Single vacancies within the dense 3O(2 x 2)/Ru(001) layer were found to exhibit a highly directional diffusion which led to self-trapping of this quasi-particle to within 3 neighboring sites. Whereas a high mobility was found within these few sites, vacancy diffusion across the surface was very slow, with a ratio of about 1000. This was expected to produce disagreements between measurements of adsorbate mobility by quasi-elastic scattering methods, and by real-space imaging (local-concentration based) techniques. The non-zero motion of the entire triangular unit proceeded in the usual stochastic manner and demonstrated that intrinsic structural defects of the substrate lattice were not involved in the self-trapping process. It was noted that mobility within one of the 2 inequivalent triangular sub-units of the (2 x 2) unit cell was strongly enhanced. It was demonstrated that an observed motional constraint was due to the local symmetry of the O adsorption sites. This implied a directional dependence of the diffusion barriers for vacancy motion within the (2 x 2) unit cell.

Directional Preference in Particle Motion: Self-Trapping of Vacancies in an Ordered Adsorbate Layer. P.Jakob, M.Gsell, D.Menzel: Physical Review B, 1999, 59[20], 13285-90