Na adsorption on the polar NbC(111) surface

Abstract

Core-level photoemission spectroscopy and angle-resolved photoemission spectroscopy with synchrotron radiation have been used to study the adsorption mechanism of Na on the NbC(111) surface. Coverage-dependent measurements of the work function and the Na core-level photoemission show that Na adsorption on NbC(111) proceeds via a polarization–depolarization transition. Valence-band photoemission measurements show that the 3s orbital of adsorbed Na is hybridized with the Nb 4d-derived surface state which is characteristic of the polar (111) surfaces of transition-metal carbides. In the initial stages of adsorption, only a bonding part of the hybridized states is found in the occupied region and an antibonding part lies above the Fermi level, which corresponds to the electronic structure for the polarized adsorbed layer. The antibonding part, mainly composed of the Na 3s orbital, is observed just below the Fermi level at around the Γ ̄ point in the high-coverage region where the work function is gradually increased after the work-function minimum. The appearance of the partially filled antibonding state indicates substantial back donation of electronic charge towards the adsorbed Na atoms, and can be viewed as direct evidence of the metallization of the overlayer in the higher-coverage region.