Measurements were made of the diffusivity of H and D, on the (111) surface, at temperatures ranging from 108 to 154K. The results for D could be described by the expression:

D (cm2/s) = 1.6 x 10-5exp-3.8(kcal/mol)/RT]

while those for H could be described by:

D (cm2/s) = 1.7 x 10-5exp-3.4(kcal/mol)/RT]

In the case of both isotopes, non-activated tunnelling was observed following thermally activated hopping at about 125K. The non-activated tunnelling rates revealed a weak isotope dependence. The non-activated tunnelling was analyzed in terms of a modified polaron model which included adatom-phonon couplings which varied quadratically in the phonon coordinates, and in terms of a conduction electron model. The weak isotope dependence was attributed to a large WKB tunnelling matrix element on Ni(111).

Observation of Diffusion of H and D on Ni(111) from Over-Barrier Hopping to Nonactivated Tunnelling. A.Lee, X.D.Zhu, A.Wong, L.Deng, U.Linke: Physical Review B, 1993, 48[15], 11256-9