Ultrafast molecular dissociation induced by intermolecular Coulombic decay in water clusters

We report a combined experimental and theoretical study on the fragmentation dynamics following the ultrafast intermolecular Coulombic decay (ICD) in (H2O)6 water clusters upon electron impact ionization. By coincident fragment ions and electron momentum spectroscopy, we show that ICD can be initiated by inner-valence ionization of a water molecule. Our ab initio molecular dynamics simulations show that ICD is followed by proton transfer leading to the formation of the (H2O)3 center dot H+ center dot center dot center dot OH center dot (H2O)2+ ion-radical complex. We propose a possible dissociative mechanism in which the system further dissociates into a pair of radical ions (H3O+/H5O2+) and neutral species of water and hydrogen peroxide (H2O2). The calculated kinetic energy spectrum of a H3O+/H5O2+ ion pair is in good agreement with experiment. Moreover, the present study of ICD in water provides an underlying production mechanism for the reactive oxygen species of H2O2 which is not considered previously in the radiolysis processes of water.