PREDISSOCIATION DYNAMICS OF OH {\it A\,}$^2 \Sigma ^+$ ($v=4$)
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Date
2005
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Publisher
Ohio State University
Abstract
Predissociation of the excited {\it A\,}$^2\Sigma^+$ electronic state of the OH radical results from curve crossings with repulsive potentials, namely $^4\Sigma^-$, $^2\Sigma^-$, $^4\Pi$, producing O($^3$P$_{J}$) + H($^2$S) fragments. Previous experimental and theoretical studies of the $v=4$ level of the OH {\it A\,}$^2\Sigma^+$ state indicate that predissociation occurs at least 10-fold faster than in lower vibrational levels, resulting in a fluorescence quantum yield that is essentially zero. In the present study, Fluorescence Depletion Infrared (FDIR) spectroscopy, a UV-IR double resonance technique, is employed to characterize the linewidths and corresponding lifetimes of highly predissociative rovibrational levels of the excited {\it A\,}$^2\Sigma^+$ electronic state of the OH radical. A least-squares fit to a Voigt line profile is used to extract the Lorentzian linewidths of the individual dips in the FDIR spectra. The homogeneous linewidth measurements, ranging from 0.23 to 0.31 cm$^{-1}$ FWHM, demonstrate that the $N=0-7$ rotational levels of the OH {\it A\,}$^2\Sigma^+$ $(v=4)$ state undergo rapid predissociation, with lifetimes $\leq 23$ ps. The experimental linewidths are in near quantitative agreement with first principle theoretical predictions.
Description
Author Institution: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323