Barium adsorption on the chemisorbed O(2 × 1)/Ni(110) surface.

Barium adsorption on the O(2 × 1)/Ni(110) surface has been studied by Auger electron spectroscopy and work function measurements in combination with photoemission measurements. The study was focused on the low coverage regime from submonolayer to double monolayer. The results show that during development of the first layer of Ba on the surface, a two-dimensional incomplete barium oxide layer, BaO, forms. This BaO layer is interspersed by Ba chemisorbed atoms reacting directly with Ni atoms. As the second layer of Ba is completed, the adsorbate approaches the metallic phase due to the Ba-Ba interaction. The low energy Auger transition lines of Ba (75 eV) and BaO (68 eV) shift towards lower energies as the Ba coverage increases. Previous photoemission measurements by synchrotron radiation are used to interpret these energy shifts, which are closely related to the barium oxidation process on the surface. The analysis shows the importance of the extra-atomic relaxation energy due to (1) the polarization of the O(2-) anions from BaO and (2) the screening from the electron density at the Fermi level of the barium overlayer and the nickel substrate.