Coverage dependence of quantum tunneling diffusion of hydrogen and deuterium on Ni(111)

We measured the diffusion rate of deuterium on Ni(111) as a function of temperature at atomic coverage of {θ}=0.3 and 0.05. In both cases, the crossovers from classical over-barrier hopping to under-barrier tunneling occurred at around T=125 K. At T=110 K, we measured the tunneling diffusion rates Dₜᵤₙₙₑₗ({θ}) for both deuterium and hydrogen in the coverage range from {θ}=0.05 to 0.45 and found that (1) Dₜᵤₙₙₑₗ({θ}) varied by less than a factor of 3; (2) the overall features of Dₜᵤₙₙₑₗ({θ}) for the two isotopes were similar; (3) Dₜᵤₙₙₑₗ({θ}) weakly peaked at around {θ}=0.25 and 0.5. Our results mostly agree with an early report by Lin and Gomer. Based on these results, we discuss the effect of the hydrogen-hydrogen interaction on hydrogen tunneling diffusion on Ni(111).