Barium and oxygen interaction on the Ni(110) surface at low coverages studied by soft x-ray photoemission spectroscopy: Ba negative binding energy shifts and their correlation with Auger electron spectroscopy shifts

In this work, barium and oxygen interaction on the Ni(110) surface is investigated, by means of soft x-ray photoemission spectroscopy (SXPS), mainly at submonolayer and monolayer coverages. The results show that oxygen interacts with barium and the substrate as well forming BaO and NiO respectively. The formation of both oxides is consistent with previous Auger electron spectroscopy (AES) results. The oxidation of barium on the surface induces negative binding energy shifts of the Ba low core atomic levels 4d, 5s and 5p. Both initial and final state effects are shown to be necessary in order to explain these peculiar energy shifts. This interpretation is based on correlating the core level binding energy shifts with previously recorded AES transition line shifts, also taking into account the changes of the work function of the surface. The analysis shows that the extra-atomic relaxation energy decreases on going from metallic Ba to BaO. This implies that the reduction of the free electron screening more than counteracts the increase in relaxation energy due to the polarizable O2- anions