Electronic-Structure Modification and the Adsorption Site of Oxygen on Alkali (Cs) Covered Surfaces Studied by Normal ExitLi−Ion Spectroscopy

Electronic-structure change of Ni(111) surfaces with low and high Cs precoverages upon exposure to oxygen has been studied by normal exit ${\mathrm{Li}}^{-}$ ion spectroscopy. It is shown that for low Cs coverages the ${\mathrm{Li}}^{-}$ yield from Ni sites decreases more rapidly than from Cs; while for Cs coverages near 1 monolayer (ML), both the yields show a maximum at low O exposures, which is explained as the extension of alkali valence-electron wave functions further towards the vacuum. The results also provide the first direct evidence that oxygen atoms lie above the Cs sites for Cs coverages near 1 ML and low O exposures.