15min:

LOGAN P. DEMPSEY, TIMOTHY D. SECHLER, CRAIG MURRAY, MARSHA I. LESTER, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323; SPIRIDOULA MATSIKA, Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122-6014.

The nascent OH X² product state distributions arising from collisional quenching of electronically excited OH A^{2 +} by H₂ and N₂ have been determined using a pump-probe technique. For both collision partners, the majority of OH X² 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''=15 with H₂ as the collision partner and N''=18 with N₂. Complementary measurements of the branching fraction into OH X² product states demonstrate that reaction is the dominant decay pathway for quenching of OH A^{2 +} by H₂, while nonreactive quenching is the dominant pathway for N₂. These observations are discussed in the context of theoretical calculations that examine the topography of the conical intersections which couple the electronically excited and ground state potential energy surfaces. The experimental observables are interpreted as dynamical signatures of nonadiabatic passage through the conical intersection regions responsible for quenching in both systems.