Theoretical studies of collisional energy transfer in highly excited molecules: The importance of intramolecular vibrational redistribution in successive collision modeling of He+CS₂

In this paper we present a new method for studying the collisional relaxation of highly excited molecules in low density gases known as the redistributed successive collisions (RSC) method, and we apply it to the relaxation of CS₂ by He at 300 K in the vibrational energy range E = 32 640-3180 cm^-1. The RSC method involves calculating sequences of collisions, subject to the assumption that rapid vibrational redistribution occurs between each collision. As a result, initial conditions for each trajectory in a sequence are sampled from a microcanonical ensemble that is defined by the final energy and angular momentum of the previous trajectory. The application to He + CS₂ leads to 〈ΔE〉's that vary linearly with E over the entire energy range considered. The agreement of these 〈ΔE〉's with measured values is good, but there is a qualitative difference in the E dependence of 〈Δ〉 over part of the range of E's. We also examine a second successive collision method that is more appropriate for high-density gases in which the internal coordinates and momenta are conserved (i.e., not redistributed) between collisions (CSC method). We find that a substantial fraction of the CSC ensembles (∼50%) exhibit extremely slow relaxation which in some cases is not complete even after 80 000 collisions. This unphysical result appears to be a classical artifact, and it leads to very small 〈ΔE〉's at medium to low E and a stronger dependence of 〈Δ〉 on E (close to quadratic) at high E. Omission of these slowly relaxing ensembles from the CSC ensemble average leads to CSC 〈ΔE〉's which are nearly identical to those from the RSC calculation. An analysis of the distribution of energy among vibrational modes in the CSC calculations indicates that the slow relaxation arises from energy becoming frozen in the asymmetric stretch of CS₂. The influence of the CS₂ intramolecular dynamics on the collisional relaxation is considered, and we find evidence of abrupt collision induced intramolecular energy redistribution due to nonlinear resonance formation.