The diffusion of K on Pd(111) was studied by means of photo-electron emission microscopy. It was shown that the method could be used to investigate diffusion at full monolayer coverage by correlating brightness with coverage in the photo-electron emission microscopic images. In the limit of zero concentration, fast diffusion was already observed at 100K. The diffusivity was equal to 8.5 x 10-9cm2/s at 135K. It was found that, in addition to a very rapid diffusion stage at the lowest concentration of less than 0.05 of a monolayer, there was a local maximum in the diffusion coefficient at 0.5 of a monolayer. Because a 2x2 over-layer was observed at this coverage, it was suggested that a phase transition was responsible for the effect. The K atoms migrated mainly across well-ordered terraces confined by the defect structures of the surface. Even in the case of diffusion on top of a well-ordered 2x2 layer of K, the defect structures of the underlying substrate were apparent in photo-electron emission microscopic images.

M.Ondrejcek, V.Cháb, W.Stenzel, M.Snábl, H.Conrad, A.M.Bradshaw: Surface Science, 1995, 331-333, 764-70