Local electrostatic potential determination of Cs/Cu(111) surfaces by negative ion spectroscopy

The local electrostatic potential change of Cs-covered Cu(111) as a function of Cs coverage has been studied with negative ion spectroscopy produced by low-energy Li+ ion impact. The results indicate that at low coverages, the valence electron of Cs is transferred to the substrate, while a considerable occupancy of the alkali slate occurs at a coverage greater than or equal to 0.35 ML. With higher coverages, the surface shows a laterally uniform electrostatic potential. The results demonstrate that site-specific negative ion spectroscopy can provide direct information on atomic-scale changes in the surface electrostatic potential.