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 2 + (H 2 O) complex ion in the visible and ultraviolet regions were studied by ion imaging experiments and theoretical calculations. At 473 nm excitation, O 2 + was the predominant photofragment ion produced. In this O 2 + 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 H 2 O + photofragment ion was mainly produced at 355 nm excitation. The kinetic energy release for the H 2 O + 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&cmb.tilde; 2 A′′ ← X&cmb.tilde; 2 A′′ and D&cmb.tilde; 2 A′′ ← X&cmb.tilde; 2 A′′ transitions, respectively, both of which were characterized by positive charge transfer from O 2 to H 2 O subunits. To further investigate the dissociation mechanisms, potential energy curves (PECs) and surfaces (PESs) for the O 2 + (H 2 O) ion were calculated for the ground and excited states. As a result, the H 2 O + channel at 355 nm excitation was explained by rapid dissociation on the repulsive PES of the D&cmb.tilde; state, while rapid electronic relaxation from the B&cmb.tilde; to X&cmb.tilde; state followed by dissociation in the ground state was inferred in the O 2 + channel at 473 nm excitation. Photodissociation dynamics of O 2 + -H 2 O in the visible and ultraviolet regions was studied by ion imaging experiments and theoretical calculations.