The migration of 1H and 2H upon the (023) surface of a field emitter tip was studied by using the fluctuation method. The surface in question consisted of (110)-oriented terraces which were separated by (100)-oriented steps. It was found that activated and tunnelling diffusion occurred parallel to, and perpendicular to, the steps. There was very little anisotropy. The activation energies and pre-exponential factors for diffusion on the present plane were very different to those for (110), and it was suggested that the present values largely reflected diffusion across steps. The activation energies for 1H were higher, than those for 2H, by about 2kcal. The pre-exponential factors for 1H were also much higher than those for 2H. It was concluded that the results were due to activated polaronic diffusion which was largely controlled by substrate deformation. The energy differences were attributed to zero-point energy differences between the 2 isotopes during lattice distortion. Under half-coverage conditions, a changeover from an initially activated process, to a slower one, was observed at temperatures of between 151 and 166K. This was attributed to the effect of a H-assisted reconstructive phase transition which involved non close-packed (100) oriented steps. It was noted that tunnelling diffusion was associated with a very small isotope effect, and it was proposed that polaronic contributions counteracted the isotope effect for pure H tunnelling.

C.Uebing, R.Gomer: Surface Science, 1991, 259[1-2], 151-61