Approximating relative rotational energy transfer in molecular collisions

Abstract

The rotational energy transfer (RET) processes taking place in an atom–diatomic gaseous mixture are analysed in detail by carrying out collision dynamical calculations of the relevant state-to-state cross-sections. Approximate schemes for the dynamics are tested along the series of He, Ne and Ar rare gases in collisions with N₂ molecules, using the IOS and CS approximate models. The corresponding average relative energy transfer (ARET) is found to be a slow function of collision energy and can be used as an efficiency index for the systems at hand. The IOS approximation, however, grossly overestimates this index especially for the more anisotropic interactions. Other dynamical models, like the hard-ellipsoid impulsive model, are also found to have the same shortcomings as the IOSA scheme. It is therefore concluded that approximate treatments of relative rotational energy-transfer processes are capable of only near quantitative predictions for systems that show very low efficiency for such processes, and fail markedly when larger amounts of energy can be transferred during collisions. The present conclusions will be of use in analysing scattering experiments.