Photodissociation processes of a water-oxygen complex cation studied by an ion imaging technique.

Photochemistry of molecular complex ions in the atmosphere affects the composition, density, and growth of chemical species. Photodissociation processes of a mass-selected O(HO) complex ion in the visible and ultraviolet regions were studied by ion imaging experiments and theoretical calculations. At 473 nm excitation, O was the predominant photofragment ion produced. In this O channel, the kinetic energy release was comparable to that estimated using a statistical dissociation model, and the anisotropy parameter was determined to be β = 1.0 ± 0.1. On the other hand, the HO photofragment ion was mainly produced at 355 nm excitation. The kinetic energy release for the HO channel was large and nonstatistical, and the anisotropy parameter was β = 1.9 ± 0.2. Theoretically, the 473 and 355 nm excitations were assigned to the B[combining tilde]A''← X[combining tilde]A'' and D[combining tilde]A''← X[combining tilde]A'' transitions, respectively, both of which were characterized by positive charge transfer from O to HO subunits. To further investigate the dissociation mechanisms, potential energy curves (PECs) and surfaces (PESs) for the O(HO) ion were calculated for the ground and excited states. As a result, the HO channel at 355 nm excitation was explained by rapid dissociation on the repulsive PES of the D[combining tilde] state, while rapid electronic relaxation from the B[combining tilde] to X[combining tilde] state followed by dissociation in the ground state was inferred in the O channel at 473 nm excitation.