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 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 B[combining tilde]²A''← X[combining tilde]²A'' and D[combining tilde]²A''← X[combining tilde]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 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 O₂⁺ channel at 473 nm excitation.