ADSORBATE-INDUCED ELECTRONIC MODIFICATION OF CS OVERLAYERS ON RU(0001)

Submonolayer amounts of Cs on Ru(0001) dosed with H-2, O-2, CO, NO and H2O were studied by means of high-resolution electron energy loss spe ctroscopy (HREELS). The energy of the Cs-Ru vibration varies with the Cs coverage theta(Cs) between 6.5 meV for theta(Cs)-->0 and 8.6 meV fo r theta(Cs) = 0.17. For theta(Cs) > 0.17 it drops again. At theta(Cs) = 0.25 the adlayer becomes metallic as indicated by screening of the C s-Ru vibration and the occurrence of an electronic excitation. Coadsor ption of O, CO, and OH with Cs at theta(Cs) = 0.25 leads to the reappe arance of the Cs-Ru vibration and the disappearance of the electronic excitation. This is interpreted as a demetallization through chemical interaction with the coadsorbed species. H is coadsorbed without any n oticeable influence on the Cs overlayer.