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.

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.

The collisional quenching of OH radicals in their excited A 2Sigma+ electronic state by molecular hydrogen is examined to determine the partitioning between reactive and nonreactive pathways. This is achieved using a pump-probe laser technique to compare the population prepared in the excited OH A 2Sigma+ state with that produced in the OH X 2Pi ground state from nonreactive quenching. Only a small fraction of the products, less than 15%, arise from nonreactive quenching; reactive quenching is the dominant product channel. The branching between the product channels provides a new dynamical signature of the conical intersection region(s) that couple the excited state potential for OH A 2Sigma++H2 with OH X 2Pi+H2 and H2O+H products.

The nascent OH X (2)Pi product state distribution arising from collisional quenching of electronically excited OH A (2)Sigma(+) by N(2) has been determined using a pump-probe technique. The majority of OH X (2)Pi products are observed in their lowest vibrational level, v(")=0, with significantly less population in v(")=1. The OH (v(")=0) products are generated with a substantial degree of rotational excitation, peaking around N(")=18, with an average rotational energy of approximately 6500 cm(-1). A preference is found for the OH Pi(A(')) Lambda-doublet, indicating some degree of ppi orbital alignment. The branching fraction into OH X (2)Pi product states demonstrates that nonreactive quenching is the dominant decay pathway for quenching of OH A (2)Sigma(+) by N(2). The topography of the conical intersection region that couples the electronically excited and ground state potential energy surfaces is also examined theoretically. The rotational excitation of the OH X (2)Pi products and branching fraction are found to be dynamical signatures of nonadiabatic passage through the conical intersection region.

Gas-liquid scattering experiments are used to investigate reactions of Cl(2) with a 2.7 M NaBr-glycerol solution at 291 K. Only the single and double halogen exchange products, BrCl and Br(2), are observed to desorb from solution. When Cl(2) molecules strike the surface at thermal collision energies, 76% desorb as Cl(2) before reacting, 1% react to form BrCl, and 23% react to form Br(2). Residence time measurements, modeled by mass-transfer equations for absorption, diffusion, reaction, and evaporation, were used to determine the time and depth scales for Cl(2) escape and BrCl and Br(2) production. This modeling indicates that Cl(2) molecules desorb from the interfacial region in less than 1 μs or are attacked within this time by Br(-) ions and irreversibly captured as Cl(2)Br(-). The products BrCl and Br(2) are created primarily within the top few monolayers of the solution and then evaporate on average 12 and 28 μs after Cl(2) initially reacts with Br(-). Notably, Br(2) is not generated from BrCl via Cl(2)Br(-) → BrCl + Cl(-) and BrCl + Br(-) → Br(2)Cl(-) but from the parallel reaction Cl(2)Br(-) + Br(-) → Br(2)Cl(-)+ Cl(-) that bypasses the BrCl intermediate. Br(2) is then likely released through two pathways, Br(2)Cl(-) → Br(2) + Cl(-) and Br(2)Cl(-) + Br(-) → Br(3)(-) + Cl(-), followed by Br(3)(-) ↔ Br(2) + Br(-). The experiments demonstrate that single and double halogen exchange reactions can occur rapidly and close to the surface even when the products are created by multiple sequential reactions.