XPS/HREELS study of NiO films grown on Ni(111)

The theoretically predicted phonon frequency dependence on film thickness f or NiO layers grown on Ni(lll) is observed experimentally. Good agreement b etween the experiment and the theoretical calculations based on the dielect ric theory is found. X-ray photoelectron spectroscopy (XPS) and high resolu tion electron energy loss spectroscopy (HREELS) data are presented in more detail for an oxygen-saturated Ni(111) surface. The third stage of this int eraction, which is the very slow thickening of the oxide layer, includes al so adsorption of oxygen on the oxide surface as revealed by the analysis of the O1s and Ni2p(3/2), photoelectron lines. The HREELS of oxygen interacti on with the Ni(111) surface exhibits a complex structure. The adsorbed oxyg en on Ni(111) is characterized by one loss feature centered at similar to 5 65 cm(-1). This single line is observed for exposures as low as 1-2 L. The starting of nickel oxidation as revealed by XPS is accompanied by the appea rance of two features in the HREELS spectrum: one at similar to 510 cm(-1) and another at;similar to 440 cm(-1) observed here as a low-energy shoulder . These two peaks are characteristic for a NiO oxide island on the Ni subst rate and they can be considered as a fingerprint of that structure. The low -energy shoulder (similar to 440 cm(-1)) decreases in intensity when OH gro ups are adsorbed on the NiO(lll) surface. The presence of OH groups produce s also a shift in the phonon peak from similar to 510 cm(-1) to similar to 550 cm(-1) [S0163-1829(99)02528-X].