Theoretical evaluation of vibrational transition rates and relaxation in CO-He

An extensive set of vibrationally inelastic transition rates k _n+Δn→n(T) is presented for CO-He. Calculations were performed over temperatures 100≤T≤3000°K, final states 0≤n≤45, and transitions Δn=1,2,3. Cross sections σ _n+Δn→n are computed by solving sets of close coupled breathing sphere equations containing 7 to 11 vibrational states. The CO-He potential surface used was computed by the method of Gordon and Kim. Sample relaxation studies are carried out to determine the roles of the different rate constants in the relaxation process. It is shown that while the conditions of the Landau-Teller theory are not satisified here, the relaxation of the total vibrational energy is still approximately described by a single relaxation time under usual experimental conditions. However, a detailed examination of the relaxation process shows that a canonically invariant temperature dependence generally does not hold. Anharmonic effects at high quantum numbers are shown to play a dominant effect in the relaxation.