Electronic structure of clean and oxygen-exposed Na and Cs monolayers on Cu (111)

The electronic structure of adsorbed Na and Cs monolayers and these alkalies coadsorbed with oxygen is studied via electron-energy-loss spectroscopy (EELS), work-function changes, Auger emission, and ultraviolet photoelectron spectroscopy. Interest is focused on the changes of the electronic structure due to a change of alkali coverage or to oxygen adsorption. Even at low coverage, when work-function values and core-level binding energies suggest that most of the alkali valence charge is transferred to the substrate, the EEL spectra show a peak due to alkali valence electron excitations. During continued alkali deposition the energy loss after an initial decrease passes a minimum and increases at high monolayer coverage. At high coverage the peak is ascribed to plasma oscillations in the monolayer. For Cs the monolayer plasmon energy loss can be varied in a continuous manner and over a wide range by changing the amount of adsorbed oxygen. We ascribe the energy-loss change to a dilution of the electron gas in the layer. The effect is observed for coverages above a critical value corresponding to around 60% of a close-packed monolayer.