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

Abstract

The occurrence of jz-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 jz 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 jz 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 jz 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.