Structural analyses of ordered rubidium phases on Ru(0001) using low-energy electron diffraction

The adsorption geometries of the p(2\ifmmode\times\else\texttimes\fi{}2) and (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} Rb/Ru(0001) phases at coverages of 0.25 and 0.33, respectively, were investigated using low-energy electron diffraction. The structure analyses revealed that Rb resides in the threefold fcc site in the p(2\ifmmode\times\else\texttimes\fi{}2) phase while for the (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} phase the hcp site is favored. The effective radius of Rb does not change with the Rb coverage within the error bars of this analysis. The concept of ``split positions'' has been used in order to take the enhanced motion of the adsorbate atoms parallel to the surface properly into account.