Measurements were made of the diffusion rate of D on (111), as a function of temperature, for atomic coverages of 0.3 or 0.05. In both cases, a change from classical over-barrier hopping to under-barrier tunnelling occurred at about 125K. At a temperature of 110K, the tunnelling diffusion rates of D and H at coverages ranging from 0.05 to 0.45 were determined. It was found that the tunnelling diffusivity, as a function of coverage, varied by less than a factor of 3. The overall features of the tunnelling diffusivity for the 2 isotopes were similar. The diffusivity peaked weakly at coverages of about 0.25 and 0.5.

A.Wong, A.Lee, X.D.Zhu: Physical Review B, 1995, 51[7], 4418-25