Photodissociation Dynamics of the [O 2 -H 2 O] + Ionic Complex.

We present an experimental study on the photodissociation dynamics of [O 2 -H 2 O] + 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 [ O 2 - H 2 O ] + ions for photodissociation. By detecting both the O 2 + and H 2 O + photofragments using the time-of-flight mass spectrometry and velocity map imaging techniques, branching ratios and total kinetic energy release distributions of the O 2 + + H 2 O and H 2 O + + O 2 product channels are experimentally measured at 16 different excitation energies. State-resolved photodissociation mechanisms of the parent [O 2 -H 2 O] + are interpreted as (1) the O 2 (X 3 Σ g - ) + H 2 O + ( X ~ 2 B 1 ) , O 2 (a 1 Δ g ) + H 2 O + ( X ~ 2 B 1 ) , and O 2 (X 3 Σ g - ) + H 2 O + ( A ~ 2 A 1 ) channels are produced from direct dissociation of [O 2 -H 2 O] + in its excited B ~ 2 A ″ , D ~ 2 A ″ , and F ~ 2 A ″ states, respectively; (2) the O 2 + (X 2 Π g ) + H 2 O ( X ~ A 1 1 ) channel is produced from nonadiabatic relaxations of the excited B ~ 2 A ″ , D ~ 2 A ″ , and F ~ 2 A ″ states to the X ~ 2 A ″ 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 D 0 = 1.05 ± 0.05 eV. This work provides important information to understand the charge-transfer dynamics in the photochemistry of [O 2 -H 2 O] + and in the ion-molecule reaction O 2 + H 2 O + → O 2 + + H 2 O.