Intramolecular water-splitting reaction in single collisions of water ions with surfaces.

We report the direct formation of H2 and O pair ions through single collisions of water ions with metal surfaces at hyperthermal energies. This unusual intramolecular reaction proceeds also for heavy and semi-heavy water, producing molecular D2 and HD ions. The selectivity of this water splitting channel is estimated at being between 9 and 13% versus complete dissociation. The collision kinematics support the hypothesis of a water molecule colliding with a single surface atom, thereby forming an excited precursor (Rydberg?) state, which dissociates subsequently to form the molecular hydrogen ion with high kinetic energy. Inelastic energy loss considerations yield an estimate for the energy of the excited precursor state of ∼7 eV and ∼11 eV at low and high incidence energies. These energies are close to the à state (1B1, 7.5 eV) and B state (1A1, 9.7 eV) of excited water (Rydberg states).