Raman spectroscopy of the N-C-O symmetric (nu(3)) and antisymmetric (nu(2)) stretch fundamentals in HNCO

We report the first gas-phase Raman spectra of the N-C-O stretching fundamentals in isocyanic acid. Using stimulated Raman excitation to prepare vibrationally excited molecules, we record spectra via two different techniques, photoacoustic Raman spectroscopy and action spectroscopy. The former detects the sound wave generated as the Stokes laser tunes through resonances and deposits heat in the gas sample. The latter detects,transitions by photodissociating the vibrationally excited stares prepared in the vibrational excitation step and detecting the photofragments by laser induced fluorescence, In analogy with the stretching modes in CO2, the N-C-O symmetric stretch (nu(3)) Raman fundamental in HNCO is strong while the antisymmetric stretch (nu(2)) is weak, although neither is symmetry forbidden. Both vibrational states are strongly perturbed. The symmetric stretch interacts with combination states that contain two quanta of bending excitation, and the antisymmetric stretch interacts with several different combination states. Both Raman spectra have strong (Q)Q branch relational structure in which the band origins for different ii sublevels in this near-prolate symmetric top follow no simple pattern. Photodissociation of the vibrationally excited states demonstrates the influence of the initial stare preparation on the rotational resonances, photofragment appearance thresholds, and Franck-Condon factors in the transition to a dissociative excited electronic state. (C) 1997 American Institute of Physics. [S0021-9606(97)02345-3].