STRUCTURAL-PROPERTIES OF ALKALI-METAL ATOMS ADSORBED ON RU(0001)

The structural properties of the ordered overlayers of Li, Na, K, Rb, and Cs on Ru(0001) are summarized. The major result is that the adsorption site depends on the coverage while the hard-sphere radii of the alkali-metal atoms do not change (if corrected for different numbers of coordination). This comparison also emphasizes the singular behavior of Cs for which adsorption takes place over single Ru atoms at a Cs coverage of 0.25. While on other close-packed substrate surfaces potassium and rubidium occupy ontop positions at low temperatures, this has not been found with Ru(0001). This finding points towards the important role of the substrate. For the ontop adsorption to be favored, an inward displacement of the substrate atoms directly underneath the alkali-metal atoms by a substantial amount is necessary which results in the formation of a quasisevenfold-coordinated bond geometry in connection with a reduction of the dipole-dipole repulsion. The stiffness of the substrate determines the energy cost for this local reconstruction, and consequently ontop adsorption on the hard Ru(0001) substrate has only been observed for the biggest alkali metal Cs where the energy difference between various adsorption sites [on the unrelaxed Ru(0001) surface] is assumed to be small. In order to force potassium to reside in the ontop position, the Ru(0001) surface has to be ''softened'' which task was accomplished by adding CO molecules to the K-(2 x 2) overlayer.