CS AND CO COADSORBED ON RU(0001) - LOW-ENERGY-ELECTRON DIFFRACTION ANALYSIS

The adsorption of CO on a precovered Cs(2 X 2)-Ru(0001) structure (theta(Cs) = 0.25) leads to the formation of two distinct (2 X 2)-Cs-CO phases with one and two CO molecules in the unit cell, respectively. The LEED structure analyses showed that Cs remains in the on-top position while CO resides in the hcp position for CO coverage below 0.25 and subsequently fills up the fee sites. Compared to the clean CO-(root 3 X root 3)R30 degrees surface, the CO molecule switches its adsite from being on-top to threefold hollow under the influence of coadsorbed Cs. The CO axis is oriented normal to the surface; no tilting has been found. In addition to the short-range interaction, the electrostatic (long-range) attraction between Cs and CO is manifested by the island growth observed for Cs coverages below 0.25 down to 0.08.