Vibrational inelasticity in H₂ collisions with He and Li⁺

The partially averaged version of classical S-matrix theory was applied to three-dimensional collisions of H$sub 2$ with He and Li$sup +$. For H$sub 2$- Li$sup +$, cross-sections for the de-excitation of H$sub 2$ from (n$sub 1$,j$sub 1$) = (1,0) to the ground vibrational manifold were computed at a total energy of 1.2 eV and compared to previously done coupled channel calculations of Schaefer and Lester. The agreement is very good. For H$sub 2$-He, the Kutzelnigg- Tsapline interaction potential was extended to small atom-diatom separations, the ab initio points were then fit to an analytic form, and cross sections for the de- excitation of H$sub 2$ from the states (n$sub 1$,j$sub 1$), n$sub 1$ = 1, j$sub 1$ = 0,2,4 to the ground vibrational manifold were computed at total energies of .9, 1.1, 1.3 and 1.5 eV. For comparison, coupled channel calculations were also performed on the system at the same energies. The agreement was poorer than in the H$sub 2$-Li$sup +$ case, for identifiable reasons. The cross sections were used to compute rate constants and relaxation times for the H$sub 2$-He system. Comparison of these results with the results of experiment and of other calculations shows good agreement, certainly within the expected errors. (7 figs., 30 refs., 3 tables) (auth