The dynamic behavior of N atoms which were adsorbed at an (00•1) surface was studied by means of scanning tunnelling microscopy. The N atoms which were formed by the dissociation of NO molecules exhibited an initial sharp concentration profile at atomic steps. Its decay was monitored as a function of time, and provided a quasi-continuum diffusion coefficient. The results could be described by:

D (cm2/s) = 0.02 exp[-0.94(eV)/kT]

The N-N interaction potential was determined independently by analyzing the equilibrium distributions in larger images. It was found that the interaction was well described by a hard sphere potential, which blocked the nearest and next-nearest sites without affecting any other sites. The site blocking that was caused by the hard sphere potentials had only a small effect because of the small coverages which were used. It was shown that the Fickian diffusion coefficient was about 20% larger than the zero-coverage value, and that the hopping frequency of the random walkers in equilibrium was about 30% smaller than in the zero-coverage limit. It was therefore concluded that no measurable differences were found because of the small coverages and the small interaction energy between N atoms.

T.Zambelli, J.Trost, J.Wintterlin, G.Ertl: Physical Review Letters, 1996, 76[5], 795-8