The adsorption of Cs on Ru(001), at coverages which varied from multi-layer to less than 0.001 of a monolayer, was studied by means of Cs 4d core level photo-emission. By using a photon energy which was optimized for adsorbate and substrate cross-sections, a detection sensitivity of 0.001 of a monolayer was obtained. The most important finding was that, as the Cs coverage decreased from a full monolayer (v3 x v3 R30º low-energy electron diffraction) to 0.001 of a monolayer, the Cs 4d level shifted slightly away from the Fermi level (the entire range was 0.25eV) until the Cs atoms on the surface were farther apart than they were in the bulk. This shift was associated with the appearance of a low binding energy shoulder which became a dominant peak at coverages which were below 0.02 of a monolayer. These results, together with low-energy electron diffraction and thermal desorption observations, were considered in the light of recent theoretical and experimental results. In this model, the charge state of alkali atoms on the surface was essentially neutral (metallic), and was insensitive to the coverage. The low binding energy peak which appeared at lower coverages appeared to originate from surface defects on a good Ru(001) monocrystal with a well-defined low-energy electron diffraction pattern.

T.K.Sham, M.L.Shek, J.Hrbek: Surface Science Letters, 1991, 241[1-2], L16-20