Identification of thes-derived Valence-Electron Level in Photoemission from Alkali-Metal Adlayers on Aluminum

Photoemission spectra from potassium adsorbed on an Al{111} surface show a K $4s$-derived state just below the Fermi level, $E_F$, which is first observed just prior to the onset of ordering in the overlayer. The feature disperses upwards with increasing ${\mathbf{k}}_{\parallel }$ and crosses $E_F$ before reaching the Brillouin-zone boundary. Its coverage dependence is indicative of a transition from strongly ionic to metallic bonding.