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, 2A′′ and 2A′) 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 2A′′ state, a proton transfer complex H3O+(OH) is only formed as a long-lived complex. On the other hand, in the ionization to the 2A′ 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 2A′′ 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.
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