Electronic quenching of OH AΣ+2 radicals in single collision events with molecular hydrogen: Quantum state distribution of the OH XΠ2 products

Abstract

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A 2Sigma+ by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X 2Pi population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A 2Sigma+ (nu'=0, N'=0), which has a significantly reduced fluorescence lifetime due to quenching by H2. The probe laser monitors the OH X 2Pi (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 2Pi 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 2Sigma+ by H2 proceeds through conical intersections between two potentials of A' reflection symmetry (in planar geometry) that correlate with the electronically excited A 2Sigma+ and ground X 2Pi 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 2Pi products.

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

“…The nonreactive quenching channel has also been investigated recently in Lester's experiments, utilizing a pump-probe approach in the collisional region of a supersonic free-jet expansion. [11][12][13][14] The OH products were generated with significant rotational excitation, but small degree of vibrational excitation, and the preference for reactive quenching over nonreactive quenching by roughly 7 to 1 was also discovered by the Lester group. 13 Theoretically, Yarkony and co-workers performed stateaveraged multiconfigurational self-consistent field/ configuration interaction calculations to determine the conical intersections ͑CoIs͒ of the potential energy surfaces ͑PESs͒ relevant to this quenching.…”

“…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.…”

“…19 The energies of the ground state potential at these configurations vary from 2.8 to 3.3 eV, relative to OH+ H 2 products. In recent joint experimental and theoretical work by Alexander and Lester and co-workers, 12 the PESs of the ground and excited AЈ electronic states were mapped out in two degrees of relevance to the OH+ H 2 products in the vicinity of the C 2v seam of intersections. These calculations were carried out using multireference configuration interaction ͑MRCI͒ method with a large basis at roughly 21 000 configurations.…”

A novel Gaussian Binning (1GB) analysis of vibrational state distributions in highly excited ${\rm H}_\text{2}$H2O from reactive quenching of OH* by ${\rm H}_\text{2}$H2

“…[39][40][41] Limited two degree of freedom potential energy surfaces were also reported which qualitatively account for the high degree of rotational excitation of the quenched OH radical in the nonreactive channel. 42 Fu et al performed QCT calculations on the ground electronic state initiated from conical intersections with the lowest energy excited electronic state at the total energy of experiment. 28 The total energy of the trajectories was 5.14 eV relative to H + H 2 O (eq).…”

An ab initio quasi-diabatic potential energy matrix for OH(2Σ) + H2

“…These reactions (and isotopic analogues) have generated substantial experimental interest in recent years. [8][9][10][11][12][13][14][15][16][17] Moreover, there has been significant theoretical interest in the electronic structure and conical intersections of this system, 10,18,19 and in the general question of adiabatic to diabatic transformations (ADTs) in molecules with conical intersections. [20][21][22][23][24] As a consequence, this system represents a benchmark test case for the development of theoretical reaction dynamics for polyatomic molecules in multiple electronic states.…”