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++ O2product channels are experimentally measured at 16 different excitation energies. State-resolved photodissociation mechanisms of the parent [O2–H2O]+are interpreted as (1) the O2(X3Σg–) + H2O+(X~2B1), O2(a1Δg) + H2O+(X~2B1), and O2(X3Σg–) + 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+(X2Πg) + 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.