Velocity map imaging studies of the photodissociation of H2O+ cations

The near-threshold photofragmentation dynamics of state-selected H2O+ cations have been investigated using velocity map ion imaging methods. The cations were prepared in the v = 0 level of the ground ((X) over tilde B-2(1)) electronic state, by 2 + 1 resonance enhanced multiphoton ionization via selected rotational levels of the H2O, (C) over tilde B-1(1), v = 0 state. Subsequent two photon excitation of the resulting H2O+ cations to (B) over tilde B-2(2) state levels lying above the lowest dissociation limit (i.e. at total energies in the range 46000-50600 cm(-1)) results in O-H bond fission and OH+ fragment ion formation. These fragments display isotropic recoil velocity distributions, which peak at low kinetic energy but extend to the highest speeds allowed by energy conservation. Ab initio calculations of key sections through the potential energy surfaces (PESs) for the ground and first few excited states of H2O+ suggest two possible mechanisms for the observed rotational and (when energetically allowed) vibrational excitation of the OH+ fragments. Both require initial non-adiabatic (vibronic coupling) from the photo-prepared (B) over tilde state level to high levels of the (A) over tilde (2)A(1) state, but involve different subsequent HO+-H bond fission mechanisms. One involves Renner-Teller coupling to the ground state PES, while the alternative requires spin-orbit induced coupling to the repulsive (a) over tilde B-4(1)((4)A '') state PES.