Competition between direct and indirect dissociation pathways in ultraviolet photodissociation of HNCO.

Photodissociation dynamics of HNCO at photolysis wavelengths between 200 and 240 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. At low photon energy excitation, the product translational energy distribution is nearly statistical and the angular distribution is isotropic, which is consistent with an indirect dissociation mechanism, i.e., internal conversion from S-1 to S-0 surface and dissociation on S-0 surface. As the photon energy increases, a direct dissociation pathway on S-1 surface opens up. The product translational energy distribution appears to be quite nonstatistical and the product angular distribution is anisotropic. The fraction of direct dissociation pathway is determined to be 36 +/- 5% at 202.67 nm photolysis. Vibrational structures are observed in both direct and indirect dissociation pathways, which can be assigned to the NCO bending mode excitation with some stretching excitation