Observation of diffusion of H and D on Ni(111) from over-barrier hopping to nonactivated tunneling

We measured the diffusion coefficients for hydrogen (H) and deuterium (D) on Ni(111) from 108 to 154 K. We observed nonactivated tunneling following thermally activated hopping at around 125 K for both isotopes. The nonactivated tunneling rates show weak isotope dependence. Our observation confirms an early report by Lin and Gomer [Surf. Sci. 225, 41 (1991)]. The nonactivated tunneling is analyzed with both a modified polaron model, which includes adatom-phonon couplings varying quadratically in phonon coordinates, and the conduction electron model. The weak isotope dependence is understood as a result of a large WKB tunneling matrix element on Ni(111).