Ionization dynamics of the small-sized water clusters: A direct ab initio trajectory study

The ionization dynamics of the water clusters (H2O)(n) (n = 3-6) have been investigated by means of the full-dimensional direct ab initio trajectory method. The static ab initio and DFT calculations were carried out at the HF/6-311G(d,p) and B3LYP/6-311G(d,p) levels, whereas the direct ab initio trajectory calculations were performed at the HF/6-311G(d) and 6-311G(d,p) levels of theory. The static ab initio and DFT calculations showed that the most stable structure is the cyclic form for all cases (n = 3-6). In the ionization of the water trimer, the complex (H3O+OH)H2O was obtained as a product (complex formation channel). In the larger clusters (n = 4-6), the OH dissociation was only found after the ionization of (H2O)n (OH dissociation channel). The OH dissociation occurs via two-step processes: the first step is a proton-transfer process from H2O+ to H2O along the hydrogen bond in the cluster, and then the (H3O+OH) complex is formed as a core in the cluster, expressed by (H2O+)-H2O-H2O --> (OH)(H3O+)-H2O. The next step is the second proton-transfer process from H3O+OH to the neighboring water molecule, which is expressed by (OH)-H3O+-H2O --> (OH)-H2O-H3O+. It was found that the OH dissociation takes place immediately after the second proton transfer. The lifetimes of the intermediate complexes are distributed in the range 50-200 fs for n = 4-6. The reaction mechanism was discussed on the basis of theoretical results.