Evidence for Local Interaction of Cs and CO on Ru(0001)

Ultra-violet photoemission spectroscopy measurements for CO
adsorption on the (2×2)-Cs precovered Ru (0001) surface were performed. The Cs 5p(5p_3/2) level (Cs state) locates at 11.3 eV in binding energy for the (2×2)-Cs overlayer on Ru (0001). Upon the CO adsorption, a new peak, which is associated with the Cs 5p state for the Cs adatoms influenced by the neighbouring 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 the (2×2)-Cs+2CO compound layer is formed. This result gives clear evidence for local interaction of Cs and CO on the Ru (0001) surface.