Photodissociation Dynamics of the [O2-H2O]+ Ionic Complex

We present an experimental study on the photodissociation dynamics of [O₂-H₂O]⁺ in the 580-266 nm wavelength range using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces mass selected and internally cold [O2-H2O]+ ions for photodissociation. By detecting both the O₂⁺ and H₂O⁺ photofragments using the time-of-flight mass spectrometry and velocity map imaging techniques, branching ratios and total kinetic energy release distributions of the O₂⁺ + H₂O and H₂O⁺ + O₂ product channels are experimentally measured at 16 different excitation energies. State-resolved photodissociation mechanisms of the parent [O₂-H₂O]⁺ are interpreted as (1) the O₂(X³Σ_g^-) + H₂O⁺(X~2B1), O₂(a¹Δ_g) + H₂O⁺(X~2B1), and O₂(X³Σ_g^-) + H₂O⁺(A~2A1) channels are produced from direct dissociation of [O₂-H₂O]⁺ in its excited B~2A″, D~2A″, and F~2A″ states, respectively; (2) the O₂⁺(X²Π_g) + H2O(X~A11) channel is produced from nonadiabatic relaxations of the excited B~2A″, D~2A″, and F~2A″ states to the X~2A″ ground state with subsequent dissociation. The latter nonadiabatic processes involve charge-transfer on the potential energy surfaces, and the charge-transfer probabilities are determined from experimental results. The dissociation energy of the ground state to the lowest dissociation limit is experimentally refined as D₀ = 1.05 ± 0.05 eV. This work provides important information to understand the charge-transfer dynamics in the photochemistry of [O₂-H₂O]⁺ and in the ion-molecule reaction O₂ + H₂O⁺ → O₂⁺ + H₂O.