Photodissociation of HNCO: Three competing pathways

The unimolecular decomposition of expansion-cooled isocyanic acid (HNCO) via channels (1) (NH)-N-3+CO, (2) H+NCO, and (3) (NH)-N-1+CO [where (NH)-N-3 and (NH)-N-1 denote NH(X(3) Sigma(-)) and NH(a(1) Delta), respectively] has been investigated following photoexcitation to the S-1((1)A '') stale in two energy regimes: (i) in the region of the (NH)-N-1+CO threshold (41 700-45 500 cm(-1); 240-220 nm), and (ii) similar to 3200 cm(-1) above D-0((NH)-N-1+CO), at around 46 000 cm(-1) (217.6 nm). Several complementary experiments are presented: NCO, (NH)-N-3 and (NH)-N-1 photofragment yield spectra and relative (NH)-N-1/(NH)-N-3 branching ratios are obtained by laser induced fluorescence (LIF); photofragment ion imaging is used to record CO angular recoil distributions, and (NH)-N-1 rotational distributions correlated with specific CO (v, J) levels. HNCO excited to S-1 undergoes complex dynamics reflecting simultaneous decomposition on several potential energy surfaces, and including internal conversion (IC) and intersystem crossing (ISC). In energy region (i), a progressive loss of structure in the (NH)-N-3 yield spectrum is observed above the opening of channel (3), and is interpreted as the imprint of short-time dynamics characteristic of the ISC step. State selectivity in the photodissociation is revealed by comparing the photofragment yield spectra of the three channels, In region (ii), product state distributions for channel (3) exhibit clear dynamical signatures, as expected for dissociation on S-1. At low excess energies channel (2) derives from dissociation on S-0, but the respective roles of S-0 and S-1 at higher energies are not well established yet. The results are discussed in terms of vibronic levels of mixed electronic character coupled directly or via radiationless decay to the various continua. The competition between the different processes depends sensitively on photolysis energy and excitation conditions.