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 collision-induced absorption spectrum of gaseous CO2 has been measured in the far infrared at temperatures of 200 K, 293 K, 323 K and 373 K. The peak of the induced absorption band is observed to shift to lower frequency and to increase in magnitude as the temperature is reduced. Values of the absorption coefficient measured at frequencies across the band, and the integrated band absorption coefficient are presented as functions of temperature. The data have been used in two ways to obtain a value of the quadrupole moment of CO2, yielding the mean values of (6.4+or-1.0)*10-26 esu cm2 and (7.6+or-1.0)*10-26 esu cm2. A discussion of possible mechanisms of absorption in addition to that due to the quadrupole- induced dipole moment is given.
