher densities. They are mainly attributed to the probable breakdown of the impact and binary collision approximations and to a number of reasons including an incorrect ECS calculation of the interbranch coupling, the nonlinear density dependence due to the finite volume of the molecules, and the neglect of the unknown imaginary part of the off-diagonal elements in the calculated relaxation matrix.

Figure 10. Absorption coefficients in the central region of the 3ν_{3} band of CO_{2 }compressed by Ar at high density (n_{1} = 4.68 Am and n_{2} = 169.4 Am). • Experimental results; Calculated with the ECS model corrected with the interbranch corrective factor b_{P-R} = 0.20. (a) Only individual lineshifts are taken into account: (b) in addition to the individual lineshifts a corrected lineshift of: -4.5 x 10^{-3}cm^{-1} Am^{-1} in the R-branch and -25 x 10^{-3}cm^{-1} Am^{-1 }in the P-branch are taken into account.

Measurements of absorption coefficients in the 3v_{3} band of CO_{2} at 1.44 μm perturbed by Ar up to 146 bar have been analyzed by using two line-mixing theoretical calculations within the impact approximation. In the first approach, the relaxation operator is treated semi-classically with adiabatic corrections. In the second, the relaxation operator is modelled with the Energy Corrected Sudden (ECS) approximation associated with a power fitting law providing the basic rotational state-to-state rates. Although the line-coupling spectroscopic cross-sections of the two models are significantly different, they both lead to satisfactory agreement with bandshapes at moderate densities (< 100 Amagat). Significant deviations between experimental and calculated spectra appear at higher densities. They are mainly attributed to the probable breakdown of the impact and binary collision approximations and to a number of reasons including an incorrect ECS calculation of the interbranch coupling, the nonlinear density dependence due to the finite volume of the molecules, and the neglect of the unknown imaginary part of the off-diagonal elements in the calculated relaxation matrix.

Figure 9. Absorption coefficients in the central region of the 3ν_{3} band of CO_{2 }compressed by Ar at high density. Experimental; Calculated with the SCA model . (a) n_{1} =4.78 Am and n_{2} = 112.3 Am. (b) n_{1} = 4.68 Am and n_{2} = 169.4 Am.

Measurements of absorption coefficients in the 3v_{3} band of CO_{2} at 1.44 μm perturbed by Ar up to 146 bar have been analyzed by using two line-mixing theoretical calculations within the impact approximation. In the first approach, the relaxation operator is treated semi-classically with adiabatic corrections. In the second, the relaxation operator is modelled with the Energy Corrected Sudden (ECS) approximation associated with a power fitting law providing the basic rotational state-to-state rates. Although the line-coupling spectroscopic cross-sections of the two models are significantly different, they both lead to satisfactory agreement with bandshapes at moderate densities (< 100 Amagat). Significant deviations between experimental and calculated spectra appear at higher densities. They are mainly attributed to the probable breakdown of the impact and binary collision approximations and to a number of reasons including an incorrect ECS calculation of the interbranch coupling, the nonlinear density dependence due to the finite volume of the molecules, and the neglect of the unknown imaginary part of the off-diagonal elements in the calculated relaxation matrix.

Figure 7. Absorption coefficients in the central region of the 3ν_{3}, band of CO_{2 }compressed by Ar at high density. Experimental; Calculated with the Lorentzian model, ECS model. (a) n_{1} =4.78 Am and n_{2} = 112.3 Am. (b) n_{1} = 4.68 Am and n_{2} = 169.4 Am.

Line mixing in CO_{2} Q branches is studied in the 13.2 μm region from balloon-borne atmospheric transmission measurements. Eighty-five spectra were recorded (at 67°N, 22°E) in the 660–960 cm^{−1} region using a Fourier transform spectrometer with an unapodized resolution (full width at half maximum) of 0.013 cm^{−1}. They involve balloon altitudes and tangent heights from 13 to 30 km and a large range of optical thicknesses. The results of computations assuming Voigt line shapes as well as accounting for line mixing are compared with measured transmissions in three CO_{2} Q branches of different intensities and symmetries. They confirm the significant effect of line mixing and validate recent line coupling models. As a consequence of the accuracy of the forward approach the Q branches can be used for remote sensing; this should lead to improved precision of pressure/temperature retrievals since absorption in the Q branch wing region is particularly pressure sensitive.

Figure 5. Measured transmission and computed results obtained accounting for line-mixing by using the models LM-R and LM-S in the 720 cm^{-1} region for spectra (a) 1 and (b) 84.
LM-R and LM-S - two models of accounting for the line-mixing. LM – line-mixing within the first order perturbation theory [Rosenkranz E.W. IEEE Trans. Antennas Propag. AP-23, 498-506, 1975, Smith E.W. J Chem Phys 74. 6658-6673. 1981]. LM-R model, parameters from Rodrigers et al JQSRT 1997 with HITRAN 1996. LM-S model, parameters from Strow L.L. anonymous ftp site, 1994. Table 1. Main characteristics of some of the balloon-borne spectra.

Line mixing in CO_{2} Q branches is studied in the 13.2 μm region from balloon-borne atmospheric transmission measurements. Eighty-five spectra were recorded (at 67°N, 22°E) in the 660–960 cm^{−1} region using a Fourier transform spectrometer with an unapodized resolution (full width at half maximum) of 0.013 cm^{−1}. They involve balloon altitudes and tangent heights from 13 to 30 km and a large range of optical thicknesses. The results of computations assuming Voigt line shapes as well as accounting for line mixing are compared with measured transmissions in three CO_{2} Q branches of different intensities and symmetries. They confirm the significant effect of line mixing and validate recent line coupling models. As a consequence of the accuracy of the forward approach the Q branches can be used for remote sensing; this should lead to improved precision of pressure/temperature retrievals since absorption in the Q branch wing region is particularly pressure sensitive.

Figure 6. Deviations between measured and computed transmissions in the 720 cm^{-1} region for spectra (a) 1 and (b) 84 of Figures 5a and 5b with a numerically downgraded resolution of 0.05 cm^{-1} (FWHM).

We present high-density experimental and theoretical results on CO_{2}---Ar gas-phase absorption in the ν_{3} and 3ν_{3} infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. A non-linear perturber density dependence of the absorption, clearly shown in the far wing, is attributed to the finite volume of the molecules. Furthermore, experiments show vibrational dephasing and narrowing effects. We have performed line-mixing computations based on the Energy Corrected Sudden approximation (ECS impact model). Significant discrepancies between experimental and calculated spectra appear when pressure increases. We then tested the influence of the finite duration of collision by using interpolations between ECS and quasi-static calculations, and we have evaluated the sensitivity of the band profiles to the interbranch mixing effects. Finally, an effective width is used in order to take other effects into account.

Figure 3. Absorption coefficients in the central region of the 3ν_{3} band: •, measured; ____, calculated with the ECS model. (a) n_{CO2}=3.26 Am and n_{Ar}=283.1 Am (n'_{Ar}=342.4 Am); (b) n_{CO2}=3.26 Am and n_{Ar}=545.5 Am (n'_{Ar}=765.6 Am).

We present high-density experimental and theoretical results on CO_{2}---Ar gas-phase absorption in the ν_{3} and 3ν_{3} infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. A non-linear perturber density dependence of the absorption, clearly shown in the far wing, is attributed to the finite volume of the molecules. Furthermore, experiments show vibrational dephasing and narrowing effects. We have performed line-mixing computations based on the Energy Corrected Sudden approximation (ECS impact model). Significant discrepancies between experimental and calculated spectra appear when pressure increases. We then tested the influence of the finite duration of collision by using interpolations between ECS and quasi-static calculations, and we have evaluated the sensitivity of the band profiles to the interbranch mixing effects. Finally, an effective width is used in order to take other effects into account.

Figure 7. ν_{3} band wing parameters B^{0}_{CO2-Ar}. Experimental results from: Refs.9,23; this work. Calculations with: the ECS impact model; the impact/quasi-static interpolation model; the Lorentzian model.
[9] Boissoles J., Menoux V., Le Doucen R., Boulet C., Robert D. Collisionally induced population transfer effects in infrared absorption spectra. II. The wing of the Ar-broadened ν_{3 }band of CO_{2}, J.Chem.Phys. 91, No.4, 2163-2171 (1989)
[23] Bulanin M.O., Dokuchaev A.B., Tonkov M.V., Filipov N.N. Influence of the line interference on the vibratio-rotation band shapes, JQSRT 31, No.6, 521-543 (1984).

We present high-density experimental and theoretical results on CO_{2}---Ar gas-phase absorption in the ν_{3} and 3ν_{3} infrared bands. Measurements have been made at room temperature for pressures up to 1000 bar in both the central and wing regions of the bands. A non-linear perturber density dependence of the absorption, clearly shown in the far wing, is attributed to the finite volume of the molecules. Furthermore, experiments show vibrational dephasing and narrowing effects. We have performed line-mixing computations based on the Energy Corrected Sudden approximation (ECS impact model). Significant discrepancies between experimental and calculated spectra appear when pressure increases. We then tested the influence of the finite duration of collision by using interpolations between ECS and quasi-static calculations, and we have evaluated the sensitivity of the band profiles to the interbranch mixing effects. Finally, an effective width is used in order to take other effects into account.

Table 2. Binary absorption coefficients B^{0}_{CO2-Ar} and density effect parameter c_{CO2-Ar }[see Eq.(11)] in the ν_{3} band wing.
Bibliography

Absorption spectra of gas-phase molecular oxygen and zero air at temperatures of 223 and 283 K have been measured in the laboratory using a coolable multipass-optics gas cell and Fourier transform spectroscopy in the wavelength range 455 to 830 nm (12,000–22,000 cm^{−1}). Net absorption cross sections of the O_{2}A−, B−, and γ-bands at <0.002 nm spectral resolution, and pressures of 100 and 1000 hPa zero air have been determined. Binary absorption cross sections of the collision-induced O_{4} bands at <0.18 nm spectral resolution and a pressure of 1000 hPa pure oxygen have been determined, with corrections for the O_{2} γ-band absorption. Calculated integrated absorption intensities and, for the O_{2}A− and B−bands, “effective” Einstein A-coefficients are compared with previous literature values.

Figure 10. Net binary absorption cross sections of the O_{4} visible bands for 1000 hPa pure oxygen at (a) 223 K and (b) 283 K. Figures 210c and 310d show the cross sections corrected for the O_{2} -γ-band absorption in the 630.0 nm region for Figures 10a and 110b, respectively. Standard deviations of the cross sections are shown on an expanded scale below each of Figures 10a-310d.

The infrared O–D stretching spectrum of fully deuterated jet-cooled water clusters is reported. Sequential red-shifts in the single donor O–D stretches, which characterize the cooperative effects in the hydrogen bond network, were accurately measured for clusters up to (D_{2}O)_{8}. Detailed comparisons with corresponding data obtained for (H_{2}O)_{n} clusters are presented. Additionally, rotational analyses of two D_{2}O dimer bands are presented. These measurements were made possible by the advent of infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) using Raman-shifted pulsed dye lasers, which creates many new opportunities for gas phase IR spectroscopy.

Figure 5. (a) D_{2}O cluster spectra as a function of source pressure. Scans are taken at 10, 15, 25, 35, and 45 p.s.i. absolute pressure. Scans are taken at 10, 15, 25, 35, and 45 p.s.i. absolute pressure. Note to two uppermost pressure scans are plotted with the same offset to highlight the ‘‘bulk ice’’ feature that appears for the highest source pressure. (b) D_{2}O cluster spectra obtained with a constant source pressure ~35 psia! and increasing concentration of water in the carrier gas.

Theoretical results for the far-wing line shapes and corresponding absorption coefficients in the high-frequency wing of the ν_{3} fundamental band of self-broadened CO_{2} are presented for a number of temperatures between 218 and 751 K. These first-principles calculations are made assuming binary collisions within the framework of a quasi-static theory with a more accurate interaction potential than in previous calculations. The theoretical results are compared with existing laboratory data and are in good agreement for all the temperatures considered.

Figure 3. Calculated absorption coefficient α(ω) in the 2400–2580 cm^{-1} spectral region of CO_{2}–CO_{2} is represented by the solid curve; the experimental data from Ref. 15 are indicated by pluses. (a) T = 296 K, (b) T = 218 K. The absorption calculated assuming a Lorentzian line shape is given by the dashed curve.
[15] R. Le Doucen, C. Cousin, C. Boulet, and A. Henry, “Temperature dependence of the absorption in the region beyond the 4.3-μm band head of CO_{2}. I: Pure CO_{2} case,” Appl. Opt. 24, 897–906 (1985).

Theoretical results for the far-wing line shapes and corresponding absorption coefficients in the high-frequency wing of the ν_{3} fundamental band of self-broadened CO_{2} are presented for a number of temperatures between 218 and 751 K. These first-principles calculations are made assuming binary collisions within the framework of a quasi-static theory with a more accurate interaction potential than in previous calculations. The theoretical results are compared with existing laboratory data and are in good agreement for all the temperatures considered.

Figure 3. Calculated absorption coefficient α(ω) in the 2400–2580 cm^{-1} spectral region of CO_{2}–CO_{2} is represented by the solid curve; the experimental data from Ref. 16 are indicated by triangles. (a) T = 291 K, (b) T = 414 K, (c) T = 534 K, (d) T = 627 K, (e) T =751 K. The absorption calculated assuming a Lorentzian line shape .
[16] J.-M. Hartmann and M.-Y. Perrin, “Measurements of pure CO_{2} absorption beyond the ν_{3} band head at high temperature,” Appl. Opt. 28, 2550–2553 (1989).ibliography

Collision-induced absorption (CIA) by CO_{2} is measured in the 1100–1600 cm^{−1}range using a Fourier-transform spectrometer with a resolution of 0.5 cm^{−1}. The current measurements, which agree well with previous ones but are more precise, reveal pronounced structures on top of both unresolved Fermi doublet bands consisting of P-, Q-,and R-like branches. Assignment of Q-branches at 1284.75 cm^{−1}and 1387.75 cm^{−1}to (CO_{2})_{2} dimers seems highly probable. The nature of other peaks observed in CIA and Raman spectra of the CO_{2 }Fermi doublet region is discussed.

Figure 2. (a) CIA spectrum in the vicinity of lower member of the Fermi doublet. (b) Same as in Fig. 2a for the upper Fermi-coupled band.

New FTIR CO_{2} high resolution spectra are recorded in argon and nitrogen matrices at 11 K at high dilution. Their evolution with CO_{2} diffusion at higher temperature is also traced. New observations are discussed in regard to previous works. Even at high dilution (1/10 000), the temperature increase causes appearance of several bands both in argon and nitrogen. Identification of carbon dioxide dimer absorptions is tentatively proposed.

