Formation Mechanism of Odd- and Even-Numbered Hydrogen Cluster Cations Using the Direct Ab Initio Molecular Dynamics...

The photoreaction of hydrogen clusters caused by cosmic-ray irradiations plays a crucial role in interstellar molecular clouds because the reaction of molecules takes place in these surrounding clusters. Previous gas-phase experiments introduced the reaction products after the ionization of the neutral hydrogen cluster. In the experiments, the odd-numbered hydrogen cluster cation, H⁺(H₂), was the main product, while the even-numbered cluster cation, (H₂)⁺, was the minor product. However, the formation yield of (H₂)⁺ was significantly low. Despite many theoretical calculations and experiments, the formation mechanism of odd- and even-numbered cluster ions from ionized hydrogen clusters is still unclear. In this study, the direct ab initio molecular dynamics (AIMD) method was applied to the reaction of the hydrogen cluster cation (H₂)⁺ to understand the abovementioned formation mechanism. The trajectories of (H₂)⁺ following the vertical ionization of the neutral cluster were calculated. Direct AIMD calculations indicated that odd-numbered cluster cations, H₃⁺(H₂) + H (dissociation), were formed directly as the main product after the ionization of (H₂). An even-numbered cluster, (H₂)⁺, was temporally formed when an H-shaped complex composed of three H₂ molecules existed within a neutral cluster. However, it was rapidly dissociated to an odd-numbered cluster. Static ab initio calculations suggested that the odd-numbered cluster complex, H₃⁺(H₂)-H, could be transferred to even-numbered ions via a transition state with a low energy barrier. The even-numbered cluster cation was formed stepwise from the odd-numbered cluster cation. The formation mechanism of odd- and even-numbered cluster cations is discussed based on these results.