Hl. Berghout et al., Relative product yields in the one-photon and vibrationally mediated photolysis of isocyanic acid (HNCO), J CHEM PHYS, 114(24), 2001, pp. 10835-10844
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.