15min:

ERIKA L. DERRO, LOGAN P. DEMPSEY, ILANA B. POLLACK, MARSHA I. LESTER, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323.

Predissociation of the excited A ^{2 +} electronic state of the OH radical results from curve crossings with repulsive potentials, namely ^{4 -}, ^{2 -}, ⁴ , producing O(³P_J) + H(²S) fragments. Previous experimental and theoretical studies of the v=4 level of the OH A ^{2 +} 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 A ^{2 +} 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 A ^{2 +} (v=4) state undergo rapid predissociation, with lifetimes \leq 23 ps. The experimental linewidths are in near quantitative agreement with first principle theoretical predictions.