Fine-structure state resolved rotationally inelastic collisions of CH(A 2Δ,v=0) with Ar: A combined experimental and theoretical study

A collaborative experimental and theoretical study of rotationally inelastic collisions of CH(A 2Δ,v=0) resolved rotational/fine-structure levels with argon is presented. Experimental state-to-state rate constants were extracted from CH A 2Δ→X 2Π fluorescence spectra upon laser excitation to individual levels in the A 2Δ state in the presence of Ar. Fluorescence detection of the collision-induced population permits resolution of the fine-structure levels at low N, but no Λ-doublet discrimination. For the lowest value of N (N=2), the dominant process is the ΔN=0 fine-structure-changing transition, and the efficiency of this transition decreases markedly with increasing N. There is an increasing preference for conservation of the fine-structure label in ΔN≠0 transitions as N increases. These rate constants have been compared to and interpreted with theoretical rate constants computed through quantum coupled-states calculations of cross sections based on ab initio CH(A 2Δ)–Ar potential energy surfaces determined by Kaledin and Heaven (to be published). The tendency to conserve the fine-structure label is attributed to the Hund’s case (b) nature of the CH(A 2Δ) state, for which the electron spin is a spectator in the collision.