A Direct ab-Initio Trajectory Study on the Ionization Dynamics of the Water Dimer

Ionization processes of a water dimer have been investigated by means of full dimensional direct ab-initio trajectory method. The structure of (H 2 O) 2 before the ionization was simulated at 10 K up to 2 ps by means of a direct ab-initio molecular dynamics (MD) method. Sixty geometrical configurations were selected from the MD calculation, and then the dynamics of (H 2 O) 2 + were calculated by means of a direct ab-initio trajectory method under a constant energy condition. The trajectories on two electronic state potential energy surfaces of (H 2 O) 2 +, i.e., the 2 A‘ ‘ and 2 A‘ states, were full-dimensionally calculated under the C s symmetry. For the 2 A‘ ‘ state, a complex composed of H 3 O + and OH, which is expressed by (H 3 O + )OH, was only formed as a long-lived complex. On the other hand, in the ionization to the 2 A‘ state, two reaction channels, the complex formation (H 3 O + )OH ( 2 A‘) and direct dissociation (H 3 O + + OH) were open as product channels. The dynamics calculations indicated that the proton of H 2 O + transferred to H 2 O within 100−150 fs for the 2 A‘ ‘ state, whereas the proton transfer at the 2 A‘ state occurs faster (30−50 fs) than that of the 2 A‘ ‘ state. The mechanism of the ionization dynamics of (H 2 O) 2 was discussed on the basis of theoretical results.