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

Abstract

Rotational energy transfer (RET) in collisions of CH A²Δ, 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 Λ-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 Λ-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 ΔN = 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 ΔN ≠ 0 collisions, previously observed for Ar, was found to extend to the other partners. The relative ΔN 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.