Figure 5. (A) Evolution in ν_{3} region of a CO_{2}/Ar mixture (1/10 000) kept at 30 K during different time lapses (t).: (a) t=0; (b) t =10 min; (c) t = 20 min; (d) t = 30 min; (e) t = 45 min; (B) comparison of a spectrum of ^{13}CO_{2}/Ar sample (1/10 000) in the ν_{3} region recorded at 11 K after deposition at 20 K with a spectrum recorded at 11 K after annealing at 30 K. M: monomer; : band due to N_{2} impurity. D', D" dimer; P pair; X not assigned.

The carbon dioxide dimer spectroscopic patterns are retrieved from the analysis of collision-induced absorption (CIA) spectral bandshape at room temperature. It is shown that the use of the simplified model based on the symmetric-top approximation allows roughly consistent simulation of the observed (CO_{2})_{2} dimer spectrum. The rotational constants obtained can be considered as effective thermally averaged constants which characterize dimeric structure, strongly distorted from the ground state. The overall CIA bandshape and the integrated intensity of absorption are broken down into partial contributions from tightly bound and metastable dimers and free-pair states. This approach is shown to be in agreement with a wide range of independent spectroscopic and thermodynamic data.

Figure 3. (b) Comparison of the calculated symmetric-top-like dimeric spectrum (solid line) with the experimental one (dots, see lower trace in Fig. 1) in the vicinity of the lower Fermi-coupled band. The dashed line shows the spectral profile which occurs provided rotational predissociation effect is neglected. Solid and dashed lines in this figure refer to the stick spectrum only and the sticks plus dash wings in the Fig. 3a, respectively. (c) Comparison of the calculated symmetric-top-like dimeric spectrum (solid line) with the experimental one (open circles, see lower trace in Fig. 1) in the vicinity of the upper Fermi coupled band.

This paper aims at examination of the (CO_{2})_{2} spectral absorption profile retrieved from the room temperature CO_{2} collision-induced absorption (CIA) spectrum in the region of the ν_{1}, 2ν_{2} Fermi doublet. The assignment of (CO_{2})_{2} vibrations is discussed, paying due attention to high-resolution CARS observations in the regions of ν_{low} and ν_{up}. A variational solution of the anharmonic vibrational problem for the (CO_{2})_{2} dimer was obtained using an extended basis set consisting of harmonic and Morse oscillator wave functions. This allowed for a satisfactory description of the vibrational dimer ν_{1}, 2ν_{2} spectrum from first principles. In particular, we have succeeded in proving that the diffuse absorption band seen in the trough between the major Fermi-coupled CIA bands belongs to combinations of CO_{2} bending vibrations in a dimer.

Typical CARS spectra taken at high resolution in the regions of ν_{low} and ν_{up} (14, 15, 23). Neat CO_{2} gas was expanded at a stagnation pressure of 1.5 bar through a ω = 90-μm-wide and l=1-mm-long slit nozzle. Both spectra were recorded at a relative distance of z/w = 4 from the nozzle exit.

14. A. A. Vigasin, A. A. Ilyukhin, L. Ramonat, V. V. Smirnov, O. M. Stelmakh, and F. Huisken, Khim. Fiz. 15, 88–95 (1996) [in Russian].
15. F. Huisken, L. Ramonat, J. Santos, V. V. Smirnov, O. M. Stelmakh, and A. A. Vigasin, J. Mol. Struct. 47, 410–411 (1997).
23. L. Ramonat, Ph.D. Thesis, University of Gottingen, 1997.

Cavity ring-down spectroscopy has been used to obtain the spectra of transient (O_{2})_{2} at 577 and 629 nm, at room temperature and pressures ranging from about 0.25 to 10.0 atm. A selection of these spectra are displayed showing the overlapping monomer and dimer features. Pressure-dependent cross sections have been obtained and Rayleigh extinction has been observed. The derived band parameters are compared to recent results from others.

Nous utilisons un spectroscomètre à cavité pour obtenir le spectre du fugace (O_{2})_{2} à 577 et 629 nm, à température de la pièce et à des pressions allant de 0,25 à 10,0 atm. Nous présentons une sélection de ces spectres qui montrent un chevauchement des caractéristiques du monomère et du dimère. Nous avons déterminé la dépendance en pression des sections efficaces et observé l’amortissement Rayleigh.

Figure 3. CRDS spectra of (O_{2})_{2 }a(0; 0)← X(0; 0) (top) and a(0; 1)← X(0; 0) (bottom) at three higher densities.

Cavity ring-down spectroscopy has been used to obtain the spectra of transient (O_{2})_{2} at 577 and 629 nm, at room temperature and pressures ranging from about 0.25 to 10.0 atm. A selection of these spectra are displayed showing the overlapping monomer and dimer features. Pressure-dependent cross sections have been obtained and Rayleigh extinction has been observed. The derived band parameters are compared to recent results from others.

Nous utilisons un spectroscomètre à cavité pour obtenir le spectre du fugace (O_{2})_{2} à 577 et 629 nm, à température de la pièce et à des pressions allant de 0,25 à 10,0 atm. Nous présentons une sélection de ces spectres qui montrent un chevauchement des caractéristiques du monomère et du dimère. Nous avons déterminé la dépendance en pression des sections efficaces et observé l’amortissement Rayleigh.

Figure 3. Portion of observed (CO_{2})_{2}+(CO_{2})_{3} infrared spectrum including central a -type Q-branch of (CO_{2})_{2}. Note resolved P-branch transitions for (CO_{2})_{3}. The P-branch transitions are labeled only for subbands where KN≥15 (conditions: 1.8% CO_{2} in He, P_{0}=1.0 bar).

Cavity ring-down spectroscopy has been used to obtain the spectra of transient (O_{2})_{2} at 577 and 629 nm, at room temperature and pressures ranging from about 0.25 to 10.0 atm. A selection of these spectra are displayed showing the overlapping monomer and dimer features. Pressure-dependent cross sections have been obtained and Rayleigh extinction has been observed. The derived band parameters are compared to recent results from others.

Nous utilisons un spectroscomètre à cavité pour obtenir le spectre du fugace (O_{2})_{2} à 577 et 629 nm, à température de la pièce et à des pressions allant de 0,25 à 10,0 atm. Nous présentons une sélection de ces spectres qui montrent un chevauchement des caractéristiques du monomère et du dimère. Nous avons déterminé la dépendance en pression des sections efficaces et observé l’amortissement Rayleigh.

Figure 4. Portion of observed (CO_{2})_{2} +(CO_{2})_{3}. Note 3:1 spacing of (CO_{2})_{3} Q-branches, and sharp appearance of the (CO_{2})_{3} rQ0-branch due to l-type doubling (conditions: 1.8% CO_{2} in He, P_{0}=1 bar).

It is the purpose of this paper to present an infrared absorption spectroscopic study of the state of aggregation of water over a wide range of temperature (25–380°C) and pressure (1–250 bar) along the liquid-gas coexistence curve, and in the supercritical domain. The evolution of the spectral profiles asociated with the internal vibrational modes and with the librational motion has been investigated. In supercritical water, at T=380°C, low pressures (densities) in the range 25–50 bar (0.01–0.05 g.cm^{−3}), only monomeric water is detected. Progressive increase of the pressure (density), from 50 to 250 bar (from 0.05 to 0.4 g.cm^{−3}), shows clearly the appearance of water dimers and trimers. Finally, upon decreasing the temperature (250-25 °C) along the liquid-gas coexistence curve, one observes a continuous evolution of the shape of the infrared spectrum characteristic of the presence of oligomers of increasing size and for temperatures lower than 200°C, the progressive appearance of the hydrogen bond network.

Figure 3. Infrared spectrum in the OH stretching region of supercritical water at T=380°C as function of pressure (density) in the range 200-250 bar (0.1-0.4 g.cm^{-3}). The calculated characteristic frequencies of the OH stretching mode of small water clusters are reported for comparison.

It is the purpose of this paper to present an infrared absorption spectroscopic study of the state of aggregation of water over a wide range of temperature (25–380°C) and pressure (1–250 bar) along the liquid-gas coexistence curve, and in the supercritical domain. The evolution of the spectral profiles asociated with the internal vibrational modes and with the librational motion has been investigated. In supercritical water, at T=380°C, low pressures (densities) in the range 25–50 bar (0.01–0.05 g.cm^{−3}), only monomeric water is detected. Progressive increase of the pressure (density), from 50 to 250 bar (from 0.05 to 0.4 g.cm^{−3}), shows clearly the appearance of water dimers and trimers. Finally, upon decreasing the temperature (250-25 °C) along the liquid-gas coexistence curve, one observes a continuous evolution of the shape of the infrared spectrum characteristic of the presence of oligomers of increasing size and for temperatures lower than 200°C, the progressive appearance of the hydrogen bond network.

Figure 2. Evolution of the OH anti-symmetric (ν_{3}) and symmetric (Vi) stretching modes, and of the bending mode (ν_{2}) of supercritical water at T=380°C as a function of pressure (density) in the range 25-180 bar (0.01-0.1 g.cm^{-3}). (B) Infrared band intensities (Km.mol^{-1}) associated with the OH stretching and bending modes of small cyclic water clusters as calculated by DFT methods (see text).

The temperature variations in collision-induced absorption (CIA) spectra of carbon dioxide in the region of the Fermi doublet are examined. New FTIR CIA spectra are recorded in the temperature range T=206–296°K. The spectra were subject to decomposition in order to separate true dimer contributions to the CIA profile from the base absorption caused by unbound pairs. The use of statistical physics theory allowed for quite nice reproduction of the observed temperature variations of the normalized dimer intensity.

Figure 2. Temperature variations of the widths (a) and the exponents (b) in the generalized Lorentz formula (1). Empty circles refer to the low-frequency component; filled circles are used for the high-frequency one.

Collision-induced absorption (CIA) cross-sections of oxygen have been measured in the UV, Visible and near-IR regions from spectra recorded by Fourier Transform Spectroscopy at different pressures and room temperature. An extensive cross-sections dataset from 42000 to 7500 cm^{−1} (238–1330 nm) is presented. The separation procedure of the discrete and diffuse absorption features is described. Pressure and foreign gas effects are discussed, and a comparison with literature data is shown. A preliminary test on the influence of the choice of the dataset on atmospheric retrievals of NO_{2}, O_{3 }, BrO, and OClO is performed.

Figure 2. The O_{2}–O_{2} collision-induced absorption cross-section at room temperature from the UV to the NIR. Assignments and vibrational levels of the bands are also shown.

Line intensities of the three lowest fundamentals of the ^{12}CH_{3}D Triad are modeled with an RMS of 3.2% using over 2100 observed values retrieved by multispectrum fitting of enriched sample spectra recorded with two Fourier transform spectrometers. The band strengths of the Triad in units of 10^{−18} cm^{−1}/(molecule cm^{−2}) at 296 K are, respectively, 2.33 for ν_{6} (E) at 1161 cm^{−1}, 1.75 for ν_{3} (A_{1}) at 1307 cm^{−1} and 0.571 for ν_{5} (E) at 1472 cm^{−1}. The total calculated absorption arising from ^{12}CH_{3}D Triad fundamentals is 4.65×10^{−18} cm^{−1}/(molecule cm^{−2}) at 296 K. In addition, some 740 intensities of nine hotbands are fitted to 8.1%; most of the hotband measurements belong to 2ν_{6}−ν_{6} and ν_{3}+ν_{6}−ν_{3} near 1160 cm^{−1}, 2ν_{3}−ν_{3} near 1290 cm^{−1} and ν_{3}+ν_{6}−ν_{6} near 1304 cm^{−1}. The other observed hotbands are ν_{5}+ν_{6}−ν_{6}, 2ν_{5}−ν_{5}, ν_{5}+ν_{6}−ν_{5}, ν_{3}+ν_{5}−ν_{3}, and ν_{3}+ν_{5}−ν_{5}.

Figure 1. Comparison of observed and simulated profile of a CO_{2} dimer band.

A new technique is introduced for the acquisition of size-selected neutral cluster spectra on the basis of argon-mediated, population-modulated electron attachment. This method is demonstrated and used to obtain the vibrational spectrum of the neutral water hexamer precursor to the (H_{2}O)_{6}^{-} cluster ion. The mid-infrared spectrum of the neutral species is dominated by four intense features above 3400 cm^{-1}, clearly indicating that significant structural rearrangements occur upon slow electron attachment to form the “magic” hexamer cluster anion. Comparison with previous spectroscopic reports and theoretical predictions indicates that the low-energy “book” isomer is most consistent with the observed band pattern and is suggested to be the species that captures a low-energy electron to form the hexamer anion.

Figure 3. Comparison between vibrational predissociation spectra of (a) (H_{2}O)_{6}^{-}‚ Ar_{7}, reproduced from ref 23, and (b) the neutral (H_{2}O)_{6} complex, obtained by argon-mediated, population-modulated electron attachment

Quantum line shape calculations of the rototranslational enhancement spectra of nitrogen-methane gaseous mixtures are reported. The calculations are based on a recent theoretical dipole function for interacting N_{2} and CH_{4} molecules, which accounts for the long-range induction mechanisms: multipolar inductions and dispersion force-induced dipoles. Multipolar induction alone was often found to approximate the actual dipole surfaces of pairs of interacting linear molecules reasonably well. However, in the case of the N_{2}-CH_{4} pair, the absorption spectra calculated with such a dipole function still show a substantial intensity defect at the high frequencies (>250 cm^{−1}) when compared to existing measurements at temperatures from 126 to 297 K, much as was previously reported.

Figure 4. Collision-induced rototranslational absorption band of interacting N_{2} and CH_{4} molecules, the binary enhancement spectrum of nitrogen- methane mixtures at the temperatures of 126°, 149°, 179°, and 212°K. Our calculations ͑thin lines͒ are compared with existing measurements ͑heavy͒ ͑Ref. 4͒.

I. R. Dagg, A. Anderson, S. Yan, W. Smith, C. G. Joslin, and L. A. A. Read, Collision-induced absorption in gaseous mixtures of nitrogen and methane, Canadian Journal of Physics, 1986, 64(11): 1467-1474, https://doi.org/10.1139/p86-260.

