Knowledge Bank

Collisional quenching of electronically excited OH $A2\Sigma +$ 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 $X2\Pi$ population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH $A2\Sigma +$ $(v\prime =0,N\prime =0)$, resulting in a significantly reduced lifetime due to quenching. The probe laser monitors the OH $X2\Pi$ $(v\prime \prime ,J\prime \prime )$ 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 $A2\Sigma +$ radicals are measured at a temperature of $\sim$50 K and fit to a single exponential decay. Lifetimes of OH $A2\Sigma +$ $(v\prime =0,N\prime )$ show a dramatic increase with rotational level $N\prime$, 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 $N\prime$ under these conditions.