A statistical model for rotationally and vibrationally inelastic collisions

Abstract

A statistical model is described which is able to treat the collisional excitation of internal degrees of freedom in collisions of large molecules in a simple way. In contrast to most previous statistical theories, this theory is able to treat direct collisions as well as collisions involving a long lived complex. A wide variety of collisional interaction strengths can be handled by the model. This is accomplished by the use of a coupling strength parameter, which limits the energy in any internal degree of freedom. This energy limit is the only adjustable parameter in the model and, when held small, allows a good description of glancing collisions and threshold effects, which are not normally considered to be amenable to statistical treatment. The results of this model are compared both with an exact theoretical treatment of the direct collinear atom–diatom collision and also with experimental ion beam data on average energy transfer in direct collisions with polyatomic molecules. In both cases good agreement is found. A simple semiempirical formula is obtained which can be used to predict the energy and angular dependence of the average relative energy transfer under conditions encountered in hot atom and in radiation chemistry and plasmas.