Negative ion formation from water molecule following absorption of vacuum ultraviolet light

Negative ion formations through ion-pair photodissociation or dissociative photoionization of H2O and D2O have been investigated by using an imaging technique in conjunction with the pulsed synchrotron light in the photon energy range between 17 and 41 eV. Cross sections for the formations of H-/D-, O-, and OH-/OD- were measured as a function of the photon energy. The large isotope effect observed in the previous mass spectroscopic study [Hunniford et al., J. Phys. B: At., Mol. Opt. Phys. 40, 1225 (2007)] were only partially reproduced in the present study. The kinetic energies of O- and H-/D- vary with increasing photon energy, which have been analyzed based on the kinematics. Through the analysis, the mechanisms of the two-body or three-body ion-pair photodissociations have been revealed. It has also been found that the axial recoil approximation breaks in the three-body ion-pair dissociation, forming O-.