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. © 1992 The American Physical Society.