Photodissociation of HNCO: Three competing pathways

The unimolecular decomposition of expansion-cooled isocyanic acid (HNCO) via channels (1) ³NH + CO, (2) H+NCO, and (3) ¹NH + CO [where ³NH and ¹NH denote NH(X³∑^-) and NH(a¹δ), respectively] has been investigated following photoexcitation to the S₁(¹A″) state in two energy regimes: (i) in the region of the ¹NH + CO threshold (41700-45500cm^-1; 240-220nm), and (ii) ∼ 3200cm^-1 above D₀(¹NH + CO), at around 46000 cm^-1 (217.6nm). Several complementary experiments are presented: NCO, ³NH and ¹NH photofragment yield spectra and relative ¹NH/³NH branching ratios are obtained by laser induced fluorescence (LIF); photofragment ion imaging is used to record CO angular recoil distributions, and ¹NH rotational distributions correlated with specific CO(v,J) levels. HNCO excited to S, undergoes complex dynamics reflecting simultaneous decomposition on several potential energy surfaces, and including internal conversion (1C) and intersystem crossing (ISC). In energy region (i), a progressive loss of structure in the ³NH 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 S₁ At low excess energies channel (2) derives from dissociation on S₀, but the respective roles of S₀ and S₁ 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.