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, D₀=1.1×{}10^-6 cm²/sec, Ediff=3.5 kcal/mol from 200 to 160 K and D₀=1.5×{}10^-9 cm²/sec, Ediff=1.2 kcal/mol from 160 to 120 K. For deuterium, D₀=5×{}10^-5 cm²/sec, Ediff=5.0 kcal/mol from 200 to 170 K and D₀=9×{}10^-10 cm²/sec, Ediff=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.