On the physical origin of propensity rules in collisions involving molecules in 2Σ electronic states

Abstract

A general quantum treatment of the collision dynamics of 2S+1Σ-state diatomic molecules is developed, with particular emphasis given to the physical origin of the collisional propensity for conservation of the e/f molecular symmetry label. State-resolved integral cross sections are expressed as a weighted sum of tensor opacities which are related to the probability that a collision will reorient the nuclear rotational angular momentum vector. Previous derivations of the propensity rules for collisions of 2S+1Σ-state diatomic molecules were restricted to the sudden or Born approximations. Here a general derivation is developed that is free of dynamical approximations, and that clearly establishes the direct connection between the observed propensity for the conservation of the e/f molecular symmetry and the collisional propensity for the conservation of the orientation of the nuclear rotational angular momentum vector. A power-law model suggested by previous semiempirical fits to cross sections for 1Σ+-state molecules is extended to open-shell molecules. This simple model allows us to predict semiquantitatively the degree to which the e/f symmetry index is conserved without performing a full dynamical calculation.