Classical molecular dynamics simulations have been carried out for gaseous CO2 starting from various anisotropic intermolecular potential energy surfaces. Through calculations for a large number of molecules treated as rigid rotors, the time evolution of the interaction-induced electric dipole vector is obtained and the Laplace transform of its autocorrelation function gives the collision-induced absorption rototranslational spectrum. The results are successfully compared with those of previous similar calculations before studies of the influences of the intermolecular potential and induced-dipole components are made. The calculated spectra show a significant sensitivity to anisotropic forces consistently with previous analyses limited to the spectral moments. The present results also demonstrate the importance of vibrational and back-induction contributions to the induced dipole. Comparisons between measured far infrared (0–250 cm−1) spectra at different temperatures and results calculated without the use of any adjustable parameter are made. When the best and more complete input data are used, the quality of our predictions is similar to that obtained by Gruszka et al. [Mol. Phys. 93, 1007 (1998)] after the introduction of ad hoc short-range overlap contributions. Our results thus largely obviate the need for such contributions the magnitudes of which remain questioned. Nevertheless, problems remain since, whereas good agreements with measurements are obtained above 50 cm−1, the calculations significantly underestimate the absorption below, a problem which is discussed in terms of various possible error sources.
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The pressure dependence of the collision‐induced spectrum in CO2 at room temperature in the frequency region 7–250 cm−1 is measured throughout this region as a function of density, ρa, from 0–85 amagat. At each frequency the density variation of the absorption is fitted by α(ν,ρa) = ρa2α2(ν) + ρa3α3(ν) to obtain values of α2(ν) and α3(ν). The coefficient α2(ν) and its temperature dependence is discussed in the previous paper of this series. The coefficient α3(ν) is negative in sign and has a band shape considerably sharper than that found for α2(ν). The Kramers–Kronig integral π−2 ∫ α3(ν)ν−2dν is in reasonable agreement with the theoretical value of the third dielectric virial coefficient. Absorption in liquid CO2 is measured at 0°C and compared with the band spectrum obtained in the gas phase at the same temperature. The peak intensity in the liquid spectrum occurs at a frequency 25 cm−1 higher than in the gas phase. The integrated intensity ρa−2 ∫ α(ν)dν in the liquid is 3.2 ± 0.1 × 10−4 cm−2⋅amagat−2, whereas that for the low density gas is 5.11 ± 0.20 × 10−3 cm−2⋅amagat−2.
