RAMAN-SPECTROSCOPY OF THE N-C-O SYMMETRICAL (NU(3)) AND ANTISYMMETRIC(NU(2)) STRETCH FUNDAMENTALS IN HNCO

We report the first gas-phase Raman spectra of the N-C-O stretching fu ndamentals in isocyanic acid. Using stimulated Raman excitation to pre pare vibrationally excited molecules, we record spectra via two differ ent techniques, photoacoustic Raman spectroscopy and action spectrosco py. The former detects the sound wave generated as the Stokes laser tu nes through resonances and deposits heat in the gas sample. The latter detects,transitions by photodissociating the vibrationally excited st ares prepared in the vibrational excitation step and detecting the pho tofragments by laser induced fluorescence, In analogy with the stretch ing modes in CO2, the N-C-O symmetric stretch (nu(3)) Raman fundamenta l in HNCO is strong while the antisymmetric stretch (nu(2)) is weak, a lthough neither is symmetry forbidden. Both vibrational states are str ongly perturbed. The symmetric stretch interacts with combination stat es that contain two quanta of bending excitation, and the antisymmetri c stretch interacts with several different combination states. Both Ra man 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 prepara tion on the rotational resonances, photofragment appearance thresholds , and Franck-Condon factors in the transition to a dissociative excite d electronic state. (C) 1997 American Institute of Physics. [S0021-960 6(97)02345-3].