Adsorbate-induced electronic modification of alkali-metal overlayers

For submonolayer coverages of Cs on Ru(0001) surfaces, the Cs-Ru vibration is observed at 7.8 meV (63 ${\mathrm{cm}}^{\mathrm{-}1}$) by means of high-resolution electron-energy-loss spectroscopy. At Cs coverages near the complete monolayer the adlayer becomes metallic as indicated by screening of the Cs-Ru vibration and the occurrence of an electronic excitation (plasmon) at 580 meV. Coadsorption of CO as well as of oxygen leads to the reappearance of the Cs-Ru vibration and the disappearance of the electronic excitation which is interpreted as a demetallization through chemical interaction with the coadsorbed species.