Geometry and unoccupied electronic states of Ba and BaO on W(001)

Abstract

A study aimed at understanding the geometrical and electronic properties of barium and oxygen coadsorbed on the tungsten (001) surface has been carried out by means of work-function measurements (\ensuremath{\Delta}\ensuremath{\varphi}), Auger-electron spectroscopy, low-energy electron diffraction, inverse photoelectron spectroscopy, and relativistic-electronic-structure calculations. A report of the experimental measurements and a comparison with theoretical results from embedded-cluster-model calculations are presented. Our experimental studies show that the work function of the W(001) surface (\ensuremath{\varphi}=4.63 eV) is lowered to approximately 2.3--2.4 eV by coadsorption of 1 ML of Ba and O regardless of the order of deposition of these two species. The technique of IPS in the isochromat mode was used to determine the unoccupied electronic-energy band structure for ordered c(2\ifmmode\times\else\texttimes\fi{}2) Ba and O layers on W(001). Several spectral features are observed above the Fermi level (${\mathit{E}}_{\mathit{F}}$), which we assign to transitions into Ba and W d states. The measured two-dimensional electronic band structure is independent of the order of Ba and O deposition. Using embedded-cluster-model calculations, we investigated two possible adsorption configurations of an ordered c(2\ifmmode\times\else\texttimes\fi{}2) adlayer of Ba and O on W(001): ``tilted,'' where Ba and O are placed on alternate fourfold-hollow sites, and ``upright,'' where the adsorbed atoms lay above the same site with Ba outermost. The calculated densities of states for the tilted geometry show distinct peaks above ${\mathit{E}}_{\mathit{F}}$ originating from Ba and W d orbitals and are in good agreement with the experimental results.