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

Abstract

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 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(X3Σg-) + H2O+, O2(a1Δg) + H2O+, and O2(X3Σg-) + H2O+ channels are produced from direct dissociation of [O2-H2O]+ in its excited , , and states, respectively; (2) the O2+(X2Πg) + channel is produced from nonadiabatic relaxations of the excited , , and states to the 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.