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Abstract

The interaction of Cs and CO was studied for submonolayer coverages of Cs on Ru(0001) by means of high-resolution electron energy-loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and low-energy electron diffraction (LEED). From the HREEL spectra and the 2x2 LEED pattern it is concluded that two-dimensional islands of Cs+2CO stoichiometry are formed first. Forhigh CO doses CO adsorbs on the bare Ru surface between the islands as concluded from then v(C–O) stretch-mode frequency and the(∛x∛)R30° LEED pattern. Besides some localinteraction between CO and Cs, there is strong evidence found for a long range, through substrateinteraction. Our data can be understood in terms of a charge redistribution model; Cs donates agiven amount of charge to the surface which is then backdonated to and shared between theadsorbed CO molecules within the (Cs+2CO)-islands and outside of them on the bare surface. The desorption temperature of CO with Ø_CO→0 increases nearly linearly with Ø_Cs indicating CO desorption out of a two dimensional Cs gas in which the lateral interaction increases with Ø_Cs.