Electronic Quenching of OH A²Σ⁺(v‘ = 0, 1) in Complexes with Hydrogen and Nitrogen

Abstract

The quenching of electronically excited OH A 2Σ+ radicals has been investigated in complexes of OH with molecular hydrogen, deuterium, and nitrogen and through complementary theoretical calculations. Many of the intermolecular vibrational levels supported by the OH A 2Σ+ (v‘ = 0, 1) + H2, D2, and N2 potentials have been characterized by laser-induced fluorescence and fluorescence depletion measurements of the complexes in the OH A 2Σ+−X 2Π 1−0 and 0−0 spectral regions. Homogeneous line broadening of the spectral features yields picosecond lifetimes for complexes prepared in levels derived from OH A 2Σ+ (v‘ = 0) as a result of electronic quenching and/or chemical reaction. More extensive line broadening is observed for complexes excited to levels correlating with OH A 2Σ+ (v‘ = 1). The corresponding decay rates are 10−75 times faster than obtained for v‘ = 0 due to the opening of the vibrational predissociation channel and/or enhancement of the quenching/reaction processes upon OH vibrational excitation. Ab initio calculations of the OH (A 2Σ+, X 2Π) + H2 and N2 potential energy surfaces reveal the minimum energy configurations, T-shaped O−H--H2 and linear O−H--N⋮N, and the large increases in interaction energy upon electronic excitation of OH. The theoretical calculations also identify specific orientations, T-shaped H−O--H2 and linear H−O--N⋮N, that lead to conical intersections between the ground- and excited-state surfaces and give rise to quenching of OH A 2Σ+ by hydrogen and nitrogen.

Reactive Quenching of OH A 2Σ+ in Collisions with Molecular Deuterium via Nonadiabatic Passage through a Conical Intersection

“…For this C 2V configuration, there is little or no barrier separating the OH A 2 Σ + + H 2 reactants from the conical intersection region. 12,18 These ab initio calculations also suggest that the water product will be produced with a large degree of internal excitation due to the considerable structural changes that must occur between the conical intersection region and the H + H 2 O asymptote. Further theoretical work by Yarkony has identified several seams of conical intersections that may influence the quenching dynamics: a symmetry-allowed C 2V seam, a symmetry-allowed C ∞V seam, and a C s seam that bridges the region between the two other seams.…”

“…11, 12 These pathways can lead to ground-state OH X 2 Π radicals (nonreactive quenching) or reaction products. Only recently have experimental and theoretical studies started to examine the outcome of these electronic quenching events.…”

Quasiclassical trajectory study of the postquenching dynamics of OH AΣ2+ by H2/D2 on a global potential energy surface

“…The quenching of OH radicals in their excited A 2 ⌺ + state by H 2 ͑D 2 ͒ to nonreactive, OH+ H 2 ͑D 2 ͒, and reactive, H 2 O+H ͑HOD+ D and D 2 O+H͒, products has been the focus of extensive experimental and theoretical studies in the past few years [8][9][10][11][12][13][14][15][16][17][18][19] and is one of the most thoroughly studied systems for nonadiabatic dynamics. In experiments, Lester and co-workers 9,10 investigated the reactive channel with H 2 and D 2 using two-photon laser-induced fluorescence ͑LIF͒ detection of the H and D atom products and analyzing the Doppler profiles.…”

The role of conical intersections in the nonadiabatic quenching of OH(A 2Σ+) by molecular hydrogen

“…[1][2][3][4][5][6][7][8][9][10][11][12] In particular, collisions between OH(A 2 ⌺ ϩ ) and molecular hydrogen have received a great deal of attention, both theoretical and experimental. 1,[4][5][6][7][8][9][10][11] The collisional quenching of OH by H 2 may exhibit two possible product channels: A reactive channel forming H 2 OϩH or a nonreactive channel forming OH(X 2 ⌸)ϩH. While the HϩH 2 O channel is not unexpected, it was only recently that experiments have examined the quenching products and established that H 2 OϩH is indeed produced in non-negligible quantities.…”