COVERAGE DEPENDENCE OF ADSORPTION-SITE GEOMETRY IN THE CS/RU(0001) SYSTEM - A LOW-ENERGY ELECTRON-DIFFRACTION ANALYSIS

The ordered overlayer structures formed by Cs adsorbed on a Ru(0001) surface were analyzed by use of low-energy electron diffraction (LEED). The phase diagram reflects the dominance of dipole-dipole repulsions between the adparticles and comprises quasiliquid configurations characterized by diffraction rings up to a coverage THETA = 0.17, followed by a (2 X 2) structure with maximum intensity of the diffraction spots at THETA = 0.23. Beyond THETA = 0.25, a series of structures with rotated unit cells is identified which are followed by a (square-root 3 X square-root 3)R 30-degrees structure around THETA = 0.33 (almost-equal-to completion of the first monolayer). In the (2 X 2) phase the Cs atoms are located in on-top sites with a Ru-Cs bond length of 3.25 +/- 0.08 angstrom, corresponding to a hard-sphere radius of 1.9 angstrom for the Cs atom. In the (square-root 3 X square-root 3)R 30-degrees structure, on the other hand, the adatoms occupy threefold hollow hcp sites with Ru-Cs bond lengths of 3.52 +/- 0.02 angstrom, corresponding to a Cs hard-sphere radius of about 2.2 angstrom. The increase in bond length and effective radius of the adparticle is paralleled by the transition of the character of bonding from more "ionic" at THETA = 0.25 (large dipole moment) to more "metallic" at THETA = 0.33 (dipole moment reduced by about 30%). The associated change of the type of adsorption site (from on-top to hollow) is qualitatively rationalized by a model according to which inherently less favorable sites may become preferred due to improved effective screening of the dipole-dipole repulsion by the location of substrate atoms in the region between neighboring adatoms.