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

Fully quantal benchmark calculations of pressure-broadening cross sections for infrared and Raman lines of CO2 perturbed by Ar are carried out using b oth close-coupling (CC) and coupled-states (CS) calculations. CS calculatio ns are found to underestimate the cross sections by up to 15%. The effect o ccurs even for isotropic Raman cross sections, which are not affected by re orientation contributions. The discrepancy arises mostly for collisions wit h large orbital angular momenta l, occurring on the long-range part of the potential. It may be attributed to collisions that are adiabatic rather tha n sudden in nature. A hybrid computational method, employing CS calculation s for low l and decoupled l-dominant (DLD) calculations for high l, offers a promising solution. (C) 1999 American Institute of Physics. [S0021-9606(9 9)00735-7].