Full dimensional ab initio direct dynamics calculations of the ionization of H2 clusters (H2)n (n = 3, 4 and 6)

Abstract

The dyamics of (H₂⁺)(H₂)_n−1 clusters following the ionization of (H₂)_n (n = 3, 4 and 6), which plays an important role in the initial processes of electron-beam irradiation of solid hydrogen, have been investigated by means of ab initio direct dynamics calculations. The full dimensional potential energy surface (PES) calculated at the UHF/311G(p) level was used throughout. One of the hydrogen molecules in the cluster was vertically ionized at time zero. The calculations showed that the H₂ molecules gradually approach the ionized hydrogen molecule H₂⁺ in the initial stage of the reaction and then one of the hydrogen molecules collides with H₂⁺ (the approach time is about 100 fs). The hydrogen atom or proton transfer reaction occurs rapidly by collision of H₂⁺ with H₂, according to the reaction H₂⁺ + H₂ → H₃⁺ + H (the reaction time is about 10 fs). The hydrogen atom leaves the cluster with large translational energy (7–26 kcal mol⁻¹, depending on the cluster size). The product ion H₃⁺ is vibrationally and rotationally hot. It was also found in some cases that H₃⁺ is solvated by neighboring H₂ molecules to form the ion–molecule cluster (H₃⁺)(H₂)_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 the theoretical results. Note that the present study is the first attempt to elucidate the ionization dynamics of hydrogen clusters.