INITIAL-STATE RESOLVED ELECTRONIC SPECTROSCOPY OF HNCO - STIMULATED RAMAN PREPARATION OF INITIAL STATES AND LASER-INDUCED FLUORESCENCE DETECTION OF PHOTOFRAGMENTS

Stimulated Raman excitation (SRE) efficiently prepares excited vibrati onal levels in the ground electronic state of isocyanic acid, HNCO. Ph otofragment yield spectroscopy measures the electronic absorption spec trum out of initially selected states by monitoring laser induced fluo rescence (LIF) of either NCO (X (2) Pi) or NH (a (1) Delta) photofragm ents. Near threshold, the N-H bond fission is predissociative, and the re is well-resolved rotational and vibrational structure in the NCO yi eld spectra that allows assignment of K-a rotational quantum numbers t o previously unidentified vibrational and rotational levels in the nu( 1) N-H stretch and nu(3) N-C-O symmetric stretch fundamentals in the g round electronic state of HNCO. The widths of NCO yield resonances dep end on the initial vibrational state, illustrating one way in which in itial vibrational state selection influences dissociation dynamics. In itial excitation of unperturbed nu(1) (N-H stretch) states leads to di ffuse NCO yield spectra compared to excitation of mixed vibrational le vels. The higher energy dissociation channel that produces NH (a (1) D elta) has coarser structure near its threshold, consistent with a more rapid dissociation, but the resonance widths still depend on the init ially selected vibrational state. (C) 1997 American Institute of Physi cs.