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 multiph oton fluorescence spectroscopy, which excites molecules cooled in a superso nic expansion, provides detailed information on the energetics and vibratio nal structure of the first excited singlet state (S-1) of isocyanic acid (H NCO). Dissociation of molecules prepared in individual vibrational states b y stimulated Raman excitation probes vibrational levels near the origin of the electronically excited state. Detection of fluorescence from dissociati on products formed by multiphoton excitation through S-1 of molecules coole d in a supersonic expansion reveals the vibrational structure at higher ene rgies. 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 harmoni c 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 calcu lations. The assigned spectra strongly suggest that the N-C stretching vibr ation is a promoting mode for internal conversion from S-1 to S-0. (C) 2000 American Institute of Physics. [S0021-9606(00)00212-9].