ADSORBATE-INDUCED ELECTRONIC MODIFICATION OF ALKALI-METAL OVERLAYERS

For submonolayer coverages of Cs on Ru(0001) surfaces, the Cs-Ru vibra tion is observed at 7.8 meV (63 cm-1) by means of high-resolution elec tron-energy-loss spectroscopy. At Cs coverages near the complete monol ayer 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 reapp earance 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.