Signatures of Quantum-Tunneling Diffusion of Hydrogen Atoms on Water Ice at 10 K

Abstract

Reported here is the first observation of the tunneling surface diffusion of a hydrogen (H) atom on water ice. Photostimulated desorption and resonance-enhanced multiphoton ionization methods were used to determine the diffusion rates at 10 K on amorphous solid water and polycrystalline ice. H-atom diffusion on polycrystalline ice was 2 orders of magnitude faster than that of deuterium atoms, indicating the occurrence of tunneling diffusion. Whether diffusion is by tunneling or thermal hopping also depends on the diffusion length of the atoms and the morphology of the surface. Our findings contribute to a better understanding of elementary physicochemical processes of hydrogen on cosmic ice dust.

Grain Surface Models and Data for Astrochemistry

“…This eliminates the effect of warm-up which can be an issue for TPD experiments. These experiments support the following picture (Watanabe et al 2010;Kuwahata et al 2015): H atoms land on the ASW surface, diffuse rapidly using the abundant shallow and middle sites, and are finally trapped in the deepest sites. Once a significant number of H atoms become trapped, the subsequent H atoms recombine with the trapped H atoms.…”

“…After experimental studies showed that the diffusion of H atoms is rather slow, thermal hopping became favored in models (Pirronello et al 1997bKatz et al 1999). More recently, the discussion on the nature of the diffusion mechanism for atoms was reopened, with experiments suggesting H atoms can quantum tunnel under particular conditions (Watanabe et al 2010;Kuwahata et al 2015). It has also been postulated that atoms as heavy as oxygen can diffuse via quantum tunneling .…”

H 2 formation on interstellar dust grains: The viewpoints of theory, experiments, models and observations

“…Recently, using a combination of laser-induced photodesorption and resonance enhanced multi-photon ionization (REMPI) methods, Watanabe and coworkers clarified how the diffusion mechanisms, either tunneling or thermal hopping, depend on the diffusion distance on ices (Kuwahata et al, 2015), and measured the activation barriers for thermal hopping of H and D atoms on ASW (Watanabe et al, 2010;Hama et al, 2012). Watanabe and coworkers demonstrated that the ASW surface contains various sites that can be categorized into at least three groups: very shallow-, middle-, and deep-potential sites, with associated diffusion activation energies of <18, 22 (23 meV for D atoms), and >30 meV, respectively.…”

Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon Surface: H2 Formation and Desorption

“…reported evidence of the quantum-tunneling diffusion of H-atoms on polycrystalline water ice (Kuwahata et al 2015), where a significant isotope effect is found in contrast to the small isotope effect in the diffusion of H-and D-atoms on ASW (Hama et al 2012). According to theoretical studies, H-atom diffusion on ASW is suppressed due to its nonperiodic potential (Smoluchowski 1979(Smoluchowski , 1981(Smoluchowski , 1983.…”

“…In other words, quantum-tunneling diffusion is thought to dominate within a single crystal, whose potential is relatively shallow and regular. Further evidence for quantumtunneling diffusion can be provided by performing the experiments and analyses developed by Kuwahata et al (Kuwahata et al 2015). In their method, the ratio of the surface number densities of H-atoms and D-atoms on the surface during atomic deposition, n(D) / n(H), is a measure of the difference in diffusion rate constants.…”