In spite of decades of extensive studies, the role of water dimers (WD) in the atmospheric radiation budget is still controversial. In order to search for evidence of the dimer in the solar near infrared, high spectral resolution pure water vapour absorption spectra were obtained in laboratory conditions for two different pressures and temperatures in the spectral region 5000–5600 cm^{−1} (1.785 to 2 µm). The residual was derived as a difference between the measured optical depth and the calculated one for water monomer, using the modified HITRAN database and two different representations of the water vapour continuum: CKD-2.4 (Clough–Kneizys–Davies) and the Ma and Tipping continuum. In both cases the residuals obtained are very similar to those expected from a recent theoretical calculation of the WD absorption. However, the WD band half-width at half maximum (HWHM) and dimerization equilibrium constant, k_{eq}, required to provide a best fit to the residual, differ for each case. To be in best agreement with the residual calculated by using the Ma and Tipping continuum, the WD bands HWHM should be ∼28 cm^{−1}, and k_{eq}=0.02±0.0035 atm^{−1}and 0.043±0.0055 atm^{−1}for temperatures 342 and 299 K respectively. For the residual calculated using the CKD-2.4 continuum the fitted value of the HWHM is ∼18cm^{−1}, and k_{eq}=0.011±0.0025 atm^{−1}(342 K) and 0.018±0.003 atm^{−1}(299 K). It is concluded that a substantial part of the WD absorption is already implicitly included within the CKD-2.4 continuum model. The increase in estimated clear-sky global mean absorption of solar radiation due to WD varies from 0.5% to 2.0%, depending on the set of WD parameters used. On the basis of a comparison of the derived k_{eq }values with others in the literature, the higher estimate is favoured.

Figure 4. Corrected residual between measurement and modified-HITRAN calculations with CKD-2.4 included. Bars show the measurement errors and errors of spectral line parameters fitting. (a) 20 hPa, 128 m, 299 K measurement. (b) 98 hPa, 9.7 m, 342 K measurement. To best fit the residual, the dimer spectrum is red-shifted by 12 cm^{-1} and 5 cm^{-1} for (a) and (b) respectively.

In spite of decades of extensive studies, the role of water dimers (WD) in the atmospheric radiation budget is still controversial. In order to search for evidence of the dimer in the solar near infrared, high spectral resolution pure water vapour absorption spectra were obtained in laboratory conditions for two different pressures and temperatures in the spectral region 5000–5600 cm^{−1} (1.785 to 2 µm). The residual was derived as a difference between the measured optical depth and the calculated one for water monomer, using the modified HITRAN database and two different representations of the water vapour continuum: CKD-2.4 (Clough–Kneizys–Davies) and the Ma and Tipping continuum. In both cases the residuals obtained are very similar to those expected from a recent theoretical calculation of the WD absorption. However, the WD band half-width at half maximum (HWHM) and dimerization equilibrium constant, k_{eq}, required to provide a best fit to the residual, differ for each case. To be in best agreement with the residual calculated by using the Ma and Tipping continuum, the WD bands HWHM should be ∼28 cm^{−1}, and k_{eq}=0.02±0.0035 atm^{−1}and 0.043±0.0055 atm^{−1}for temperatures 342 and 299 K respectively. For the residual calculated using the CKD-2.4 continuum the fitted value of the HWHM is ∼18cm^{−1}, and k_{eq}=0.011±0.0025 atm^{−1}(342 K) and 0.018±0.003 atm^{−1}(299 K). It is concluded that a substantial part of the WD absorption is already implicitly included within the CKD-2.4 continuum model. The increase in estimated clear-sky global mean absorption of solar radiation due to WD varies from 0.5% to 2.0%, depending on the set of WD parameters used. On the basis of a comparison of the derived k_{eq }values with others in the literature, the higher estimate is favoured.

Figure 5. Corrected residual between measurement and modified-HITRAN calculations with Ma and Tipping continuum included. Bars show the measurement errors and errors of spectral line parameters fitting. (a) 20 hPa, 128 m, 299°K measurement. (b) 98 hPa, 9.7 m, 342°K measurement. To best fit the residual, the dimer spectrum is red-shifted by 9 cm^{-1} and 5 cm^{-1} for (a) and (b) respectively.

We carried out a spectroscopic field experiment designed to measure water vapor continuum absorption in the visible and near-infrared spectral regions. Atmospheric spectra at ∼1 nm resolution were recorded using direct sunlight at high solar zenith angles during sunrise. Simultaneously radiosonde soundings and a network of geodetic Global Positioning System (GPS) receivers were deployed to constrain the water vapor amount along the absorption path. The solar spectra were analyzed using the Differential Optical Absorption Spectroscopy technique, while the GPS and radiosonde observations were used as input data to a line-by-line radiative transfer model to compute theoretical differential absorption spectra. The difference between the measurements and the simulated spectra provides information regarding the additional absorption owing to the H_{2}O continuum. The data are compared to predictions of the widely used Clough-Kneizys-Davies continuum model as well as with theoretically derived spectra of water dimer. The results show that continuum absorption contributes significantly to solar absorption even in highly saturated H_{2}O bands. The comparisons provide the needed observations to improve future continuum parameterizations.

Figure 15. Differential continuum in terms of optical depth computed from the transmission differences plotted in Figure 14. The estimated uncertainty of the retrieval is indicated by the grey shaded area. At large SZA the retrieved continuum becomes negative due to saturation effects. Both CKD models predict higher continuum absorption than observed, showing good agreement in the band wings.

We carried out a spectroscopic field experiment designed to measure water vapor continuum absorption in the visible and near-infrared spectral regions. Atmospheric spectra at ∼1 nm resolution were recorded using direct sunlight at high solar zenith angles during sunrise. Simultaneously radiosonde soundings and a network of geodetic Global Positioning System (GPS) receivers were deployed to constrain the water vapor amount along the absorption path. The solar spectra were analyzed using the Differential Optical Absorption Spectroscopy technique, while the GPS and radiosonde observations were used as input data to a line-by-line radiative transfer model to compute theoretical differential absorption spectra. The difference between the measurements and the simulated spectra provides information regarding the additional absorption owing to the H_{2}O continuum. The data are compared to predictions of the widely used Clough-Kneizys-Davies continuum model as well as with theoretically derived spectra of water dimer. The results show that continuum absorption contributes significantly to solar absorption even in highly saturated H_{2}O bands. The comparisons provide the needed observations to improve future continuum parameterizations.

Figure 17. Differential continuum in terms of optical depth computed from the transmission differences plotted in Figure 16. The estimated uncertainty of the retrieval is indicated by the grey shaded area. While CKD 2.4.1. agrees well with the observation, the continuum computed by MT_CKD_1.0 yields lower continuum than observed. The dimer model predicts an isolated feature near 746 nm, while the retrieval exhibits enhanced absorption around 750 nm.

From the spectrum of water (H_{2}^{16}O and H_{2}^{18}O) trapped in neon matrix recorded between 50 and 9000 cm^{−1} 29 vibrational transitions from the ground state have been identified for the water dimer. Twenty measured in the mid- and near infrared have been assigned to one-, two- and three-quanta transitions of the intramolecular modes, five in the far infrared to intermolecular modes and four in the mid infrared to binary intra + inter combinations. These assignments are based on ^{16}O/^{18}O isotopic shifts and on the comparison with the spectrum of the acetonitrile:water one to one complex in which the vibrational properties of the water subunit are very close to that of the proton donor molecule in (H_{2}O)_{2}. The comparison of the results in the mid- and far infrared with those obtained in the gas phase shows that the Ne matrix induced perturbations are very small for the intramolecular vibrations and do not exceed 20% for the intermolecular ones. Accordingly this set of data can be used to test the ability for a new version of the Gaussian program to account for the anharmonicity of vibrations and its evolution upon hydrogen-bonding. The results show that the anharmonicity coefficients are generally well reproduced for the intramolecular modes and that the highly anharmonic low frequency intermolecular modes are calculated less than 20% higher than observed in the gas phase.

Figure 1. Temperature effect on some bands of (H_{2}^{18}O)_{2} trapped in Ne (Ne/H_{2}^{18}O=800). Spectra recorded at 8°K (lower traces) and 4°K (upper traces) after annealing at 10 K. M: Q(1)-type line of 2ν_{1} of the monomer. Plot 1. and 301. - (ν_{1}+ν_{2})/(ν_{1}+ν_{3}) PD; Plot 601. and 401. - ν_{1}/2ν_{2} PA; Plot 201. and 501. - ν_{1}/2ν_{1 }PD.

Hydrogen bonding among water molecules is largely responsible for non-ideality in water vapor. Spectroscopic observations in pressurized water vapor using either IR or Raman techniques provide evidence of substantial molecular aggregation up to the critical point and above. The mean size or the molar fractions of the individual aggregates which are formed are, however, very difficult to determine from the analysis of spectra. Present paper aims at the spectral bandshape modeling in sub-critical water vapor making use of available IR absorption spectra in the OH fundamental and overtone. The modeling is supported by quantum-chemical ab initio and anharmonic vibrational calculations for the water dimer, the results of which are compared with previous ab initio and low-temperature laboratory data.

Figure 2. Water vapor absorption in the OH stretching fundamental. Points with error bars stand for experimental data from [9]. Light, dash, and solid traces refer to the monomer, dimer, and resulting spectral profiles, respectively.

The molecular complexes formed between a nitric oxide molecule and the various deuterated isotopomers of the methane molecule have been studied in a supersonic jet expansion. The electronic spectrum arising from the transition corresponding to a 3s←π∗ excitation (Ã Σ+2←X̃ Π2) located on the NO chromophore has been recorded employing resonance-enhanced multiphoton ionization spectroscopy, with each of CH_{4}, CH_{3}D, CH_{2}D_{2}, CHD_{3}, and CD_{4} as the complexing partner. Rich spectra are obtained, whose appearance changes in a systematic way as the amount of deuteration increases. Unexpectedly, it was possible to record spectra not only in the parent mass channel, but also in various fragment channels; this also led to the identification of some O atom resonances; and their origin is discussed. Discussion is presented of the structure in the spectra, and its possible sources including hindered internal rotation of the methane and NO moieties, overall rotation of the complex, and tunneling. In addition, some guidance has been gleaned from ab initio calculations, and these are discussed in the light of the experimental results.

Figure 3. (1+1) REMPI spectra recorded under three different conditions. In (a) only NO and Ar were present and the spectrum was recorded in the m/z=16 mass channel; in (b) NO, CH_{4}, and Ar were present, and again the spectrum was recorded in the m/z=16 mass channel; in (c) NO, CH_{4} and Ar were present, with the spectrum being recorded in the m/z=46 mass channel. See text for details.

The molecular complexes formed between a nitric oxide molecule and the various deuterated isotopomers of the methane molecule have been studied in a supersonic jet expansion. The electronic spectrum arising from the transition corresponding to a 3s←π∗ excitation (Ã Σ+2←X̃ Π2) located on the NO chromophore has been recorded employing resonance-enhanced multiphoton ionization spectroscopy, with each of CH_{4}, CH_{3}D, CH_{2}D_{2}, CHD_{3}, and CD_{4} as the complexing partner. Rich spectra are obtained, whose appearance changes in a systematic way as the amount of deuteration increases. Unexpectedly, it was possible to record spectra not only in the parent mass channel, but also in various fragment channels; this also led to the identification of some O atom resonances; and their origin is discussed. Discussion is presented of the structure in the spectra, and its possible sources including hindered internal rotation of the methane and NO moieties, overall rotation of the complex, and tunneling. In addition, some guidance has been gleaned from ab initio calculations, and these are discussed in the light of the experimental results.

Figure 4. (1+1) REMPI spectra of the five different isotopomers of NO·methane. Whole scans. Spectra recorded in the parent mass channel, except for the CH_{3}D isotopomer, which was recorded in the CH_{3}D^{+} masschannel (where a small bleed in of O+ signal is seen from the m/z=16 channel, cf. Fig. 3) —see text.

The molecular complexes formed between a nitric oxide molecule and the various deuterated isotopomers of the methane molecule have been studied in a supersonic jet expansion. The electronic spectrum arising from the transition corresponding to a 3s←π∗ excitation (Ã Σ+2←X̃ Π2) located on the NO chromophore has been recorded employing resonance-enhanced multiphoton ionization spectroscopy, with each of CH_{4}, CH_{3}D, CH_{2}D_{2}, CHD_{3}, and CD_{4} as the complexing partner. Rich spectra are obtained, whose appearance changes in a systematic way as the amount of deuteration increases. Unexpectedly, it was possible to record spectra not only in the parent mass channel, but also in various fragment channels; this also led to the identification of some O atom resonances; and their origin is discussed. Discussion is presented of the structure in the spectra, and its possible sources including hindered internal rotation of the methane and NO moieties, overall rotation of the complex, and tunneling. In addition, some guidance has been gleaned from ab initio calculations, and these are discussed in the light of the experimental results.

Figure 5. (1+1) REMPI spectra of the five different isotopomers of NO·methane. Low wavenumber regions. Spectra recorded in the parent mass channel, except for the CH_{3}D isotopomer, which was recorded in the CH_{3}D^{+} mass channel—see text.

Existing measurements of the collision-induced rototranslational absorption spectra of gaseous mixtures of methane with helium, hydrogen, or nitrogen are compared to theoretical calculations, based on refined multipole-induced and dispersion force-induced dipole moments of the interacting molecular pairs CH_{4}–He, CH_{4}–H_{2}, and CH_{4}–N_{2}. In each case the measured absorption exceeds the calculations substantially at most frequencies. We present the excess absorption spectra, that is the difference of the measured and the calculated profiles, of these supramolecular CH_{4}–X systems at various gas temperatures. The excess absorption spectra of CH_{4}–X pairs differ significantly for each choice of the collision partner X, but show common features (spectral intensities and shape) at frequencies from roughly 200 to 500 cm^{−1}. These excess spectra seem to defy modeling in terms of ad hoc exchange force–induced dipole components attempted earlier. We suggest that besides the dipole components induced by polarization in the electric molecular multipole fields and their gradients, and by exchange and dispersion forces, other dipole induction mechanisms exist in CH_{4}–X complexes that presumably are related to collisional distortion of the CH_{4} molecular frame.

Figure 2. Dots: measurements of the rototranslational enhancement absorption spectra of hydrogen-methane gas mixtures at various temperatures (Refs. 4 and 8); solid lines: calculations of the binary rototranslational spectra.

4. G. Birnbaum, A. Borysow, and H. G. Sutter, J. Quant. Spectrosc. Radiat. Transf. 38, 189 (1987)
8. P. Codastefano, P. Dore, and L. Nencini, J. Quant. Spectrosc. Radiat. Transf. 36, 239 (1986).

The collision-induced, rototranslational absorption spectrum of compressed methane gas is computed, based purely on the reliably known, leading multipole-induced dipole components of CH_{4} molecular pairs. In contrast to previous work of the kind no ad hoc empirical corrections of unknown exchange force-induced dipole components are attempted. Not surprisingly, the calculated spectra show a sizeable absorption defect at virtually all frequencies, when compared to existing laboratory measurements. The defect suggests the presence of dipole-induction mechanisms in addition to those due to the leading multipole-induced dipole terms. The excess absorption, the differences between measured and calculated spectra, resembles in certain ways the excess absorption spectra seen at the same frequencies in methane-X gas mixtures, where X stands for helium, hydrogen, or nitrogen, respectively [ Buser and Frommhold J. Chem. Phys. 122 024301 (2005)]. To a large extent, the excess absorption seems to be related to collisional distortions of the tetrahedral frame of the unperturbed CH_{4} molecule.

