Internal state distributions of CO from HNCO photodissociation

Abstract

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 statepotential energy surface.