The inelastic scattering of 2Π [case (b)] molecules and an understanding of the differing Λ doublet propensities for molecules of π vs π3 orbital occupancy

Abstract

The quantum formalism for the scattering of a diatomic molecule in a 2Π electronic state which is well described by Hund’s case (b) limit is investigated here. For a particular JFi→J′F′1 transition, quantum interference effects will lead to preferential population of one of the final state Λ doublet levels. The nonstatistical population of final state Λ doublet levels arises from an interference between terms in the expansion of the two electrostatic potential energy surfaces, of A′ and A″ reflection symmetry, which describe the interaction between a molecule in a Π electronic state and a closed-shell partner. The particular Λ doublet level preferred is opposite for molecules of π1 vs π3 electron occupancy. The physical origin of this reversal in the Λ doublet propensity is a direct reflection of the fact that for the former the A′ potential surface is more repulsive since the sole π electron lies in the triatomic plane in this case, whereas for molecules of π3 electron occupancy the A′ surface is less repulsive than the A″ surface since for the A′ surface only one of the three π electrons lies in the triatomic plane. The magnitude of these Λ doublet propensities is illustrated by calculated cross sections for the CH(X 2Π)–He system using the ab initio potential energy surfaces calculated by the Argonne theoretical group, and these cross sections are compared to those of the crossed molecular study of Liu and Macdonald [J. Chem. Phys. 91, xxxx (1989)]. A similar analysis is carried out for collisions of a molecule of π3 electron occupancy and is illustrated by inelastic collisions of OH(X2Π).