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 (2)Delta,upsilon = 0) resolved rotational/fine-structure levels with argon is presented. Experimental state-to-state rate constants were extracted from CH A (2)Delta --> X (2)Pi fluorescence spectra upon laser excitation to individual levels in the A (2)Delta 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 Lambda -doublet discrimination. For the lowest value of N (N = 2), the dominant process is the DeltaN = 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 DeltaN not equal 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 (2)Delta)-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 (2)Delta) state, for which the electron spin is a spectator in the collision. (C) 2001 American Institute of Physics.