RAMAN-SPECTROSCOPY OF THE NU(1) N-H STRETCH FUNDAMENTAL IN ISOCYANIC ACID (HNCO) - STATE MIXING PROBED BY PHOTOACOUSTIC-SPECTROSCOPY AND BYPHOTODISSOCIATION OF VIBRATIONALLY EXCITED-STATES

We report the first gas phase Raman spectrum of isocyanic acid. Using stimulated Raman excitation (SRE) to prepare vibrationally excited sta tes, we detect transitions by both photoacoustic Raman spectroscopy (P ARS) and action spectroscopy. In this paper we present results on the v(1) N-H stretch fundamental, leaving the spectra of the N-C-O symmetr ic and antisymmetric stretch modes for a separate publication. The Ram an spectrum shows extensive state mixing in the vl fundamental, in agr eement with previous infrared work. Measurement of the effective b-axi s rotational constants for different mixed vibrational states in this near-prolate symmetric top limits the number of candidates for perturb ing states and shows which vibrational modes participate. Double reson ance photodissociation further probes the vibrational spectroscopy of isocyanic acid. The scheme is first to prepare a vibrationally excited state by SRE, then photodissociate only the molecules prepared in the first step, and finally probe the decomposition products by laser-ind uced fluorescence (LIF). An action spectrum, obtained by scanning the vibrational excitation laser (Stokes) wavelength with the photolysis l aser wavelength fixed and the probe laser tuned to a LIF transition in one of the photofragments, is the key to unraveling the spectroscopy. The intensity differences between PARS and action spectrum transition s reveal the vibrational state mixing and provide the Franck-Condon fa ctors for transitions to the excited electronic state. (C) 1997 Americ an Institute of Physics.