Line shape, transport and relaxation properties from intermolecular potential energy surfaces: The test case of CO2–Ar

Abstract

Two new potential surfaces for CO2–Ar, obtained principally from the spectra of van der Waals complexes, are tested against measurements of pressure broadening and of transport and relaxation properties, none of which was used in the determination of the surfaces. Pressure broadening of both infrared and Raman lines is considered. The coupled states (CS) approximation is used for all line shape calculations. Thermally averaged infrared and Raman cross sections at 523, 296, 160 and 77 K (infrared) and 295 K (Raman) show good agreement with the experimental data available. Generalized transport and relaxation cross sections are obtained via full classical trajectory and classical CS calculations. Properties tested include diffusion, viscosity and nuclear spin relaxation. They provide a different test of the surfaces and agree well with experiment.

A failing of coupled-states calculations for inelastic and pressure-broadening cross sections: Calculations on CO2–Ar

“…7 We therefore carried out extensive CS calculations of the pressure broadening, which have been reported separately. 4 It would be in principle be preferable to carry out benchmark quantal calculations using close-coupling ͑CC͒ calculations, 8 which make no dynamical approximations at all. Unfortunately, for a system as heavy as CO 2 -Ar, it is currently prohibitively expensive to carry out such calculations at the very large number of energies needed for full line shape calculations.…”

“…For other parameters, such as those characterizing line shifts and line mixing, the situation is even less satisfactory. As a first step to allow us to assess the various theoretical methods in use, we have recently carried out benchmark calculations 4 of pressure broadening of CO 2 infrared lines by Ar, using fully quantal scattering calculations on potential energy surfaces fitted to the spectra of van der Waals complexes.…”

Energy corrected sudden calculations of linewidths and line shapes based on coupled states cross sections: The test case of CO2–argon

“…13 The resulting potentials have been shown to reproduce experimental transport properties, NMR relaxation times, and pressure-broadened linewidths, 12 none of which was used in fitting the surfaces. In view of the similarity of the linewidth calculations for the two potentials, we have considered only the ''single repulsion'' potential in the present work.…”

“…Section III presents the results of CS calculations of the basic rates Q L Ј(T) from the accurate potential of Ref. 12. Section IV compares ECS pressure-broadened linewidths and synthetic profiles obtained from the calculated basic rates with experiment.…”

“…However, these ''experimental'' basic rates are quite strongly model-dependent, and it is of great interest to compare them with values of Q L Ј(T) calculated directly from a reliable intermolecular potential. Hutson et al 12,13 have recently developed such a potential for the system CO 2 -Ar, and this gives us the opportunity to test the ECS approach for a relatively heavy system for which a large body of experimental results exists. This is the purpose of the present paper.…”