Figure 1. Existing measurements [17] at temperatures of 163°, 195°, 243°, and 297°K (dots) are compared to calculated absorption spectra (solid line) in methane.

P. Codastefano, P. Dore, L. Nencini, Temperature dependence of the far-infrared absorption spectrum of gaseous methane, Journal of Quantitative Spectroscopy and Radiative Transfer, Volume 35, Issue 4, April 1986, Pages 255-263, https://doi.org/10.1016/0022-4073(86)90079-8.

Two collision-induced absorption features of oxygen have been investigated by means of the laser-based cavity ring-down technique at pressures between 0 and 1000 hPa and at temperatures in the range 184–294 K. Peak cross sections, resonance widths and integrated cross sections, as well as spectral profiles, have been determined for the broad O_{2}O2–O_{2}O2 resonances centered at 477 and 577 nm. Results are compared with previous measurements to establish an updated temperature dependence for the cross sections of both resonances, yielding integrated cross sections, that exhibit a minimum near 200 K and that increase in a near-linear fashion in the atmospherically relevant range of 200–300 K. A significant increase in the widths of the resonance profiles upon temperature increase is firmly established. Parameters and temperature-dependent trends for the shape and strengths of the resonances are produced, that can be implemented in cloud retrieval in atmospheric Earth observation.

Figure 4. The absorption profiles of the O_{2} + O_{2} feature near 477 nm at 294 K, 230°K and 184°K. Residuals from a comparison to a model function are shown as well. Further details are given in Section 2.5.

Two collision-induced absorption features of oxygen have been investigated by means of the laser-based cavity ring-down technique at pressures between 0 and 1000 hPa and at temperatures in the range 184–294 K. Peak cross sections, resonance widths and integrated cross sections, as well as spectral profiles, have been determined for the broad O_{2}O2–O_{2}O2 resonances centered at 477 and 577 nm. Results are compared with previous measurements to establish an updated temperature dependence for the cross sections of both resonances, yielding integrated cross sections, that exhibit a minimum near 200 K and that increase in a near-linear fashion in the atmospherically relevant range of 200–300 K. A significant increase in the widths of the resonance profiles upon temperature increase is firmly established. Parameters and temperature-dependent trends for the shape and strengths of the resonances are produced, that can be implemented in cloud retrieval in atmospheric Earth observation.

Figure 5. The absorption profiles of the O_{2}-O_{2} feature near 577 nm at 294 K taken from Ref. [2], 268 and 190 K. Residuals from a fit to a model function as discussed in Section 2.5 are shown as well.

Helium droplet technique has been used in order to measure the strength of the infrared absorption in small ammonia and water clusters as a function of size. Hydrogen bonding in ammonia and water dimers causes an enhancement of the intensity of the hydrogen stretching bands by a factor of four and three, respectively. Two types of the hydrogen bonded clusters show different size dependence of the infrared intensity per hydrogen bond. In ammonia (NH_{3})_{2} and (NH_{3})_{3} it is close to the crystal value. In water clusters, it increases monotonically with cluster size being in tetramers, a factor of two smaller than in the ice. The measured infrared intensity in water clusters is found to be a factor of two to three smaller as compared to the results of numerical calculations.

Figure 1. Depletion spectra of NH_{3} panel a and H_{2}O panel b single molecules and clusters in He droplets of about 3500 atoms with an average number of about 1.5 and 2 captured molecules per droplet, respectively. Vertical dashed lines give the frequencies of the vibrational band origins of monomers. Sharp rovibrational lines of monomers are assigned Refs. 25 and 26. Additional strong bands are assigned to the absorption of the (NH_{3})_{k} and (H_{2}O)_{k} clusters, which have been enhanced by hydrogen bonding. Cluster size, k, is indicated by numbers. Spectral peaks a – d in panel b are assigned to the dangling OH bonds of water clusters of different size.

The IR spectra of complexes of water with nitrogen molecules in the range of the symmetric (ν_{1}) and antisymmetric (ν_{3}) bands of H_{2}O have been studied in helium droplets. The infrared intensities of the ν_{3} and ν_{1} modes of H_{2}O were found to be larger by factors of 1.3 and 2, respectively, in the N_{2}−H_{2}O complexes. These factors are smaller than those obtained in recent theoretical calculations. The conformation of the N_{2}−H_{2}O complex was estimated. Spectra and IR intensities of the (N_{2})_{2}−H_{2}O and N_{2}−(H_{2}O)_{2} complexes were also obtained and their structures are discussed.

Figure 1. Depletion spectra of the N_{2}-H_{2}O complexes in the range of the ν_{3} band of H_{2}O in He droplets. Pickup pressures: panel (a) P_{H2O} ) 3*10^{-6} mbar, P_{N2} = 0; panel (b) P_{H2O}=3 10^{-6} mbar, P_{N2} = 9*10^{-6} mbar. Panel (c) shows spectrum (b) with the contribution of spectrum (a) subtracted. This has been scaled to eliminate H_{2}O linesfrom the spectrum. Smooth curves are Gaussian fits as described in the text. The origin of the ν_{3} band of H_{2}O in He droplets is marked by the vertical dashed line. Pickup pressure dependences of the intensity of the peaks marked in panel (b) by arrows are shown in Figure 2.

The IR spectra of complexes of water with nitrogen molecules in the range of the symmetric (ν_{1}) and antisymmetric (ν_{3}) bands of H_{2}O have been studied in helium droplets. The infrared intensities of the ν_{3} and ν_{1} modes of H_{2}O were found to be larger by factors of 1.3 and 2, respectively, in the N_{2}−H_{2}O complexes. These factors are smaller than those obtained in recent theoretical calculations. The conformation of the N_{2}−H_{2}O complex was estimated. Spectra and IR intensities of the (N_{2})_{2}−H_{2}O and N_{2}−(H_{2}O)_{2} complexes were also obtained and their structures are discussed.

Figure 3. Depletion spectra of the N_{2}-H_{2}O complexes in the range of the î1 band of H_{2}O in He droplets. Pickup pressures: panel (a) P(H_{2}O) = 6*10^{-6} mbar, P(N_{2}) = 0; panel (b) P(H_{2}O) = 6*10^{-6 }mbar, P(N_{2}) = 9*10-6 mbar; panel (c) P(H_{2}O) = 6*10^{-6} mbar, P(N_{2}) = 1.8*10^{-5} mbar. Smooth curves are Gaussian fits as described in the text. The spectra were measured with a collimated laser beam, which gave about a factor of 5 larger effective laser energy flux, as compared with measurements in Figure 1. The origin of the î1 band of H_{2}O in He droplets is marked by the vertical dashed line.

We have built new global fits for the ground state potential energy surfaces (PES) of N_{2}–H_{2} and N_{2}–N_{2} complexes using ab initio perturbative and supermolecular methods. The analytical expressions used in the four-dimensional fitting procedure require the knowledge of the multipole moments, the static and dynamic multipolar polarizabilities of each monomer, from which long-range electrostatic, induction and dispersion coefficients are evaluated. In agreement with previous work, we have found the most stable conformation of N_{2}–H_{2} to be linear and that of N_{2}–N_{2} to have a 45/50° canted parallel shape. The quality of present PESs have been checked by comparing between calculated and experimental second virial coefficients and integral scattering cross-sections, which are found to be in good agreement.

Figure 1. Potential energy as a function of the intermolecular distance R for some selected conformations of the dimer, (θ^{N2}_{a}; θ^{H2/N2}_{b}; Φ) = (90°; 90°; 0°) for H, (0°, 0°, 0°) for L, (90°, 90°, 90°) for X, (90°, 0°, 0°) for T_{a} and (0°, 90°, 0°) for T_{b}, of N_{2}–H_{2} (left panel) and N_{2}–N_{2} (right panel).

The present paper considers the density effect on the water vapour absorption profile in the OH stretch range. The density evolution of the water vapour OH fundamental band shape is interpreted in terms of the monomer–dimer equilibrium. In contrast to previous works we adopt experimental values for the frequencies and IR intensities of the dimer vibrations in the vicinity of 3µm. This made it possible to reduce the number of unknown parameters required in the course of our spectral fit.

Figure 2. Typical examples of the spectral fit for the 650K isotherm

Based on first-principle molecular dynamic simulations, we calculate the far-infrared spectra of small water clusters (H_{2}O)_{n} (n=2,4,6) at frequencies below 1000 cm^{-1} and at 80 K and at atmospheric temperature (T > 200 K). We find that cluster size and temperature affect the spectra significantly. The effect of the cluster size is similar to the one reported for confined water. Temperature changes not only the shape of the spectra but also the total strength of the absorption, a consequence of the complete anharmonic nature of the classical dynamics at high temperature. In particular, we find that in the frequency region up to 320 cm^{-1}, the absorption strength per molecule of the water dimer at 220 K is significantly larger than that of bulk liquid water, while tetramer and hexamer show bulklike strengths. However, the absorption strength of the dimer throughout the far-infrared region is too small to explain the measured vapor absorption continuum, which must therefore be dominated by other mechanisms.

Figure 3. Calculated far-IR absorption spectra per molecule for the dimer. The spectrum obtained by Scribano and Leforestier (Ref. 26) (dotted line) is also shown, for the comparison.

[26] Y. Scribano and C. Leforestier, J. Chem. Phys. 126, 234301 (2007).

Based on first-principle molecular dynamic simulations, we calculate the far-infrared spectra of small water clusters (H_{2}O)_{n} (n=2,4,6) at frequencies below 1000 cm^{-1} and at 80 K and at atmospheric temperature (T > 200 K). We find that cluster size and temperature affect the spectra significantly. The effect of the cluster size is similar to the one reported for confined water. Temperature changes not only the shape of the spectra but also the total strength of the absorption, a consequence of the complete anharmonic nature of the classical dynamics at high temperature. In particular, we find that in the frequency region up to 320 cm^{-1}, the absorption strength per molecule of the water dimer at 220 K is significantly larger than that of bulk liquid water, while tetramer and hexamer show bulklike strengths. However, the absorption strength of the dimer throughout the far-infrared region is too small to explain the measured vapor absorption continuum, which must therefore be dominated by other mechanisms.

Figure 4. Calculated far-IR absorption spectra per molecule for the tetramer and the hexamer

Intermolecular potentials for the three lowest multiplet states (singlet, triplet and quintet) of the O_{2}(^{3}Σ^{−}_{g})–O_{2}(^{3}Σ^{−}_{g}) dimer have been investigated in detail by means of high level ab initio calculations. The methods used include MRCI, ACPF, CASPT2, using different active spaces and basis sets. The results for the quintet state are compared with benchmark CCSD(T) calculations. As expected, the former methods do not account accurately for dispersion interactions, although the CASPT2 method performs better than the CI based ones. On the other hand, it is shown that highly correlated methods are necessary to accurately describe the splittings among the multiplet states. We propose to obtain singlet and triplet interaction potentials by combining CCSD(T) quintet potentials and multiconfigurational singlet–quintet and triplet–quintet splittings, respectively. The calculated splittings are quite stable regarding the method employed, except for the well region of the singlet and triplet states within the rectangular configuration, which corresponds to the absolute minima of these multiplet states. Nevertheless, we have been able to assess adequate upper and lower bounds to the interaction potential for this particular region.

Figure. 4 Intermolecular potentials for the the O_{2}–O_{2} dimer singlet and triplet states as obtained for the four limiting geometries by combining CCSD(T) quintet energies and multireference splittings

We present a spectroscopic study of the water vapor continuum absorption in the far-IR region from 10 to 90 cm^{-1} (0.3–2.7 THz). The experimental technique combines a temperature-stabilized multipass absorption cell, a polarizing (Martin–Puplett) interferometric spectrometer, and a liquid-He-cooled bolometer detector. The contributions to the absorbance resulting from the structureless H_{2}O–H_{2}O and H_{2}O–N_{2} continua have been measured in the temperature range from 293 to 333 K with spectral resolution of 0.04–0.12 cm^{-1}. The resonant water vapor spectrum was modeled using the HITRAN04 database and a Van Vleck–Weisskopf lineshape function with a 100 cm^{-1} far-wing cut-off. Within experimental uncertainty, both the H_{2}O–H_{2}O and H_{2}O–N_{2} continua demonstrate nearly quadratic dependencies of absorbance on frequency with, however, some deviation near the 2.5 THz window. The absorption coefficients of 3.83 and 0.185 (dB/km)/(kPa THz)^{2} were measured for self- and foreign-gas continuum, respectively. The corresponding temperature exponents were found to be 8.8 and 5.7. The theoretically predicted foreign continuum is presented and a reasonable agreement with experiment is obtained.

Podobedov V.B., Plusquellic D.F., Siegrist K.M., Fraser G.T., Ma Q., Tipping R.H. Continuum and magnetic dipole absorption of the water vapor-oxygen mixtures from 0.3 to 3.6 THz. J. Molec. Spectrosc., 2008. V.251, 203-209, doi:10.1016/j.jms.2008.02.021.

The measurement of absorbance in water vapor-oxygen mixtures is reported for the far-IR region from 10 to 120 cm^{-1} (0.3–3.6 THz). The experiments were performed in a temperature-stabilized multipass absorption cell coupled to a far-infrared Fourier transform spectrometer with a liquid-He-cooled bolometer detector. The absorbance components due to both the H_{2}O–O_{2} continuum and the oxygen magnetic dipole discrete lines have been measured in the temperature range from 294 to 333 K with a spectral resolution of 0.03 to 0.07 cm^{-1. In the range up to 2.5 THz, the H}_{2}O–O_{2} continuum demonstrates a nearly quadratic dependence of absorbance on frequency, while in the 3.43 THz window deviation from this dependence was detected. The series of rotational lines associated with magnetic dipole transitions was measured in pure oxygen. In the mixture of 1.36 kPa of water vapor and 79.2 kPa of oxygen, a comparable contribution from continuum and discrete lines associated with magnetic dipole transitions of O_{2} was observed. The absorption coefficient of 0.066 (dB/km)/(kPa THz)^{2} and its temperature exponent of 4.7 were measured for the H_{2}O–O_{2} continuum. Experimental continua data compared to theoretically predicted values exhibit good agreement. The modeling of the resonant water vapor spectrum was performed using a Van Vleck–Weisskopf lineshape with a 215 cm^{-1} far-wing cut-off and the HITRAN2004 database.

Figure 2. Self-continuum absorption of water vapor at P = 2.13 kPa at different temperatures. Data for the 84.1 cm^{-1} window are not included in the ν2-fit presented by solid curves. The absorbance, A, is expressed as A= log_{10}(1/T), where the maximum transmittance, T, is equal to unity. For convenience, the second scale in dB/km is also presented. For the pathlength used here (23.3 m), the absorbance of A = 1 corresponds to absorption coefficient of 429 dB/km.

