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

Abstract

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.

Rotationally resolved quenching and relaxation of CH(A2Δ,v=0,N) in the presence of CO

“…6 Ground state CH(X 2 ⌸) was first generated in the pulsed 248 nm KrF laser photolysis of bromoform. 15 The laser beam was mildly focused to enhance the nonlinear photodissociation process.…”

193nm photolysis of CHCl3: Probe of the CH product by CRDS

“…This simulation indicates that the rotational and vibrational populations probed by the CRDS detection are well-reproduced by Boltzmann distributions at T rot = 300 ± 20 K and T vib = 1900 ± 50 K, respectively. Concerning the rotational temperature, collisions of CH radicals with argon induce an efficient and complete rotational thermalization of the CH nascent population to room temperature, within a few hundreds of nanoseconds after the production [24], prior to any CRDS measurement. As far as vibration is concerned, thermalization is much slower and is not expected to be completed within the time scale of the ring-down [23] with T vib = 1900 K and T rot = 300 K (see text for details).…”

Spectroscopic and theoretical characterization of the AΔ2-XΠ2 transition of CH–Ne

“…24͒ were used to analyze similar rotational energy-transfer data for CH͑A͒ + Ar collisions. 16,25 Energy-transfer data for CH+ Ne collisions have not been reported, but the complex has been examined using the B-X transition. 26,27 The potential energy surfaces of CH͑X͒ − Ne and CH͑B͒ − Ne resemble those of the −Ar complex, the primary difference being that the binding energies are appreciably smaller for Ne.…”

“…From a theoretical perspective, the collisions of CH with rare-gas atoms present the most tractable model systems. 7,8,16,17 To date, collisional energy-transfer processes have been characterized for CH͑X͒ + He, 2,3 CH͑X͒ + Ar, 18,19 CH͑A͒ + He, 13,14 CH͑A͒ + Ar, 9,11,13,14 CH͑B͒ + He, 13,14 and CH͑B͒ + Ar. 10,14,15 All aspects of collision dynamics are governed by the potential energy surfaces for the interaction of a colliding pair.…”