Ionization dynamics of a water dimer: specific reaction selectivity

Ionization dynamics of a water dimer have been investigated by means of a direct ab initio molecular dynamics (MD) method. Two electronic state potential energy surfaces of (H2O)(2)(+) (ground and first excited states, (2)A '' and (2)A') were examined as cationic states of (H2O)(2)(+). Three intermediate complexes were found as product channels. One is a proton transfer channel where a proton of H2O+ is transferred into the H2O and then a complex composed of H3O+(OH)was formed. The second is a face-to-face complex channel denoted by (H2O-OH2)(+) where the oxygen-oxygen atoms directly bind each other. Both water molecules are equivalent to each other. The third one is a dynamical complex where H2O+ and H2O interact weakly and vibrate largely with a large intermolecular amplitude motion. The dynamics calculations showed that in the ionization to the (2)A '' state, a proton transfer complex H3O+(OH) is only formed as a long-lived complex. On the other hand, in the ionization to the (2)A' state, two complexes, the face-to-face and dynamical complexes, were found as product channels. The proton of H2O+ was transferred to H2O within 25-50 fs at the (2)A '' state, meaning that the proton transfer on the ground state is a very fast process. On the other hand, the decay process on the first excited state is a slow process due to the molecular rotation. The mechanism of the ionization dynamics of (H2O)(2) was discussed on the basis of theoretical results.