Electronic quenching of OH A²Σ+ radicals in single collision events with molecular hydrogen: Quantum state distribution of the OH X²Π products

Cleary, P.A., Dempsey, L.P., Murray, C., Lester, M.I., Kłos, J. and Alexander, M.H. (2007) Electronic quenching of OH A²Σ+ radicals in single collision events with molecular hydrogen: Quantum state distribution of the OH X²Π products. Journal of Chemical Physics, 126(20), p. 204316. (doi: 10.1063/1.2730505)

Abstract

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A ²Σ+ by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X ²Π population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A ²Σ+ (ν′ = 0, N′ = 0), which has a significantly reduced fluorescence lifetime due to quenching by H₂. The probe laser monitors the OH X ²Π (ν″, N″) population via laser-induced fluorescence on various A-X transitions under single collision conditions. The experiments reveal a high degree of rotational excitation (N″) of the quenched OH X ²Π products observed in ν″ = 1 and 2 as well as a pronounced propensity for quenching into the Π(A′) Λ-doublet level. These experiments have been supplemented by extensive multireference, configuration-interaction calculations aimed at exploring the topology of the relevant potential energy surfaces. Electronic quenching of OH A ²Σ+ by H₂ proceeds through conical intersections between two potentials of A′ reflection symmetry (in planar geometry) that correlate with the electronically excited A ²Σ+ and ground X ²Π states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H₂. Corroborating and extending earlier work of Hoffman and Yarkony [J. Chem. Phys. 113, 10091 (2000) ], these calculations reveal a steep gradient away from the OH–H₂ conical intersection as a function of both the OH orientation and interfragment distance. The former will give rise to a high degree of OH rotational excitation, as observed for the quenched OH X ²Π products.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Murray, Dr Craig
• Authors: Cleary, P.A., Dempsey, L.P., Murray, C., Lester, M.I., Kłos, J., and Alexander, M.H.
• Subjects: Q Science > QD Chemistry
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Journal of Chemical Physics
• Journal Abbr.: J. Chem. Phys.
• ISSN: 00219606
• Published Online: 31 May 2007