Evidence for local interaction of Cs and CO on Ru(0001)

Ultra-violet photoemission spectroscopy measurements for CO adsorption on t he (2 x 2)-Cs precovered Ru (0001) surface were performed. The Cs 5p (5p(3/ 2)) level (Cs state) locates at 11.3eV in binding energy for the (2 x 2)-Cs overlayer on nu (0001). Upon the CO adsorption, a new peak, which is assoc iated with the Cs 5p state for the Cs adatoms influenced by the neighbourin g CO adsorbed (the Cs + CO state), appears at the lower binding energy side of the Cs state. This new peak is at 10.8 eV for the CO saturation where t he (2 x 2)-Cs+2CO compound layer is formed. This result gives clear evidenc e for local interaction of Cs and CO on the Ru (0001) surface.