ENERGY CORRECTED SUDDEN CALCULATIONS OF LINEWIDTHS AND LINE-SHAPES BASED ON COUPLED STATES CROSS-SECTIONS - THE TEST-CASE OF CO2-ARGON

The accuracy of the energy-corrected sudden (ECS) formalism for line s hape calculations is investigated, using coupled states calculation fo r CO2-Ar collisions on the recently developed ''single repulsion'' pot ential of Hutson er al. [J. Chem. Phys. 107, 1824 (1997); 105, 9130 (1 996)]. Inelastic cross sections sigma(0)(L-->0,E) = Q(L)'(E) are calcu lated using the MOLSCAT program, and then averaged over Maxwell-Boltzm ann kinetic energy distributions to give the thermally averaged ''basi c rates'' Q(L)'(T) needed in the ECS formalism. The ECS linewidths for low initial J, J(i) less than or equal to 16, are sensitive only to t he low-L basic rates, for which the CS calculations are converged; com paring them with directly calculated CS linewidths thus gives a string ent test of the ECS model, and it works well (within 10%). However, fo r higher J(i) lines and for band shape calculations, basic rates for h igher L are needed for convergence. These are obtained by an extrapola tion procedure based on experimental data, using an exponential power law and the adiabaticity factor recently suggested by Bonamy ct al. [J . Chem. Phys. 95, 3361 (1991)] ECS calculations using the resulting ba sic rates are designated ''extrapolated CS-ECS calculations,'' and are found to give accurate results for high-J linewidths, for near-wing a bsorption and for band profiles over a very wide range of perturber pr essures (up to 1000 arm). (C) 1998 American Institute of Physics. [S00 21-9606(98)02138-2].