Electronic quenching of OH A ²Σ⁺radicals in single collision events with H₂and D₂: a comprehensive quantum state distribution of the OH X ²Π products

Abstract

A pump-probe laser-induced fluorescence technique has been used to examine the nascent OH X (2)Pi product state distribution arising from non-reactive quenching of electronically excited OH A (2)Sigma(+) by molecular hydrogen and deuterium under single-collision conditions. The OH X (2)Pi products were detected in v''=0, 1 and 2; the distribution peaks in v''=0 and decreases monotonically with increasing vibrational excitation. In all vibrational levels probed, the OH X (2)Pi products are found to be highly rotationally excited, the distribution peaking at N''=15 when H(2) was used as the collision partner and N''=17 for D(2). A marked propensity for production of Pi(A') Lambda-doublet levels was observed, while both OH X (2)Pi spin-orbit manifolds were equally populated. These observations are interpreted as dynamical signatures of the nonadiabatic passage of the OH + H(2)/D(2) system through the seams of conical intersection that couple the excited state (2 (2)A') and ground state (1 (2)A') surfaces.

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

“…Despite the very different collision conditions, the vibration-rotation distribution of Fig. 10 is qualitatively similar to that observed in the low energy experiments of Dempsey et al 12 The ground (v = 0) vibrational state is the dominant product in both calculation and experiment. The relative population of v = 1 is somewhat higher here than in the low energy experiment.…”

“…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.…”

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.…”

“…14 The calculated vibrational state occupations, summed over rotational states, are 71% v = 0, 21% v = 1, and 8% v = 2, which are in good accord with the corresponding experimental estimates of 70%, 25%, and 5%. 14 We also calculated the D 2 rovibrational distribution and present the results in Fig. 14. This distribution, which has not been measured experimentally, shows a moderate degree of rotational excitation and, as with OH, the vibrational ground state dominates, with some appreciable vibrational excitation, as predicted previously.…”

“…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.…”