Rotational energy transfer in collisions of CH A²Δ, v = 0 with Ar, N² and CO²

Rotational energy transfer (RET) in collisions of CH A²?, v=0 with Ar, N² and CO² has been investigated experimentally. Laser excitation was used to prepare selected initial levels N = 2,3,5 (all colliders), 8 (Ar and CO²) and 14 (Ar only). Additional fine-structure state selection was achieved for the lower levels, and ?-doublet selection for the higher levels. Total and state-to-state RET rate constants were extracted from time-resolved dispersed fluorescence spectra. Partial fine-structure-state resolution was possible for N'=4, and ?-doublet resolution for the highest levels. For Ar, good absolute agreement was found with previous state-selective experimental work and ab initio theoretical predictions. RET is substantially more efficient for N² and CO², for which the first reliable absolute rate constants are provided. A change of fine-structure state with ?N=0 is the most probable single state-to-state channel at low N for all three partners. Partial conservation of the fine-structure label during ?N?0 collisions, previously observed for Ar, was found to extend to the other partners. The relative ?N propensities are similar for all three partners. This is consistent with a simple impulsive model of the collision dynamics, in which energy constraints dominate because of the large rotational energy spacing characteristic of hydrides. This model does not, however, provide a straightforward explanation for the substantial differences in RET efficiency for Ar, N² and CO².