15min:

LOGAN P. DEMPSEY, PATRICIA A. CLEARY, CRAIG MURRAY, MARSHA I. LESTER, Department of Chemistry, University of Pennsylvania, Philadelphia, PA, 19104.

Collisional quenching of electronically excited OH A^{2 +} radicals by molecular hydrogen is known to be an efficient process. Quenching can proceed through a reactive channel, producing water and atomic hydrogen, or through a nonreactive channel. The goal of this work is to study the outcome of the nonreactive quenching channel, and thereby extract information about the dynamical pathways that lead to quenching. A pump-probe scheme is utilized to determine the OH X² population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A^{2 +} (v'=0, N'=0), resulting in a significantly reduced lifetime due to quenching. The probe laser monitors the OH X² (v'', J'') population via laser-induced fluorescence (LIF) on various A-X transitions. To convert observed LIF intensities to a population distribution, it is necessary to account for the fluorescence quantum yield and lifetime of the emitting state. Quenching lifetimes for OH A^{2 +} radicals are measured at a temperature of 50 K and fit to a single exponential decay. Lifetimes of OH A^{2 +} (v'=0, N') show a dramatic increase with rotational level N', corresponding to a decrease in quenching efficiency. The experimental lifetimes are used to extract pseudo first-order rate constants for quenching as a function of N' under these conditions.