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

The theoretically predicted phonon frequency dependence on film thickness for NiO layers grown on Ni(111) is observed experimentally. Good agreement between the experiment and the theoretical calculations based on the dielectric theory is found. X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS) data are presented in more detail for an oxygen-saturated Ni(111) surface. The third stage of this interaction, which is the very slow thickening of the oxide layer, includes also adsorption of oxygen on the oxide surface as revealed by the analysis of the O$1s$ and Ni2p3/2 photoelectron lines. The HREELS of oxygen interaction with the Ni(111) surface exhibits a complex structure. The adsorbed oxygen on Ni(111) is characterized by one loss feature centered at ~565 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 appearance of two features in the HREELS spectrum: one at ~510 cm^-1 and another at ~440 cm^-1 observed here as a low-energy shoulder. These two peaks are characteristic for a NiO oxide island on the Ni substrate and they can be considered as a fingerprint of that structure. The low-energy shoulder (~440 cm^-1) decreases in intensity when OH groups are adsorbed on the NiO(111) surface. The presence of OH groups produces also a shift in the phonon peak from ~510 cm^-1 to ~550 cm^-1.