We present a spectroscopic study of the water vapor continuum absorption in the far-IR region from 10 to 90 cm^{-1} (0.3–2.7 THz). The experimental technique combines a temperature-stabilized multipass absorption cell, a polarizing (Martin–Puplett) interferometric spectrometer, and a liquid-He-cooled bolometer detector. The contributions to the absorbance resulting from the structureless H_{2}O–H_{2}O and H_{2}O–N_{2} continua have been measured in the temperature range from 293 to 333 K with spectral resolution of 0.04–0.12 cm^{-1}. The resonant water vapor spectrum was modeled using the HITRAN04 database and a Van Vleck–Weisskopf lineshape function with a 100 cm^{-1} far-wing cut-off. Within experimental uncertainty, both the H_{2}O–H_{2}O and H_{2}O–N_{2} continua demonstrate nearly quadratic dependencies of absorbance on frequency with, however, some deviation near the 2.5 THz window. The absorption coefficients of 3.83 and 0.185 (dB/km)/(kPa THz)^{2} were measured for self- and foreign-gas continuum, respectively. The corresponding temperature exponents were found to be 8.8 and 5.7. The theoretically predicted foreign continuum is presented and a reasonable agreement with experiment is obtained.

Podobedov V.B., Plusquellic D.F., Siegrist K.M., Fraser G.T., Ma Q., Tipping R.H. Continuum and magnetic dipole absorption of the water vapor-oxygen mixtures from 0.3 to 3.6 THz. J. Molec. Spectrosc., 2008. V.251, 203-209, doi:10.1016/j.jms.2008.02.021.

The measurement of absorbance in water vapor-oxygen mixtures is reported for the far-IR region from 10 to 120 cm^{-1} (0.3–3.6 THz). The experiments were performed in a temperature-stabilized multipass absorption cell coupled to a far-infrared Fourier transform spectrometer with a liquid-He-cooled bolometer detector. The absorbance components due to both the H_{2}O–O_{2} continuum and the oxygen magnetic dipole discrete lines have been measured in the temperature range from 294 to 333 K with a spectral resolution of 0.03 to 0.07 cm^{-1. In the range up to 2.5 THz, the H}_{2}O–O_{2} continuum demonstrates a nearly quadratic dependence of absorbance on frequency, while in the 3.43 THz window deviation from this dependence was detected. The series of rotational lines associated with magnetic dipole transitions was measured in pure oxygen. In the mixture of 1.36 kPa of water vapor and 79.2 kPa of oxygen, a comparable contribution from continuum and discrete lines associated with magnetic dipole transitions of O_{2} was observed. The absorption coefficient of 0.066 (dB/km)/(kPa THz)^{2} and its temperature exponent of 4.7 were measured for the H_{2}O–O_{2} continuum. Experimental continua data compared to theoretically predicted values exhibit good agreement. The modeling of the resonant water vapor spectrum was performed using a Van Vleck–Weisskopf lineshape with a 215 cm^{-1} far-wing cut-off and the HITRAN2004 database.

Figure 4. Foreign continuum data for two different mixtures of H_{2}O/N_{2}: 1.43/78.5 kPa (triangles) and 0.67/70 kPa (rhombs). For clarity, only the end-temperature data are shown. The curves present the ν_{2}-fit of absorbance data over all available windows.

The nature of the water vapour continuum absorption and the possible contribution of water dimers (WD) to this phenomenon have been a matter of debate for many years. The current work presents an overview and analysis of a number of experiments, both recent and old, where spectral signatures, similar to recent theoretical predictions for WD, have been observed in equilibrium laboratory conditions within near-infra-red (IR) water vapour absorption bands. These experiments, in contrast to those where water complexes are usually studied in non-equilibrium and low-temperature conditions, can give direct information about the possible WD amount in atmospheric conditions. Intercomparison of the results of these works and the recent ab initio prediction for WD band intensities and positions testifies in favour of a significant contribution of WD absorption to the water vapour self-continuum in the centre of the strongest near-IR water vapour absorption bands

Figure 7. The spectra of water vapour in 5300 cm^{-1} (a), and 3700 cm^{-1} (b) absorption bands, obtained in the experiments of Poberovsky [73, 74] for the cases (1): H_{2}O + N_{2}, and (2): pure H_{2}O (see text); the difference spectrum (2)-(1), attributed by Poberovsky to water clusters; the difference spectrum of Poberovsky, modified in this work; WD according to the model [52]. (a) Experiment [73]: T=530°K, spectral resolution FWHM=15 cm^{-1}, L=4.83 cm (1) and 0.49 cm (2); WD simulation: line strengths and positions from [52], HWHM=28 cm^{-1} [54], K_{eq}(530°K)=0.0023 atm^{-1} (Curtiss et al. [59] extrapolation). (b): Experiment [74]: T=503°K, spectral resolution FWHM=10 cm^{-1}; L=0.88 cm (1) and 0.08 cm (2); WD simulation: HWHM=25 cm^{-1}, K_{eq}(503 K)=0.0032 atm^{-1}.

[52] Schofield D.P., Kjaergaard H.G. Calculated OH-stretching and HOH-bending vibrational transitions in the water dimer. Phys. Chem. Chem. Phys., 2003;5:3100–5.
[54] Ptashnik I.V., Smith K.M., Shine K.P., Newnham D.A. Laboratory measurements of water vapour continuum absorption in spectral region 5000–5600 cm^{-1}: evidence for water dimers. Q. J. R. Meteorol Soc 2004;130:2391–408.
[59] Curtiss L.A, Frurip DJ, Blander M. Studies of molecular association in H_{2}O and D_{2}O vapors by measurement of thermal conductivity. J. Chem. Phys. 1979;71:2703–11
[73] Poberovsky A.V. Problemy fiziki atmosfery. Sbornik Trudov Univ Leningrad 1976;13:81–7 (in Russian).
[74] Poberovsky A.V. Diss. kand. fiz-mat nauk (PhD. thesis), Leningrad University, 1976 (in Russian).

We report three modifications to recent ab initio, full-dimensional potential energy surfaces (PESs) for the water dimer [X. Huang et al., J. Chem. Phys.128, 034312 (2008)]. The first modification is a refit of ab initio electronic energies to produce an accurate dissociation energy D_{e}. The second modification adds replacing the water monomer component of the PES with a spectroscopically accurate one and the third modification produces a hybrid potential that goes smoothly in the asymptotic region to the flexible, Thole-type model potential, version 3 dimer potential (denoted TTM3-F) [G. S. Fanourgakis and S. S. Xantheas, J. Chem. Phys.128, 074506 (2008)]. The rigorous D_{0} for these PESs, obtained using diffusion Monte Carlo calculations of the dimer zero-point energy, and an accurate zero-point energy of the monomer, range from 12.5 to 13.2 kJ/mol (2.99–3.15 kcal/mol), with the latter being the suggested benchmark value. For TTM3-F D_{0} equals 16.1 kJ/mol. Vibrational calculations of monomer fundamental energies using the code MULTIMODE are reported for these PESs and the TTM3-F PES and compared to experiment. A classical molecular dynamics simulation of the infrared spectra of the water dimer and deuterated water dimer at 300 K are also reported using the ab initio dipole moment surface reported previously [X. Huang, B. J. Braams, and J. M. Bowman, J. Phys. Chem. A110, 445 (2006)].

Figure 4. (Color online) Expanded portion of the (H_{2}O)_{2} spectrum shown in Fig. 3. The smooth curve is a smoothed representation of the structured classical spectrum. The vertical lines represent the nearly exact quantum energies (and not intensities) from Ref. 13, described in detail in the text.

Atmospheric emission in the ν_{2} band of water vapor by the foreign-broadened continuum (1300–2000 cm^{−1}) is important for retrievals of upper tropospheric water vapor. Previous work reported continuum coefficients retrieved from two downwelling emission measurements made with the Polar Atmospheric Emitted Radiance Interferometer (PAERI) at temperatures characteristic of the upper troposphere (below −25 °C) at Dome C, Antarctica. These results are improved upon here using 19 different measurements. Improvements have been made to the PAERI radiance calibration, the radiance simulations, and the error analysis. Compared to the Mlawer, Tobin, Clough, Kneizys, Davies continuum, the retrieved continuum is found to be 20% to 50% lower from 1350 to 1490cm^{−1} and 0% to 20% higher from 1850 to 1980 cm^{−1}.

Figure 6, Continuum coefficients retrieved from Polar Atmospheric Infrared Radiance Interferometer (PAERI) measurements in (a) the low-wavenumber wing of the ν_{2} band, (b) the center, and (c) the high-wavenumber wing. The results of previous work (RWW06) [9] and the MTCKD continuum are shown.

[9] P. M. Rowe, V. P. Walden and S. G. Warren, “Measurements of the foreign-broadened continuum of water vapor in the 6.3-μm band at -30^{o}C,” Appl. Opt. 45, 4366-4382 (2006).

The infrared spectrum of the water trimer trapped in solid neon has been identified. Eighteen groups of absorptions between 1600 and 11,000 cm^{−1} were assigned to one-, two- and three-quanta transitions of the intramolecular modes. Because of the near equivalence of the three molecules and their weak interactions most of these modes correspond to quasi degenerate vibrations involving the bending δ, free OH stretching (OH_{f}) and bonded OH stretching (OH_{b}) of the three subunits at 1608, 3725 and 3525–3473 cm^{−1}, respectively. In the last case the 52 cm^{−1} splitting is due to the coupling between the OH_{b} oscillators. Calculated anharmonic frequencies correctly agree with these observations and allow to propose a new assignment of the intermolecular modes. Finally combinations of intra + intermolecular transitions were identified and assigned on the basis of calculated anharmonicity coefficients.

Figure 3. Parts of the infrared spectra of H_{2}O/Ne = 1/140 matrix recorded at 3 K before and after annealing at 12.5 K (lower and upper trace, respectively). Sample deposited at 6 K for 6 h at 15 mmol/h. (a–d) Regions 3d, mOHb + d, 2msOHb and 3mOHf, respectively. Note that the mOHb + d region (b) is subdivided in two parts, one relating to ms, the other to ma. The decrease of the signal to noise ratio in (d) after annealing is due to noticeable decrease of the transmission. PA, PD: dimer; Tri: trimer; P: (H_{2}O)n, n > 3.

XVIth Symposium on High Resolution Molecular Spectroscopy (HighRus-2009)

In a previous series of papers, a model for the calculation of CO_{2}-air absorption coefficients taking line-mixing into account and the corresponding database/software package were described and widely tested. In this study, we present an update of this package, based on the 2008 version of HITRAN, the latest currently available. The spectroscopic data for the seven most-abundant isotopologues are taken from HITRAN. When the HITRAN data are not complete up to J″=70, the data files are augmented with spectroscopic parameters from the CDSD-296 database and the high-temperature CDSD-1000 if necessary. Previously missing spectroscopic parameters, the air-induced pressure shifts and CO_{2} line broadening coefficients with H_{2}O, have been added. The quality of this new database is demonstrated by comparisons of calculated absorptions and measurements using CO_{2} high-pressure laboratory spectra in the 1.5–2.3 μm region. The influence of the imperfections and inaccuracies of the spectroscopic parameters from the 2000 version of HITRAN is clearly shown as a big improvement of the residuals is observed by using the new database. The very good agreements between calculated and measured absorption coefficients confirm the necessity of the update presented here and further demonstrate the importance of line-mixing effects, especially for the high pressures investigated here. The application of the updated database/software package to atmospheric spectra should result in an increased accuracy in the retrieval of CO_{2} atmospheric amounts. This opens improved perspectives for the space-borne detection of carbon dioxide sources and sinks.

Figure 3. Measured absorption coefficients (bottom) and associated square density normalized values (top) for pure CO_{2} at 295.15K and seven total pressures from 15 to 44atm: (b) Dens normalized Abs (10^{-6} cm^{-1}/atm^{2}) and (a) Abs Coeff (10^{-2} cm^{-1}).

Water dimers have been assembled in He droplets and studied by infrared laser depletion spectroscopy. All four OH stretching bands of the dimer have been identified in the spectral range 3590–3800 cm^{−1}. Infrared intensities of the bands are also reported. The results are compared with previous measurements and theoretical calculations.

Figure 2. Spectra of the OH-stretching bands of water molecules and clusters in He droplets of about 3500 atoms at spectral resolution of 1 cm^{-1} upon capture of one and three water molecules on average per droplet in panels a and b, respectively.

From 1984 to 1996 there was rapid progress in the spectroscopy of CO_{2} dimers and trimers, but since then little work has appeared. Here we report a number of new high-resolution infrared results. For the dimer, a combination band provides the first experimental intermolecular vibrational frequency, and ^{13}C isotopic spectra clarify the role of resonant and non-resonant vibrational shifts. For the cyclic trimer, a new parallel combination band involving an out-of-plane intermolecular mode is observed, and various ^{13}C isotope results are analysed. In addition to these dimer and trimer results, the spectra also contain features which must be due to larger (CO_{2})_{N} clusters in the range N ≈ 5–15.

Figure 1. Part of the fundamental band of the CO_{2} dimer for (^{12}C^{16}O_{2})_{2}, (^{13}C^{16}O_{2})_{2}, and ^{12}C^{16}O_{2}–^{13}C^{16}O_{2}. An analogous range is covered for each isotopomer, but this is not so obvious since the nuclear spin statistics are different. The asterisks mark lines that are partly or entirely due to the He–CO_{2} complex. The simulations use parameters from Table 2, an effective rotational temperature of 3 K, and an assumed Gaussian linewidth of 0.0017 cm^{-1}.

From 1984 to 1996 there was rapid progress in the spectroscopy of CO_{2} dimers and trimers, but since then little work has appeared. Here we report a number of new high-resolution infrared results. For the dimer, a combination band provides the first experimental intermolecular vibrational frequency, and ^{13}C isotopic spectra clarify the role of resonant and non-resonant vibrational shifts. For the cyclic trimer, a new parallel combination band involving an out-of-plane intermolecular mode is observed, and various ^{13}C isotope results are analysed. In addition to these dimer and trimer results, the spectra also contain features which must be due to larger (CO_{2})_{N} clusters in the range N ≈ 5–15.

Figure 2. Portions of the observed and simulated combination bands of (a) the CO_{2} dimer and (b) the CO_{2} trimer.

From 1984 to 1996 there was rapid progress in the spectroscopy of CO_{2} dimers and trimers, but since then little work has appeared. Here we report a number of new high-resolution infrared results. For the dimer, a combination band provides the first experimental intermolecular vibrational frequency, and ^{13}C isotopic spectra clarify the role of resonant and non-resonant vibrational shifts. For the cyclic trimer, a new parallel combination band involving an out-of-plane intermolecular mode is observed, and various ^{13}C isotope results are analysed. In addition to these dimer and trimer results, the spectra also contain features which must be due to larger (CO_{2})_{N} clusters in the range N ≈ 5–15.

