The influence of molecular rotation on vibration–translation energy transfer

Abstract

The role of molecular rotations in the exchange of vibrational and translational energy is investigated for collisions between anharmonic diatomic molecules and structureless atoms. A three-dimensional, semiclassical, impact parameter description is applied with emphasis directed towards the influence of rotational coupling on the net rate of vibrational energy transfer summed over all final rotational states. These results are then related to the predictions of an equivalent collinear collision model, and their comparison allows an evaluation of the collinear approximation. The mechanisms of vibrational energy transfer including rotational transitions are shown to be separable into three classes, with the molecules belonging to each class identified first and foremost by their ratio of fundamental vibrational and rotational frequencies, ωe/Be, and second by the proximity of their initial state to a near-resonant vibration–rotation transition with a small change in angular momentum. While the dynamics of molecules with ωe/Be ratios that are comparable to the range of angular momentum transitions having strong coupling are found to require a complete three-dimensional description, the rates of vibrational energy transfer in molecules with large ωe/Be ratios appear to be well approximated by a collinear collision model.