Photodissociation Dynamics of the [O-HO] Ionic Complex.

We present an experimental study on the photodissociation dynamics of [O-HO] 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 O and HO photofragments using the time-of-flight mass spectrometry and velocity map imaging techniques, branching ratios and total kinetic energy release distributions of the O + HO and HO + O product channels are experimentally measured at 16 different excitation energies. State-resolved photodissociation mechanisms of the parent [O-HO] are interpreted as (1) the O(XΣ) + HO, O(aΔ) + HO, and O(XΣ) + HO channels are produced from direct dissociation of [O-HO] in its excited , , and states, respectively; (2) the O(XΠ) + 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 = 1.05 ± 0.05 eV. This work provides important information to understand the charge-transfer dynamics in the photochemistry of [O-HO] and in the ion-molecule reaction O + HO → O + HO.