On j z-preserving propensities in molecular collisions. I. Quantal coupled states and classical impulsive approximations

Abstract

The occurrence of j_z‐preserving propensities in atom–linear molecule collisions is considered within the contexts of the quantum mechanical CS approximation and of a classical model collision system. The latter involves an impulsive interaction which is the extreme limit of the class of potentials for which the CS approximation is expected to be valid. The classical model results in exact conservation of j_z along a ’’kinematic apse.’’ Quantum mechanically, the CS approximation is reformulated in a manner that clearly shows the relationship between the l̄ choice and the degree and direction of j_z preservation. Away from the forward direction, the simplest choice obeying time reversal symmetry l̄=(l+l′)/2, is shown to result in a propensity for preserving j_z along a ’’geometric apse’’ which coincides with the kinematic apse in the energy sudden limit, and for nonenergy sudden systems only differs significantly from it close to the forward direction.