PHOTOEMISSION-STUDY OF THE CS/GAP(110) INTERFACE AT LOW-TEMPERATURES

The formation of the Cs/GaP(110) interface at low temperature has been studied using core and valence level photoemission. It is found that a nonmetallic first adsorption layer is followed by a transition to a metallic film. Valence level spectra show that Cs induces a new peak above the valence band maximum, which from the appearance of the core level spectra is assigned to adsorbate-substrate charge transfer. At higher coverages an intermediate species is identified on the basis of Cs 5p core level spectra. The metallic layer of Cs is characterised by metallic surface and bulk core level emission, the occurrence of plasmon satellites, and the emergence of a metallic Fermi edge. The relative separation of the different Cs core level line contributions are analysed through a Born-Haber cycle and qualitative arguments based on final state screening.