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

Abstract

Photochemistry of molecular complex ions in the atmosphere affects the composition, density, and growth of chemical species. Photodissociation processes of a mass-selected O₂⁺(H₂O) complex ion in the visible and ultraviolet regions were studied by ion imaging experiments and theoretical calculations. At 473 nm excitation, O₂⁺ was the predominant photofragment ion produced. In this O₂⁺ 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 H₂O⁺ photofragment ion was mainly produced at 355 nm excitation. The kinetic energy release for the H₂O⁺ 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 ²A′′ ← ²A′′ and ²A′′ ← ²A′′ transitions, respectively, both of which were characterized by positive charge transfer from O₂ to H₂O subunits. To further investigate the dissociation mechanisms, potential energy curves (PECs) and surfaces (PESs) for the O₂⁺(H₂O) ion were calculated for the ground and excited states. As a result, the H₂O⁺ channel at 355 nm excitation was explained by rapid dissociation on the repulsive PES of the state, while rapid electronic relaxation from the to state followed by dissociation in the ground state was inferred in the O₂⁺ channel at 473 nm excitation.