ELECTRONIC-STRUCTURE OF C(2X2) BA ADSORBED ON W(001)

We have used the ab initio full-potential linearized-plane-wave method to investigate the electronic properties of a c (2 X 2) overlayer of barium adsorbed onto a W(001) surface. We have determined the equilibrium height of the barium overlayer to be 2.48 angstrom above the tungsten surface by minimizing the total energy of the Ba/W(001) film. We calculate that the work function of the tungsten substrate (4.66 eV) is lowered by approximately 1.9 eV by the adsorption of the barium overlayer, a result which is in excellent agreement with experiment. We find that the Ba atoms are not ionized but rather form a metallic-covalent layer. In contrast to earlier studies of cesium adsorption on W(001) and Mo(001) surfaces, we find a very strong covalent component to the bonding between the tungsten substrate and the barium adsorbates. We find that the dominant substrate-adsorbate interaction involves the d states of the Ba adsorbates and the d-like surface states of the tungsten substrate. The Ba s states play only a minor role in this interaction and the bonding appears to be much more covalent than polar metallic. The work-function lowering is explained by multiple surface dipoles resulting from the interaction of the valence electrons of the adsorbate and the localized d-like surface states near the Fermi level of the tungsten substrate, and from the interaction of the 5p semicore states of the adsorbate overlayer with the s-like states at the bottom of the tungsten conduction band. The strong covalent contribution to the Ba-W bond is suggested as a reason to explain why Ba adsorption is not as effective as Cs in lowering the work function of the tungsten substrate and why Ba films are more stable than their cesium counterparts.