Photoemission observation of Na-induced states in the band gap and evidence for charge transfer in Na/GaP(110)

Na adsorption on GaP(110) is shown to induce a photoemission peak in the region of the band gap above the valence-band maximum. This peak is interpreted as arising from the formerly empty Ga dangling-bond surface state, which is occupied and lowered in energy by interaction with the Na 3s level. The interaction between Ga and the Na substrate atoms is reflected in the appearance of a shoulder on the Ga 3d core-level line and two separate Na 2p emission lines. These data, which are interpreted by recourse to recent self-consistent electronic-structure calculations, provide strong evidence for charge transfer in the bonding of Na to GaP.