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

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Direct observation of all three photofragmentation channels of HNCO gives the relative yields of NH+CO3, H+NCO, and NH+CO1 at nine different photolysis energies for both thermal and vibrationally excited molecules. Each higher energy channel dominates as it becomes accessible, but vibrational excitation changes the relative yield of NH1 markedly. Photolysis of HNCO(3ν1) at the same total energy yields up to 2.5 times more NCO with a corresponding reduction in NH1. The relative yield of NH3 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 S1 surface produces NH+CO1 and in which internal conversion to S0 leads to H+NCO, by spin-allowed unimolecular decay, and to NH+CO3, by intersystem crossing and decomposition on T1. The observed vibrational enhancement of the NCO yield is consistent with vibrational excitation impeding the decomposition to NH+CO1 on S1 and, thus, increasing the number of molecules that cross to S0 and decay to H+NCO.