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 r esolution electron energy loss spectroscopy. Measurements were perform ed for a definite sodium precoverage, as a function of oxygen exposure and temperature. For a coverage of two layers of sodium, the loss spe ctra 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 decomp osition of these complexes and the reappearance of the stretching freq uencies of isolated Na-O and O-Ni species. At 550 K, only the oxidatio n of the Ni substrate occurs. The spectra are correlated to work funct ion 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 un derneath the Na layer.