Figure 3. Three CO_{2} cluster absorption features that can be assigned as parallel bands of symmetric or nearly symmetric tops. The bands in the upper two panels have similar, but not identical, rotational constants which are appropriate for (CO_{2})_{n} with n≈5–7. The band in the lower panel corresponds to n≈10.

The interaction-induced dipole moment surface of the van der Waals CH_{4}–N_{2} complex has been calculated for a broad range of intermolecular separations R and configurations in the approximation of the rigid interacting molecules at the MP2 and CCSD(T) levels of theory using the correlation-consistent aug-cc-pVTZ basis set with the basis set superposition error correction. The simple model to account for the exchange effects in the range of small overlap of the electron shells of interacting molecules and the induction and dispersion interactions for large R has been suggested. This model allows describing the dipole moment of van der Waals complexes in analytical form both for large R, where induction and dispersion have the key role, and for smaller R including whole ranges of their potential wells, where the exchange effects are important. The proposed model was tested on a number of configurations of the CH_{4}–N_{2} complex and was applied for the analytical description of the dipole moment surface for the family of the most stable configurations of the CH_{4}–N_{2} complex.

Figure 3. The ab initio calculation of the dipole moment (a) μ_{x} and (b) μ_{y} for six configurations of the CH_{4}–N_{2} complex. Solid line—MP2 calculations; circles—CCSDT calculations. The numbers indicate the configurations.

The interaction-induced dipole moment surface of the van der Waals CH_{4}–N_{2} complex has been calculated for a broad range of intermolecular separations R and configurations in the approximation of the rigid interacting molecules at the MP2 and CCSD(T) levels of theory using the correlation-consistent aug-cc-pVTZ basis set with the basis set superposition error correction. The simple model to account for the exchange effects in the range of small overlap of the electron shells of interacting molecules and the induction and dispersion interactions for large R has been suggested. This model allows describing the dipole moment of van der Waals complexes in analytical form both for large R, where induction and dispersion have the key role, and for smaller R including whole ranges of their potential wells, where the exchange effects are important. The proposed model was tested on a number of configurations of the CH_{4}–N_{2} complex and was applied for the analytical description of the dipole moment surface for the family of the most stable configurations of the CH_{4}–N_{2} complex.

Figure 4. Dipole moment components (a) μ_{x} and (b) μ_{y} for configurations 3–5 of the CH_{4}–N_{2} complex.

Two quanta transitions involving the vibrational excitation of both proton donor (PD) and proton acceptor (PA) molecules of the water dimer trapped in inert matrices, a particular case of similtaneous transitions, have been identified. They are characterized by weaker intensity and smaller anharmonicity than the usual combinations of PD or PA. In some cases their intensity is strongly enhanced by quasi perfect resonances with PD combinations, as proved by decoupling effects in ^{18}O/^{16}O isotopic mixtures.

Figure 1. Evolution of the spectrum of water trapped in N_{2} in the regions 5120–5170 (lower frame) and 7190–7150 cm^{-1} (upper frame) as a function of the ^{18}O/^{16}O isotopic ratio: (a) 0, (b) 1, (c) 3. Lower frame: the isotopic varieties of PA and PD are reported in this order. Upper frame: for natural water the fit of the dimer absorptions in the range 7185–7175 cm^{-1} has been reported.

Two quanta transitions involving the vibrational excitation of both proton donor (PD) and proton acceptor (PA) molecules of the water dimer trapped in inert matrices, a particular case of similtaneous transitions, have been identified. They are characterized by weaker intensity and smaller anharmonicity than the usual combinations of PD or PA. In some cases their intensity is strongly enhanced by quasi perfect resonances with PD combinations, as proved by decoupling effects in ^{18}O/^{16}O isotopic mixtures.

Figure 2. Identification of ST’s in Ne matrix at 3K. Lower trace: ^{16}O/^{18}O decoupling effect for the resonant (ν_{2} + ν_{3})PDMST(ν_{2}PA + ν_{3}PD) transitions. ^{18}O/^{16}O isotopic ratios of (a) 0, (b) 0.2, (c) 5. The isotopic varieties of PA and PD are reported in this order. Upper trace: ST(ν_{1}PA + ν_{1}PD) observed at 7251.5 cm^{-1} in absence of resonance. R(0), P(1), Q(1): rovibrational transitions of H_{2}O monomer.

Recent laboratory observations and advances in theoretical quantum chemistry allow a reappraisal of the fundamental mechanisms that determine the water vapour self-continuum absorption throughout the infrared and millimetre wave spectral regions. By starting from a framework that partitions bimolecular interactions between water molecules into free-pair states, true bound and quasi-bound dimers, we present a critical review of recent observations, continuum models and theoretical predictions. In the near-infrared bands of the water monomer, we propose that spectral features in recent laboratory-derived self-continuum can be well explained as being due to a combination of true bound and quasi-bound dimers, when the spectrum of quasi-bound dimers is approximated as being double the broadened spectrum of the water monomer. Such a representation can explain both the wavenumber variation and the temperature dependence. Recent observations of the self-continuum absorption in the windows between these near-infrared bands indicate that widely used continuum models can underestimate the true strength by around an order of magnitude. An existing far-wing model does not appear able to explain the discrepancy, and although a dimer explanation is possible, currently available observations do not allow a compelling case to be made. In the 8–12 μm window, recent observations indicate that the modern continuum models either do not properly represent the temperature dependence, the wavelength variation, or both. The temperature dependence is suggestive of a transition from the dominance of true bound dimers at lower temperatures to quasi-bound dimers at higher temperatures. In the mid- and far-infrared spectral region, recent theoretical calculations indicate that true bound dimers may explain at least between 20% and 40% of the observed self-continuum. The possibility that quasi-bound dimers could cause an additional contribution of the same size is discussed. Most recent theoretical considerations agree that water dimers are likely to be the dominant contributor to the self-continuum in the mm-wave spectral range.

Figure 10. Smoothed spectra of WD absorption cross-section from Victorova and Zhevakin (V&Zh) [75], Scribano and Leforestier (S&L) [12] and Lee et al. [13] for temperatures around 296°K. The MTCKD [15] continuum model and empirical self-continuum of Burch [45] are also presented for comparison: (A) mm-wave region and (B) mid-to-far infrared spectral regions. The dashed lines in (B) show ratios (right hand axis) of the S&L and Lee et al. calculated spectra to the experimental continuum [45] (above 340 cm^{-1}) and to the MTCKD-1.3 model (below 340cm^{-1}). The equilibrium constant used by S&L: Keq,S&L(~296°K)=0.05 atm^{-1}, is adopted here to represent WD absorption cross-section for all three works. This causes the cross-section curve from Lee et al. to be a factor 1.86 lower than in the original work. The absorption in dB/km from V&Zh and S&L is converted to cm^{2} molec^{-1} atm^{-1} units using formula: 2:3026 x 10^{-6} [dB/km]* T *K_{eq},_{S&L}/ (273.15* N_{A}* P_{WD}), where N_{A}=2.687x10^{19} molec cm^{-3}; T~296°K; P_{WD} is WD partial pressure (atm) assumed in the particular work:8x10^{-6} in V&Zh and 2.2x10^{-5} in S&L. The implicit term K_{eq}, _{S&L}/K_{eq} (where K_{eq} is equilibrium constant applied for calculation in particular work) is used in the formula above to bring different calculations to similar conditions.

[12] Scribano Y, Leforestier C. Contribution of water dimers absorption to the millimeter and far infrared atmospheric water continuum. J Chem Phys2007;126:234301.
[13] Lee M-S, Baletto F, Kanhere DG, Scandolo S. Far-infrared absorption of water clusters by first-principles molecular dynamics. J Chem Phys 2008; 128:214506.
[15] MTCKD Available from web-site: /http:// rtweb.aer.com/ continuum_frame.htmlS.
[45] Burch D. Continuum absorption by H_{2}O, Air Force Geophysics Laboratory report,AFGL-TR-81-0300, Hanscom AFB, MA,1981.
[75] Viktorova AA, Zhevakin SA. Microradiowave absorption by dimers of atmospheric water vapor. Sov Phys Dokl 1970; 15:852–5.

Recent laboratory observations and advances in theoretical quantum chemistry allow a reappraisal of the fundamental mechanisms that determine the water vapour self-continuum absorption throughout the infrared and millimetre wave spectral regions. By starting from a framework that partitions bimolecular interactions between water molecules into free-pair states, true bound and quasi-bound dimers, we present a critical review of recent observations, continuum models and theoretical predictions. In the near-infrared bands of the water monomer, we propose that spectral features in recent laboratory-derived self-continuum can be well explained as being due to a combination of true bound and quasi-bound dimers, when the spectrum of quasi-bound dimers is approximated as being double the broadened spectrum of the water monomer. Such a representation can explain both the wavenumber variation and the temperature dependence. Recent observations of the self-continuum absorption in the windows between these near-infrared bands indicate that widely used continuum models can underestimate the true strength by around an order of magnitude. An existing far-wing model does not appear able to explain the discrepancy, and although a dimer explanation is possible, currently available observations do not allow a compelling case to be made. In the 8–12 μm window, recent observations indicate that the modern continuum models either do not properly represent the temperature dependence, the wavelength variation, or both. The temperature dependence is suggestive of a transition from the dominance of true bound dimers at lower temperatures to quasi-bound dimers at higher temperatures. In the mid- and far-infrared spectral region, recent theoretical calculations indicate that true bound dimers may explain at least between 20% and 40% of the observed self-continuum. The possibility that quasi-bound dimers could cause an additional contribution of the same size is discussed. Most recent theoretical considerations agree that water dimers are likely to be the dominant contributor to the self-continuum in the mm-wave spectral range.

Figure 5. (A, B)The experimental water vapour self-continuum, derived from measurements made by Paynter et al. [41], in the 1600 and 3600 cm^{-1} water vapour bands at 295°K as compared to the expected spectra for bound and quasi- bound water dimers. The former is simulated using WD band intensities and positions from [9] (VPT2), K^{b}_{eq} (296°K)=0.03 atm^{-1} and Lorentzian profile FWHM=60 cm^{-1} for every WD subband. The spectrum of quasi-bound dimers is simulated using WM lines from HITRAN-2008 [52] with doubled intensities and Lorentzian width FWHM=20 cm^{-1} for every line (seeEq.(1)). The thick line demonstrates the total simulated spectrum of water dimers. Error bars show the experimental uncertainty of the continuum retrieval. (C, D) Averaged spectra of the retrieved self-continuum C_{s} at different temperatures (left-hand axis) and the ratio of the spectra C_{s} (296°K)/C_{s}(351°K) (solid line; right-hand axis), illustrating the temperature dependence of the continuum.

[9] Kjaergaard H, Garden A, Chaban G, Gerber R, Matthews D, Stanton J. Calculation of vibrational transition frequencies and intensities in water dimer: comparison of different vibrational approaches. J Phys Chem A2008;112:4324–35 [HG Kjaergaard, personal communications, 2010].
41] Paynter DJ, Ptashnik IV, Shine KP, Smith KM, McPheat R, Williams RG. Laboratory measurements of the water vapor continuum in the 1200–8000 cm^{-1} region between 293 K and 351 K. J Geophys Res 2009;114:D21301.
[52] Rothman LS, Gordon IE, Barbe A, Chris Benner A, Bernath PF, et al. The HITRAN 2008 molecular spectroscopic database. JQSRT 2009;110:533–72.

Recent laboratory observations and advances in theoretical quantum chemistry allow a reappraisal of the fundamental mechanisms that determine the water vapour self-continuum absorption throughout the infrared and millimetre wave spectral regions. By starting from a framework that partitions bimolecular interactions between water molecules into free-pair states, true bound and quasi-bound dimers, we present a critical review of recent observations, continuum models and theoretical predictions. In the near-infrared bands of the water monomer, we propose that spectral features in recent laboratory-derived self-continuum can be well explained as being due to a combination of true bound and quasi-bound dimers, when the spectrum of quasi-bound dimers is approximated as being double the broadened spectrum of the water monomer. Such a representation can explain both the wavenumber variation and the temperature dependence. Recent observations of the self-continuum absorption in the windows between these near-infrared bands indicate that widely used continuum models can underestimate the true strength by around an order of magnitude. An existing far-wing model does not appear able to explain the discrepancy, and although a dimer explanation is possible, currently available observations do not allow a compelling case to be made. In the 8–12 μm window, recent observations indicate that the modern continuum models either do not properly represent the temperature dependence, the wavelength variation, or both. The temperature dependence is suggestive of a transition from the dominance of true bound dimers at lower temperatures to quasi-bound dimers at higher temperatures. In the mid- and far-infrared spectral region, recent theoretical calculations indicate that true bound dimers may explain at least between 20% and 40% of the observed self-continuum. The possibility that quasi-bound dimers could cause an additional contribution of the same size is discussed. Most recent theoretical considerations agree that water dimers are likely to be the dominant contributor to the self-continuum in the mm-wave spectral range.

Figure 8. Comparison of temperature dependencies for pure water vapour continuum absorption in mm-wave and mid-IR spectral ranges. Experimental data for 190 and 239 GHz are from [67] and [68], respectively; CO_{2} laser absorptions are from [69,70]. Solid lines show the result of calculations using formula (2a) and universal parameters n=1.63 and D_{o}=1700 K. The data for infrared continuum (right axis) are shown in arbitrary units. The Figure is adopted from [63].

[63] Vigasin AA. Water vapor continuous absorption in various mixtures: possible role of weakly bound complexes. JQSRT 2000; 64: 25–40.
[67] Bauer A, Godon M. Temperature dependence of water vapor absorption in line wings at 190 GHz. JQSRT 1991; 46:211–20.
[68] Bauer A, Godon M, Carlier J, Ma Q. Water vapor absorption in the atmospheric window at 239 GHz. JQSRT 1995; 53:411–23.
[69] Aref’ev VN. Molecular absorption of radiation by water vapour in the relative transparency window of the atmosphere at 8–13 μm. Opt Atmos 1989; 2:1034 inRussian.
[70] Hinderling J, Sigrist MW, Kneubuhl FK. Laser-photoacoustic spectroscopy of water-vapor continuum and line absorption in the 8 to 14 μm atmospheric window. Infrared Phys 1987; 27:63–120.

