Hydrogen Dissociation Dynamics from Water Clusters on Triplet-State Energy Surfaces.

The ice surface provides two-dimensional reaction fields for bimolecular collisions in interstellar space. As HO molecules on the surface are typically exposed to cosmic rays, HO in the excited state is easily dissociated into H + OH, where a H atom is released from the surface to the gas phase. In the present study, the reaction dynamics of small-sized water clusters on the triplet-state potential energy (T) surface, following the vertical electronic excitation from the ground state (S), were investigated using direct ab initio molecular dynamics to provide insights into the generation mechanism of H atoms from an irradiated ice surface. In all clusters, that is, (HO) ( = 2-6), the H atom was directly dissociated from one of the HO molecules in the clusters (direct dissociation), whereas the OH radical remained in the cluster. In the branched form of HO tetramer ( = 4) and the book form of HO hexamer ( = 6), the dissociated hydrogen atom (H') collided with the neighboring HO molecule, and the exchange of H atoms occurred as H' + HO → H'-HO (collision) → H'OH + H (hydrogen exchange). The translational energy of the exchanged H atom decreases significantly (by approximately 50%) because the kinetic energy of the H' atom is efficiently transferred to the vibrational modes of the cluster during the H-exchange reaction. The mechanism of H atom dissociation is discussed based on theoretical results.