Ionization dynamics of a water dimer: specific reaction selectivity

Abstract

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.