Full dimensional ab initio direct dynamics calculations of the ionization of H-2 clusters (H-2)(n) (n=3, 4 and 6)

The dyamics of (H-2(+))(H-2)(n-1) clusters following the ionization of (H-2 )(n) (n = 3, 4 and 6), which plays an important role in the initial process es of electron-beam irradiation of solid hydrogen, have been investigated b y means of ab initio direct dynamics calculations. The full dimensional pot ential energy surface (PES) calculated at the UHF/311G(p) level was used th roughout. One of the hydrogen molecules in the cluster was vertically ioniz ed at time zero. The calculations showed that the H-2 molecules gradually a pproach the ionized hydrogen molecule H-2(+) in the initial stage of the re action and then one of the hydrogen molecules collides with H-2(+) (the app roach time is about 100 fs). The hydrogen atom or proton transfer reaction occurs rapidly by collision of H-2(+) with H-2, according to the reaction H -2(+)+H-2 --> H-3(+) + H (the reaction time is about 10 fs). The hydrogen a tom leaves the cluster with large translational energy (7-26 kcal mol(-1), depending on the cluster size). The product ion H-3(+) is vibrationally and rotationally hot. It was also found in some cases that H-3(+) is solvated by neighboring H-2 molecules to form the ion-molecule cluster (H-3(+))(H-2) (m) (m = 1-3). Similar dynamics features were obtained for each cluster (n = 3, 4 and 6). The mechanism of the reaction is discussed on the basis of t he theoretical results. Note that the present study is the first attempt to elucidate the ionization dynamics of hydrogen clusters.