Na adsorption on Ru(0001): a low-energy electron-diffraction analysis of three ordered phases

Sodium adsorbed on the Ru(0001) surface below room temperature forms a number of ordered phases, commensurate as well as incommensurate. They are investigated by low-energy electron-diffraction (LEED) and work-function measurements (Δφ). The structural analyses for the p(2 × 2)-, (√3 × √3)R30°- and (3 × 3)-phases at coverages of 1 4 , 1 3 and 4 9 , respectively, reveal coverage-dependent adsorption sites. As for K adsorbed on Ru(0001), Na atoms occupy threefold fcc-sites in the p(2 × 2) phase while for the (√3 × √3)R30° phase the hcp site is favoured. A structural analysis of the (3 × 3) phase, including 4 adsorbate atoms within the unit cell, revealed that nearest-neighbour distances are maximized at the cost of occupying low symmetry sites. This conclusion is supported by comparison of relative theoretical and experimental adsorbate-induced spot intensities. For all three phases the Na hard-sphere radii are nearly the same and close to the covalent Pauling radius.