Collision-induced vibrational energy transfer in small polyatomic molecules

Recent theoretical and experimental progress has helped to clarify what determines the rates at which small molecules relax from vibrationally excited states via intermolecular vibration-rotation, translation (V-(R,T)), and intramolecular vibration-vibration (V-V) energy transfer. In this article, we examine how the influence of some of these important factors can be identified by comparing experimental data for similar molecular systems and can be understood on the basis of simple theoretical models. The role of molecular rotation in collision-induced vibrational energy transfer is considered by comparing experimental results for V-(R,T) relaxation for non-hydride and hydride molecules. It appears that in hydrides rotation can quite strongly accelerate vibrational energy transfer not only by absorbing significant amounts of energy in rotation but also by increasing the effective impact velocity in collisions. This latter effect can be reproduced reasonably well by using theoretical models based on first-order perturbation theory. We also consider, in some detail, what effect the mixing of rovibrational states by Fermi resonance or Coriolis coupling has on energy-transfer rates, especially of intramolecular V-V processes. Collisional processes involving CO₂, OCS, HCN, and H₂CO are considered. It is shown that, contrary to what has often been assumed, coupling between vibrational states will not invariably lead to efficient V-V transfer between them because of the possibility of quantum mechanical interferences associated with the necessary orthogonality of molecular eigenstates.