The mechanism of surface diffusion of h and datoms on amorphous solid water: Existence of various potential sites

To understand elementary processes leading to H₂ formation, and the hydrogenation and deuteration reactions of adsorbed species on dust grains in dense clouds, we experimentally investigated the diffusion of atomic hydrogen and deuterium on amorphous solid water (ASW) at temperatures of 8-15K. The present study extended our previous study for selective detections of H and Datoms, and of H₂ (J = 0 and 1) and D₂ (J = 0 and 1) molecules adsorbed on ASW using both photo-stimulated desorption and resonance-enhanced multiphoton ionization, to investigate potential sites on ASW for diffusion, recombination dynamics, and the diffusion mechanism of H and Datoms. Our results demonstrate that the ASW surface contains various potential sites that can be categorized into at least three groups: very shallow, middle-, and deep-potential sites, with diffusion activation energies of ≤18, 22 (23meV for Datoms), and ≥30meV, respectively. The present study pictured the outline of H₂ formation on cosmic ice dust at low temperatures: Hatoms landing on the dust will diffuse rapidly at the abundant shallow and middle sites on ASW, and finally become trapped at deep sites. The Hatoms that arrive next recombine with such trapped Hatoms to yield H₂ molecules. The small isotopic difference between the diffusion of H and Datoms on ASW indicates that the diffusion mechanism can be explained by thermal hopping, at least at middle-potential sites.