Discrete Valence-Electron States in Thin Metal Overlayers on a Metal

We have measured photoelectron energy spectra from thin Na and Ba overlayers on Cu(111). The spectra show extremely narrow adsorbate-induced peaks, much narrower than observed for any other adsorbate system. These features arise from electrons trapped in the potential well between the vacuum barrier and the Cu(111) surface which has a high electron reflectivity for energies within the band gap producing the necks of the Cu Fermi surface.