Hreel study of the coadsorption of sodium and oxygen on Ni(111)

The coadsorption of oxygen and sodium on Ni(111) was studied by high resolution electron energy loss spectroscopy. Measurements were performed for a definite sodium precoverage, as a function of oxygen exposure and temperature. For a coverage of two layers of sodium, the loss spectra show the vibrational features related to Na-O-2 and O-O bonds of superoxide NaO2 molecules. The annealing up to 450 K causes the decomposition of these complexes and the reappearance of the stretching frequencies of isolated Na-O and O-Ni species. At 550 K, only the oxidation of the Ni substrate occurs. The spectra are correlated to work function measurements, and the behavior of the work function versus oxygen exposure, for high alkali coverages, is ascribed to a reionization of the metallic Na layer, rather than to a penetration of oxygen atoms underneath the Na layer.