The transition from oxygen chemisorption to oxidation of ultra-thin Ni layers on Cu(111)

X-ray photoelectron spectroscopy (XPS) was used to study the initial stages of surface oxidation of pseudomorphic Ni monolayers on Cu(111). Oxygen was adsorbed at 150 K followed by annealing the sample to 300 K and 600 K, res pectively. For oxygen coverages between 0.4 ML and 2.0 ML we find little ch ange in the peak shapes of the O 1s XPS signal. The Ni 2p(3/2) spectra chan ge, however, drastically: the onset of the oxidation is marked by the appea rance of a peak doublet shifted with respect to the peak of metallic Ni. Ba sed on these spectra we find a minimum oxygen coverage of 0.7 ML necessary for the onset of oxidation. The oxidation is nearly complete after the adso rption of about 2.0 ML oxygen. The exposure of different Ni coverages (0.5- 2.0 ML) to oxygen shows that oxidation takes place only in the top-most Ni layer. (C) 2001 American Institute of Physics.