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 H₂O molecules on the surface are typically exposed to cosmic rays, H₂O 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 (T₁) surface, following the vertical electronic excitation from the ground state (S₀), 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, (H₂O) (n = 2-6), the H atom was directly dissociated from one of the H₂O molecules in the clusters (direct dissociation), whereas the OH radical remained in the cluster. In the branched form of H₂O tetramer (n = 4) and the book form of H₂O hexamer (n = 6), the dissociated hydrogen atom (H') collided with the neighboring H₂O molecule, and the exchange of H atoms occurred as H' + H₂O → H'-H₂O (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.