Ionization Dynamics of the Small-Sized Water Clusters: A Direct Ab Initio Trajectory Study

The ionization dynamics of the water clusters (H 2 O) 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/6-31G(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 (H 3 O + OH)H 2 O 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 (H 2 O) n (OH dissociation channel). The OH dissociation occurs via two-step processes: the first step is a proton-transfer process from H 2 O + to H 2 O along the hydrogen bond in the cluster, and then the (H 3 O + OH) complex is formed as a core in the cluster, expressed by (H 2 O + )−H 2 O−H 2 O → (OH)(H 3 O + )−H 2 O. The next step is the second proton-transfer process from H 3 O + OH to the neighboring water molecule, which is expressed by (OH)−H 3 O + −H 2 O → (OH)−H 2 O−H 3 O + . 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.