Structural analyses of Cs+CO coadsorbed on Ru(0001)

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Abstract
Adsorption of CO onto a Ru(0001) surface precovered with Cs (coverage 0.25) leads to the formation of two distinct (2×2)-Cs-CO phases with one and two CO molecules in the unit cell. Structural analysis by means of low-energy electron diffraction reveals that the Cs atoms remain in on-top positions, while the CO molecules are located in threefold sites, contrary to the occupation of on-top sites if adsorbed alone. This effect of site switching is attributed to the enhanced electron density of the Ru substrate leading to more pronounced back donation in bonding the CO molecules.