Relative product yields in the one-photon and vibrationally mediated photolysis of isocyanic acid (HNCO)

Direct observation of all three photofragmentation channels of HNCO gives t he relative yields of (NH)-N-3+CO, H+NCO, and (NH)-N-1+CO at nine different photolysis energies for both thermal and vibrationally excited molecules. Each higher energy channel dominates as it becomes accessible, but vibratio nal excitation changes the relative yield of (NH)-N-1 markedly. Photolysis of HNCO(3 nu (1)) at the same total energy yields up to 2.5 times more NCO with a corresponding reduction in (NH)-N-1. The relative yield of (NH)-N-3 and NCO, however, is indifferent to vibrational excitation. The dependence of the yields on vibrational excitation supports a picture in which direct decomposition on the S-1 surface produces (NH)-N-1+CO and in which internal conversion to S-0 leads to H+NCO, by spin-allowed unimolecular decay, and to (NH)-N-3+CO, by intersystem crossing and decomposition on T-1. The obser ved vibrational enhancement of the NCO yield is consistent with vibrational excitation impeding the decomposition to (NH)-N-1+CO on S-1 and, thus, inc reasing the number of molecules that cross to S-0 and decay to H+NCO. (C) 2 001 American Institute of Physics.