Recent laboratory observations and advances in theoretical quantum chemistry allow a reappraisal of the fundamental mechanisms that determine the water vapour self-continuum absorption throughout the infrared and millimetre wave spectral regions. By starting from a framework that partitions bimolecular interactions between water molecules into free-pair states, true bound and quasi-bound dimers, we present a critical review of recent observations, continuum models and theoretical predictions. In the near-infrared bands of the water monomer, we propose that spectral features in recent laboratory-derived self-continuum can be well explained as being due to a combination of true bound and quasi-bound dimers, when the spectrum of quasi-bound dimers is approximated as being double the broadened spectrum of the water monomer. Such a representation can explain both the wavenumber variation and the temperature dependence. Recent observations of the self-continuum absorption in the windows between these near-infrared bands indicate that widely used continuum models can underestimate the true strength by around an order of magnitude. An existing far-wing model does not appear able to explain the discrepancy, and although a dimer explanation is possible, currently available observations do not allow a compelling case to be made. In the 8–12 μm window, recent observations indicate that the modern continuum models either do not properly represent the temperature dependence, the wavelength variation, or both. The temperature dependence is suggestive of a transition from the dominance of true bound dimers at lower temperatures to quasi-bound dimers at higher temperatures. In the mid- and far-infrared spectral region, recent theoretical calculations indicate that true bound dimers may explain at least between 20% and 40% of the observed self-continuum. The possibility that quasi-bound dimers could cause an additional contribution of the same size is discussed. Most recent theoretical considerations agree that water dimers are likely to be the dominant contributor to the self-continuum in the mm-wave spectral range.

Figure 10B. Smoothed spectra of WD absorption cross-section from Scribano and Leforestier (S&L) [12] and Lee et al. [13] for temperatures around 296 K. The MTCKD [15] continuum model and empirical self-continuum of Burch [45] are also presented for comparison in the mid-to-far infrared spectral regions.

[12] Scribano Y., Leforestier C. Contribution of water dimers absorption to the millimeter and far infrared atmospheric water continuum. J. Chem. Phys., 2007;126:234301.
[13] Lee M.-S., Baletto F., Kanhere D.G., Scandolo S. Far-infrared absorption of water clusters by first-principles molecular dynamics. J. Chem. Phys., 2008; 128:214506.
[15] MTCKD Available from web-site: /http:// rtweb.aer.com/ continuum_frame.htmlS.
[45] Burch D. Continuum absorption by H_{2}O, Air Force Geophysics Laboratory report,AFGL-TR-81-0300, Hanscom AFB, MA,1981.

Recent laboratory observations and advances in theoretical quantum chemistry allow a reappraisal of the fundamental mechanisms that determine the water vapour self-continuum absorption throughout the infrared and millimetre wave spectral regions. By starting from a framework that partitions bimolecular interactions between water molecules into free-pair states, true bound and quasi-bound dimers, we present a critical review of recent observations, continuum models and theoretical predictions. In the near-infrared bands of the water monomer, we propose that spectral features in recent laboratory-derived self-continuum can be well explained as being due to a combination of true bound and quasi-bound dimers, when the spectrum of quasi-bound dimers is approximated as being double the broadened spectrum of the water monomer. Such a representation can explain both the wavenumber variation and the temperature dependence. Recent observations of the self-continuum absorption in the windows between these near-infrared bands indicate that widely used continuum models can underestimate the true strength by around an order of magnitude. An existing far-wing model does not appear able to explain the discrepancy, and although a dimer explanation is possible, currently available observations do not allow a compelling case to be made. In the 8–12 μm window, recent observations indicate that the modern continuum models either do not properly represent the temperature dependence, the wavelength variation, or both. The temperature dependence is suggestive of a transition from the dominance of true bound dimers at lower temperatures to quasi-bound dimers at higher temperatures. In the mid- and far-infrared spectral region, recent theoretical calculations indicate that true bound dimers may explain at least between 20% and 40% of the observed self-continuum. The possibility that quasi-bound dimers could cause an additional contribution of the same size is discussed. Most recent theoretical considerations agree that water dimers are likely to be the dominant contributor to the self-continuum in the mm-wave spectral range.

Figure 10. Smoothed spectra of WD absorption cross-section from Victorova and Zhevakin (V&Zh) [75], Scribano and Leforestier (S&L) [12] and Lee et al. [13] for temperatures around 296°K. The MTCKD [15] continuum model and empirical self-continuum of Burch [45] are also presented for comparison: (A) mm-wave region and (B) mid-to-far infrared spectral regions. The dashed lines in (B) show ratios (right hand axis) of the S&L and Lee et al. calculated spectra to the experimental continuum [45] (above 340 cm^{-1}) and to the MTCKD-1.3 model (below 340cm^{-1}). The equilibrium constant used by S&L: Keq,S&L(~296 K)=0.05 atm^{-1}, is adopted here to represent WD absorption cross-section for all three works. This causes the cross-section curve from Lee et al. to be a factor 1.86 lower than in the original work. The absorption in dB/km from V&Zh and S&L is converted to cm^{2} molec^{-1} atm^{-1} units using formula: 2:3026 x 10^{-6} [dB/km]* T *K_{eq},_{S&L}/ (273.15* N_{A}* P_{WD}), where N_{A}=2.687x10^{19} molec cm^{-3}; T~296°K; P_{WD} is WD partial pressure (atm) assumed in the particular work:8x10^{-6} in V&Zh and 2.2x10^{-5} in S&L. The implicit term K_{eq}, _{S&L}/K_{eq} (where K_{eq} is equilibrium constant applied for calculation in particular work) is used in the formula above to bring different calculations to similar conditions.

[12] Scribano Y., Leforestier C. Contribution of water dimers absorption to the millimeter and far infrared atmospheric water continuum. J. Chem. Phys., 2007; 126: 234301.
[13] Lee M.-S., Baletto F., Kanhere D.G., Scandolo S. Far-infrared absorption of water clusters by first-principles molecular dynamics. J. Chem. Phys., 2008; 128: 214506.
[15] MTCKD Available from web-site: /http:// rtweb.aer.com/ continuum_frame.htmlS.
[45] Burch D. Continuum absorption by H_{2}O, Air Force Geophysics Laboratory report,AFGL-TR-81-0300, Hanscom AFB, MA,1981.
[75] Viktorova A.A., Zhevakin S.A. Microradiowave absorption by dimers of atmospheric water vapor. Sov. Phys. Dokl., 1970; 15:852–855.

Thirteen specific infrared bands in the 2350 cm^{−1} region are assigned to carbon dioxide clusters, (CO_{2})_{N}, with N = 6, 7, 9, 10, 11, 12 and 13. The spectra are observed in direct absorption using a tuneable infrared laser to probe a pulsed supersonic jet expansion of a dilute mixture of CO_{2} in He carrier gas. Assignments are aided by cluster structure calculations made using two reliable CO_{2} intermolecular potential functions. For (CO_{2})_{6}, two highly symmetric isomers are observed, one with S_{6} symmetry (probably the more stable form), and the other with S_{4} symmetry. (CO_{2})_{13} is also symmetric (S_{6}), but the remaining clusters are asymmetric tops with no symmetry elements. The observed rotational constants tend to be slightly (≈2%) smaller than those from the predicted structures. The bands have increasing vibrational blueshifts with increasing cluster size, similar to those predicted by the resonant dipole-dipole interaction model but significantly larger in magnitude.

Figure 3. Observed and simulated spectra showing bands assigned to the S_{4} isomer of (CO_{2})_{6}. The perpendicular band in the lower panel is overlapped by bands of (CO_{2})_{2} and of the cyclic (brown) and noncyclic (blue) isomers of (CO_{2})_{3} as shown, while the Q- and R-branches of the parallel band in the upper panel are relatively clear. Observed and simulated spectra showing bands assigned to the S_{4} isomer of (CO_{2})_{6}. The perpendicular band in the lower panel is overlapped by bands of (CO_{2})_{2} and of the cyclic (brown) and noncyclic (blue) isomers of (CO_{2})_{3} as shown, while the Q- and R-branches of the parallel band in the upper panel are relatively clear.

The infrared spectrum of the water dimer trapped in solid neon has been recorded up to the visible by improving significantly the experimental technique used in a previous paper [Y. Bouteiller, J.P. Perchard, Chem. Phys. 305 (2004) 1]. A total of 22 intramolecular transitions of the proton donor (PD) and 23 of the proton acceptor (PA) are now identified and assigned on the basis of ^{16}O/^{18}O isotopic shifts and of realistic anharmonicity corrections. From an ab initio determination of the potential energy a perturbation-resonance treatment has been carried out for each polyad P_{n}, n = 2–8. Finally combinations of intra + intermolecular transitions were identified and assigned on the basis of calculated anharmonicity coefficients.

Figure 1. Parts of the infrared spectrum of a H_{2}O/Ne = 1/150 matrix recorded at 3 K at the resolution of 1 cm^{-1} in the P6 and P8 domains. Sample deposited at 5.8 K for 6 h at a rate of 7 mmol/h. Tri: water trimer. (b) Spectra recorded before (upper trace) and after annealing at 12 K (lower trace).

The infrared spectrum of the water dimer trapped in solid neon has been recorded up to the visible by improving significantly the experimental technique used in a previous paper [Y. Bouteiller, J.P. Perchard, Chem. Phys. 305 (2004) 1]. A total of 22 intramolecular transitions of the proton donor (PD) and 23 of the proton acceptor (PA) are now identified and assigned on the basis of ^{16}O/^{18}O isotopic shifts and of realistic anharmonicity corrections. From an ab initio determination of the potential energy a perturbation-resonance treatment has been carried out for each polyad P_{n}, n = 2–8. Finally combinations of intra + intermolecular transitions were identified and assigned on the basis of calculated anharmonicity coefficients.

Figure 3. Spectrum of H_{2}O trapped in Ne in the P5 region. Same conditions of deposition as for Fig. 1. (a) Spectra recorded at the resolution of 0.5 cm^{-1}. (b) Spectra recorded before (upper trace) and after annealing at 12 K (lower trace).

We employ recent flexible ab initio potential energy and dipole surfaces [Y. Wang, X. Huang, B. C. Shepler, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 134, 094509 (2011)10.1063/1.3554905] to the calculation of IR spectra of the intramolecular modes of water clusters. We use a quantum approach that begins with a partitioned normal-mode analysis of perturbed monomers, and then obtains solutions of the corresponding Schrödinger equations for the fully coupled intramolecular modes of each perturbed monomer. For water clusters, these modes are the two stretches and the bend. This approach is tested against benchmark calculations for the water dimer and trimer and then applied to the water clusters (H_{2}O)_{n} for n = 6–10 and n = 20. Comparisons of the spectra are made with previous ab initio harmonic and empirical potential calculations and available experiments.

Figure 4. Local-monomer IR spectra in the OH-stretch region of the indicated isomers of the hexamer obtained with the local-monomer model

We employ recent flexible ab initio potential energy and dipole surfaces [Y. Wang, X. Huang, B. C. Shepler, B. J. Braams, and J. M. Bowman, J. Chem. Phys. 134, 094509 (2011)10.1063/1.3554905] to the calculation of IR spectra of the intramolecular modes of water clusters. We use a quantum approach that begins with a partitioned normal-mode analysis of perturbed monomers, and then obtains solutions of the corresponding Schrödinger equations for the fully coupled intramolecular modes of each perturbed monomer. For water clusters, these modes are the two stretches and the bend. This approach is tested against benchmark calculations for the water dimer and trimer and then applied to the water clusters (H_{2}O)_{n} for n = 6–10 and n = 20. Comparisons of the spectra are made with previous ab initio harmonic and empirical potential calculations and available experiments.

Figure 6. Local-monomer IR spectra of intramolecular modes of the lowest energy isomer of the indicated cluster.

The origin of the line shape of the O−H stretch vibrational spectrum is analyzed for supercritical water in the low- and medium-density region by using classical molecular dynamics simulation for the flexible point-charge model, SPC/Fw. The spectrum calculated for the water model is in good agreement with the experimental one in the low-density region. The spectral origins in the low-density region of 0.01–0.04 g cm^{−3} are assigned to a sharp peak due to the bond oscillation along the O−H vector and two broad bands due to the rotational coupling, by taking an isolated single molecule as a reference in the low-density limit. The bands due to the rotational coupling reduce in intensity with increasing density as the rotations are more hindered by the hydrogen-bonding interactions, and their intensities increase with increasing temperature due to the accelerated rotational motion. The O−H stretch oscillation in the time correlation function attenuates in a timescale comparable with the lifetime of the hydrogen bonds, and the spectra conditioned by the number of hydrogen bonds are dominantly controlled by the local solvation structure.

Figure 1. Comparison of the spectrum of the dipole time correlation function ◦calculated at 400^{◦}C with the experimental IR spectrum. The experimental data in (a) by Tassaing et al. (Ref. 6) are taken at 380^{◦}C with 0.009 g cm^{−3} and those by Vigasin et al. (Ref. 7) at 377^{◦}C with 0.0105 g cm^{−3}. In (b), the data by Tassaing et al. are at 380^{◦}C with 0.04 g cm^{−3} and those by Vigasin et al. are at 377^{◦}C with 0.0405 g cm^{−3}. The calculated spectrum is shifted by 50 cm^{−1} to the higher frequency for comparison of the line shape to the experimental one.

A recently introduced bond–bond formulation of the intermolecular interaction has been extended to six-atom systems to the end of assembling a new potential energy surface (PES) and has been incorporated into a grid empowered simulator able to handle the modeling of the CO_{2} + CO_{2} processes. The proposed PES is full dimensional and accounts for the dependence of the intermolecular interaction on some basic physical properties of the colliding partners, including modulations induced by the monomer deformation. The used analytical formulation of the interaction involves a limited number of parameters, each having a clear physical meaning. Guess values for these parameters can also be obtained from analytical correlation formulae. Such estimates can then be fine tuned by exploiting experimental and theoretical information. The resulting PES well describes stretched and bent asymptotic CO_{2} monomers as well as the CO_{2}–CO_{2} interaction in the most and less stable configurations. On this potential massive quasiclassical elastic and inelastic detailed scattering trajectories have been integrated, by exploiting the innovative computational technologies of the grid. The efficiency of the approach used and the reliability of the estimates of the dynamical properties obtained in this way is such that we can now plan a systematic evaluation of the state specific rate coefficient matrix elements needed for space craft reentry modeling. Here, we present probabilities and cross sections useful to rationalize some typical mechanisms characterizing the vibrational transitions of the CO_{2} + CO_{2} system on the flexible monomer proposed PES. On such PES, the key dynamical outcomes are: (a) there is a strong energy interchange between symmetric stretching of the reactants and bending of the products (and viceversa) while asymmetric stretching is strongly adiabatic (b) reactant energy is more efficiently allocated (with respect to the rigid monomers PES) as product vibration when reactant stretching modes are excited while the contrary is true when the reactant bending mode is excited.

Figure 4. Comparison of the interaction energies for selected dimer geometries of a pair of rigid monomers (black lines) and of a rigid plus a stretched (both CAO bonds of monomer a have been symmetrically stretched of 10% with respect to equilibrium) monomer (red dashed lines). The T_{a} configuration refers to that with the stretched monomer a and the unstretched monomer b perpendicular to it and lying on the line connencting the two molecular centers of mass. Bond-bond results are plotted in the lower panel while those corresponding to the ab initio calculations are plotted in the upper panel.

