Observation of a transition from over-barrier hopping to activated tunneling diffusion: H and D on Ni(100)

Using a linear optical diffraction technique, we measured the surface diffusion coefficients for hydrogen (H) and deuterium (D) on Ni(100) at coverage θ=0.7. From 200 K down to 120 K, the diffusion for both H and D evolves from a high-temperature activated behavior to a low-temperature activated behavior. For hydrogen, ${\mathit{D}}_0$=1.1×${10}^{\mathrm{-}6}$ ${\mathrm{cm}}^2$/sec, ${\mathit{E}}_{\mathrm{diff}}$=3.5 kcal/mol from 200 to 160 K and ${\mathit{D}}_0$=1.5×${10}^{\mathrm{-}9}$ ${\mathrm{cm}}^2$/sec, ${\mathit{E}}_{\mathrm{diff}}$=1.2 kcal/mol from 160 to 120 K. For deuterium, ${\mathit{D}}_0$=5×${10}^{\mathrm{-}5}$ ${\mathrm{cm}}^2$/sec, ${\mathit{E}}_{\mathrm{diff}}$=5.0 kcal/mol from 200 to 170 K and ${\mathit{D}}_0$=9×${10}^{\mathrm{-}10}$ ${\mathrm{cm}}^2$/sec, ${\mathit{E}}_{\mathrm{diff}}$=1.05 kcal/mol from 170 to 120 K. Our preliminary analysis based upon small-polaron theories and available surface-phonon data suggests that the observed transition is most likely to be from an over-barrier hopping to an activated tunneling diffusion.