Electronic quenching of OH A 2Σ+ radicals in single collision events with molecular hydrogen:: Quantum state distribution of the OH X 2Π products

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A (2)Sigma(+) by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X (2)Pi population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A (2)Sigma(+) (nu(')=0, N-'=0), which has a significantly reduced fluorescence lifetime due to quenching by H-2. The probe laser monitors the OH X (2)Pi (nu '', 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 (2)Pi products observed in nu ''=1 and 2 as well as a pronounced propensity for quenching into the Pi(A(')) Lambda-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 (2)Sigma(+) by H-2 proceeds through conical intersections between two potentials of A(') reflection symmetry (in planar geometry) that correlate with the electronically excited A (2)Sigma(+) and ground X (2)Pi states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H-2. 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-H-2 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 (2)Pi products. (C) 2007 American Institute of Physics.