Internal state distributions of CO from HNCO photodissociation

The internal state distributions of CO produced by photodissociation of HNCO at 1930 (230.1 nm) and at 10 200 cm−1 (193.3 nm) in excess of the dissociation energy are determined from multiphoton ionization spectra of the CO fragment measured under collision-free conditions. The rotational state distribution of the CO produced at the lower photolysis energy is characterized by a temperature of (491±23) K. The rotational state distribution of CO produced by photodissociation at the higher photon energy in not well characterized by a temperature. This latter distribution has maximum population near J=37, extends beyond J=65, and accounts for ∼20% of the available energy in excess of that necessary to rupture the HN–CO bond. An impulsive dissociation model assuming that dissociation occurs from an excited HNCO complex containing a nonlinear N–C–O configuration accounts for the average CO rotational excitation while phase-space theory does not agree with the observed distributions. Fitting a semiclassical model to the data using a logically constructed potential energy surface and a ground-state-dependent function provides a useful parametrization for the impulsive dissociation. Although not absolute, this analysis suggests that the dissociation occurs directly on a repulsive excited state potential energy surface.