A recently introduced bond–bond formulation of the intermolecular interaction has been extended to six-atom systems to the end of assembling a new potential energy surface (PES) and has been incorporated into a grid empowered simulator able to handle the modeling of the CO_{2} + CO_{2} processes. The proposed PES is full dimensional and accounts for the dependence of the intermolecular interaction on some basic physical properties of the colliding partners, including modulations induced by the monomer deformation. The used analytical formulation of the interaction involves a limited number of parameters, each having a clear physical meaning. Guess values for these parameters can also be obtained from analytical correlation formulae. Such estimates can then be fine tuned by exploiting experimental and theoretical information. The resulting PES well describes stretched and bent asymptotic CO_{2} monomers as well as the CO_{2}–CO_{2} interaction in the most and less stable configurations. On this potential massive quasiclassical elastic and inelastic detailed scattering trajectories have been integrated, by exploiting the innovative computational technologies of the grid. The efficiency of the approach used and the reliability of the estimates of the dynamical properties obtained in this way is such that we can now plan a systematic evaluation of the state specific rate coefficient matrix elements needed for space craft reentry modeling. Here, we present probabilities and cross sections useful to rationalize some typical mechanisms characterizing the vibrational transitions of the CO_{2} + CO_{2} system on the flexible monomer proposed PES. On such PES, the key dynamical outcomes are: (a) there is a strong energy interchange between symmetric stretching of the reactants and bending of the products (and viceversa) while asymmetric stretching is strongly adiabatic (b) reactant energy is more efficiently allocated (with respect to the rigid monomers PES) as product vibration when reactant stretching modes are excited while the contrary is true when the reactant bending mode is excited.

Figure 5. Comparison of the interaction energies for selected dimer geometries of a pair of rigid monomers (black lines) and of a rigid plus a stretched (the two C-O bonds of monomer a have been respectively stretched and shrunk of 10% with respect to equilibrium) monomer (red dashed lines). In particular the stretched C-O bond is the one lying on the intermolecular separation R and pointing closer to the monomer b in the L configuration. The Ta configuration refers to that with the stretched monomer a and the unstretched monomer b perpendicular to it and lying on the line connecting the two molecular CM. Bond-bond results are plotted in the lower panel while those corresponding to present ab initio calculations are in the upper panel.

Water dimers (H_{2}O)_{2} are believed to affect Earth’s radiation balance and climate, homogeneous condensation, and atmospheric chemistry. Moreover, the pairwise interaction which binds the dimer appears to be of paramount importance for expounding a complete molecular description of the liquid and solid phases of water. However, there have been no secure, direct observations of water dimers at environmentally relevant temperatures despite decades of studies. We report the first unambiguous observation of the dimer spectrum recorded in equilibrium water vapor at room temperature.

Figure 2. The observed dimer spectrum (a). The ab intio calculated [21] dimer spectrum [(b) upper trace]. The water vapor continuum absorption measured in moist nitrogen [28] at atmospheric pressure (d). The calculated collision-induced absorption [32] [(c) lower trace], the absorption is shown multiplied by factor of 1000 to make it visible). All spectra correspond to water vapor at 13 Torr and 296 K.

[21] Y. Scribano and C. Leforestier, J. Chem. Phys. 126, 234301 (2007).
[28] M. A. Koshelev, E. A. Serov, V. V. Parshin, and M. Yu. Tretyakov, J. Quant. Spectrosc. Radiat. Transfer 112, 2704 (2011).
[32] C. Leforestier, R. H. Tipping, and Q. Ma, J. Chem. Phys. 132, 164302 (2010).

The collisions between two oxygen molecules give rise to O4 absorption in the Earth atmosphere. O_{4} absorption is relevant to atmospheric transmission and Earth’s radiation budget. O_{4} is further used as a reference gas in Differential Optical Absorption Spectroscopy (DOAS) applications to infer properties of clouds and aerosols. The O_{4} absorption cross section spectrum of bands centered at 343, 360, 380, 446, 477, 532, 577 and 630 nm is investigated in dry air and oxygen as a function of temperature (203–295°K), and at 820 mbar pressure. We characterize the temperature dependent O_{4} line shape and provide high precision O_{4} absorption cross section reference spectra that are suitable for atmospheric O_{4} measurements. The peak absorption cross-section is found to increase at lower temperatures due to a corresponding narrowing of the spectral band width, while the integrated cross-section remains constant (within o3%, the uncertainty of our measurements). The enthalpy of formation is determined to be ΔH^{250} = -0.12±0.12 kJ mol^{-1}, which is essentially zero, and supports previous assignments of O_{4} as collision induced absorption (CIA). At 203°K, van der Waals complexes (O_{2-dimer}) contribute less than 0.14% to the O_{4} absorption in air. We conclude that O_{2-dimer} is not observable in the Earth atmosphere, and as a consequence the atmospheric O_{4} distribution is for all practical means and purposes independent of temperature, and can be predicted with an accuracy of better than 10^{-3} from knowledge of the oxygen concentration profile.

Figure 4. Comparison of peak and integrated cross-sections to available literature values at four wavelengths

We present (far-infrared) Collision Induced Absorption (CIA) spectra calculations for pure gaseous N_{2} made for the first time, from first-principles. They were carried out using classical molecular dynamics simulations based on ab initio predictions of both the intermolecular potential and the induced-dipole moment. These calculations reproduce satisfactory well the experimental values (intensity and band profile) with agreement within 3% at 149 K. With respect to results obtained with only the long range (asymptotic) dipole moment (DM), including the short range overlap contribution improves the band intensity and profile at 149 K, but it deteriorates them at 296 K. The results show that the relative contribution of the short range DM to the band intensity is typically around 10%. We have also examined the sensitivity of the calculated CIA to the intermolecular potential anisotropy, providing a test of the so-called isotropic approximation used up to now in all N_{2} CIA calculations. As all these effects interfere simultaneously with quantitatively similar influences (around 10%), it is rather difficult to assert which one could explain remaining deviations with the experimental results. Furthermore, the rather large uncertainties and sometimes inconsistencies of the available measurements forbid any definitive conclusion, stressing the need for new experiments.

Figure 2. Collision-induced absorption spectrum (in cm^{−1} /amagat^{2} ) of N_{2} by N_{2} (a) at 149 K, (b) at 228 K, and (c) at 296 K, as obtained from measurements12, 13 (circles) and from the CMDS calculations using the present ab initio dipole moment and the intermolecular potential of Ref. 16 (continuous lines). The error bars correspond to ± 10%.

The water vapour continuum is characterised by absorption that varies smoothly with wavelength, from the visible to the microwave. It is present within the rotational and vibrational–rotational bands of water vapour, which consist of large numbers of narrow spectral lines, and in the many ‘windows’ between these bands. The continuum absorption in the window regions is of particular importance for the Earth’s radiation budget and for remote-sensing techniques that exploit these windows. Historically, most attention has focused on the 8–12 lm (mid-infrared) atmospheric window, where the
continuum is relatively well-characterised, but there have been many fewer measurements within bands and in other window regions. In addition, the causes of the continuum remain a subject of controversy. This paper provides a brief historical overview of the development of understanding of the continuum and then reviews recent developments, with a focus on the near-infrared spectral region. Recent laboratory measurements in near-infrared windows, which reveal absorption typically an order of magnitude stronger than in widely used continuum models, are shown to have important consequences for remote-sensing techniques that use these windows for retrieving cloud properties.

Figure 3. Self- and foreign-continuum cross-section (upper and lower panels respectively) as retrieved from laboratory measurements in Ptashnik et al. (2011, 2012) (CAVIAR), along with their associated uncertainties, compared to the far-wing model of Tipping and Ma (1995) and the MTCKD-2.5 continuum model.

Ptashnik I.V., McPheat R.A., Shine K.P., Smith K.M., Williams R.G. (2011b) Water vapor continuum absorption in near-infrared windows derived from laboratory measurements. J Geophys Res 116:D16305
Ptashnik I.V., McPheat R.A., Shine K.P., Smith K.M., Williams R.G. (2012) Water vapour foreign continuum absorption in near-infrared windows from laboratory measurements. Phil Trans Roy Soc A (to appear). doi:10.1098/rsta.2011.0218
Tipping R.H., Ma Q. (1995) Theory of the water vapor continuum and validations. Atmos Res 36:69–94

Cw-CRDS spectra of water-rare gas supersonic expansions were recorded between 7229 and 7262 cm^{−1}. The effective absorption pathlength was about 1 km in jet-cooled gas and the resolution about 1 × 10^{−4} cm^{−1}. Many well-resolved structures are observed that could be assigned from experimental evidence to H2O-Ar/Kr bands. Eight broader unresolved features are more specifically reported and assigned to small H2O multimers, in good agreement and refining previous observations by Nizkorodov et al. [J. Chem. Phys.122, 194316 (2005)]. Among these, the band at 7256.5 cm^{−1} is shown to be a Q branch of the water dimer with accompanying R and very weak P lines. The band is assigned to a K a = 0 ← 1 transition and rotationally analyzed, leading to a restricted set of upper state rotational constants. The upper state lifetime (60 ± 3 ps) is extracted from the linewidths.

Figure 2. Focus on the rotational structure of the (H_{2}O)_{2} band observed around 7256.5 cm^{−1} in the cw-CRDS spectra of a supersonic expansion of H_{2}O in Ne, Ar, and Kr carrier gases. Monomer lines and H_{2}O-rare gas lines are also observed. The simulated structure is presented at the bottom.

We present an extensive study of the four-dimensional potential energy surface (4D-PES) of the carbon dioxide dimer, (CO_{2})_{2}. This PES is developed over the set of intermolecular coordinates. The electronic computations are carried out at the explicitly correlated coupled cluster method with single, double, and perturbative triple excitations [CCSD(T)-F12] level of theory in connection with the augmented correlation-consistent aug-cc-pVTZ basis set. An analytic representation of the 4D-PES is derived. Our extensive calculations confirm that “Slipped Parallel” is the most stable form and that the T-shaped structure corresponds to a transition state. Later on, this PES is employed for the calculations of the vibrational energy levels of the dimer. Moreover, the temperature dependence of the dimer second virial coefficient and of the first spectral moment of rototranslational collision-induced absorption spectrum is derived. For both quantities, a good agreement is found between our values and the experimental data for a wide range of temperatures. This attests to the high quality of our PES. Generally, our PES and results can be used for modeling CO_{2} supercritical fluidity and examination of its role in planetary atmospheres. It can be also incorporated into dynamical computations of CO_{2} capture and sequestration. This allows deep understanding, at the microscopic level, of these processes.

Figure 3. Potential energy cuts along the R coordinate for Linear 0-0-0 (i.e.,θ_{1}=θ_{2}=0^{0},φ=0^{0}; left panel) and near-equilibrium Slipped Parallel (i.e., θ_{1}=θ_{2}=60^{0}, φ=0^{0}; right panel) configurations. These cuts are computed using CCSD(T)/aug-cc-pVXZ (X=D,T,Q,CBS), CCSD(T)-F12(a,b)/aug-cc-pVTZ, MP2/aug-cc-pVQZ. We give also the SAPT potential of Bukowski et al.35 for comparison. 1a_{0}=1 bohr=0.529177 A.

35. S.R. Bukowski, J. Sadlej, B. Jeziorski, P. Jankowski, K. Szalewicz, S. A. Kucharski, H. L. Williams, and B. M. Rice, J. Chem. Phys. 110, 3785 (1999).
48. K. A. Peterson, D. E. Woon, and T. H. Dunning, Jr., J. Chem. Phys. 100, 7410 (1994).
49 A. Halkier, T. Helgaker, P. Jørgensen, W. Klopper, and J. Olsen, Chem. Phys. Lett. 302, 437 (1999).

We present an extensive study of the four-dimensional potential energy surface (4D-PES) of the carbon dioxide dimer, (CO_{2})_{2}. This PES is developed over the set of intermolecular coordinates. The electronic computations are carried out at the explicitly correlated coupled cluster method with single, double, and perturbative triple excitations [CCSD(T)-F12] level of theory in connection with the augmented correlation-consistent aug-cc-pVTZ basis set. An analytic representation of the 4D-PES is derived. Our extensive calculations confirm that “Slipped Parallel” is the most stable form and that the T-shaped structure corresponds to a transition state. Later on, this PES is employed for the calculations of the vibrational energy levels of the dimer. Moreover, the temperature dependence of the dimer second virial coefficient and of the first spectral moment of rototranslational collision-induced absorption spectrum is derived. For both quantities, a good agreement is found between our values and the experimental data for a wide range of temperatures. This attests to the high quality of our PES. Generally, our PES and results can be used for modeling CO_{2} supercritical fluidity and examination of its role in planetary atmospheres. It can be also incorporated into dynamical computations of CO_{2} capture and sequestration. This allows deep understanding, at the microscopic level, of these processes.

FIGURE 4. Potential energy cuts along the R coordinate for Linear 0-0-0 (left panel) and Slipped Parallel 60-60-60 (right panel) configurations. These cuts are computed using CCSD(T)-F12b/aug-cc-pVTZ. We give also the individual contribution of each multipolar expansion term. See text for more details. 1a_{0} = 1 bohr = 0.529177 Å.

The rotational spectrum of the van der Waals complex CH_{4}–CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110–145 GHz. Newly observed and assigned transitions belong to the K = 2–1 subband correlating with the rotationless j_{CH4} = 0 ground state and the K = 2–1 and K = 0–1 subbands correlating with the j_{CH4} = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH_{4}–CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH_{4}–CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH_{4} face closest to the CO subunit and binding energy D_{e} = 177.82 cm^{−1}. The bound rovibrational levels of the CH_{4}–CO complex were calculated for total angular momentum J = 0–6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D_{0} are 91.32, 94.46, and 104.21 cm^{−1} for A (j_{CH4} = 0), F (j_{CH4} = 1), and E (j_{CH4} = 2) nuclear spin modifications of CH_{4}–CO, respectively.

Figure 2. CH_{4}–CO interaction energies for different basis sets and methods as a function of the intermolecular distance for selected angular orientations.

A line-by-line calculation of the continuum absorption coefficient with the wing line contour describing the absorption in different windows outside of the water vapor bands in the 4000–8000 cm^{-1} spectral region is presented. The continuum absorption calculated with the line wing contour characterizing the absorption in the 5800–6400 cm^{-1} window is shown to be close to the total absorption. Hence it follows that the absorption in the 5000–5500 cm^{-1} and 6900–7700 cm^{-1} bands is almost entirely due to metastable dimers and free complexes.

Table 6. Experimental Н_{2}О self-continuum (T =296 K) versus predicted absorption spectra of bound and metastable dimers^{1} and present calculations for the 5200–5500 cm^{-1} band. The black dots are the experimental data, the triangles are the stable dimer absorption coefficients, and the gray curves represent the calculated results using the κ_{Lor}χ line shape. The frequency step is 50 cm^{-1}.