The electronic origin and vibrational levels of the first excited singlet state of isocyanic acid (HNCO)

The combination of vibrationally mediated photofragment yield spectroscopy, which excites molecules prepared in single vibrational states, and multiphoton fluorescence spectroscopy, which excites molecules cooled in a supersonic expansion, provides detailed information on the energetics and vibrational structure of the first excited singlet state (S-1) of isocyanic acid (HNCO). Dissociation of molecules prepared in individual vibrational states by stimulated Raman excitation probes vibrational levels near the origin of the electronically excited state. Detection of fluorescence from dissociation products formed by multiphoton excitation through S-1 of molecules cooled in a supersonic expansion reveals the vibrational structure at higher energies. Both types of spectra show long, prominent progressions in the N-C-O bending vibration built on states with different amounts of N-C stretching excitation and H-N-C bending excitation. Analyzing the spectra locates the origin of the S-1 state at 32 449 +/- 20 cm(-1) and determines the harmonic vibrational frequencies of the N-C stretch (omega(3)=1034 +/- 20 cm(-1)), the H-N-C bend (omega(4)=1192 +/- 19 cm(-1)), and the N-C-O bend (omega(5)=599 +/- 7 cm(-1)), values that are consistent with several ab initio calculations. The assigned spectra strongly suggest that the N-C stretching vibration is a promoting mode for internal conversion from S-1 to S-0. (C) 2000 American Institute of Physics. [S0021-9606(00)00212-9].