X-RAY PHOTOELECTRON DIFFRACTION AND LOW-ENERGY ELECTRON-DIFFRACTION STUDY OF THE INTERACTION OF OXYGEN WITH THE NI(001) SURFACE - C(2X2) TO SATURATED OXIDE

We have carried out a combined X-ray photoelectron diffraction (XPD) and low-energy electron diffraction (LEED) study of the interaction of oxygen with Ni(001) at ambient temperature from the c(2 x 2) structure up to the saturated oxide. Several structural conclusions are possible based on an R-factor comparison of an extensive series of azimuthal- and polar-O 1s XPD data to theoretical simulations based on a single-scattering cluster (SSC) model with spherical wave scattering. A new method for normalizing experimental and theoretical intensities for XPD R-factor analyses is also used. For the c(2 x 2) structure, we find that the oxygen sits in the fourfold hollows site, at a vertical distance (z) of approximately 0.75 angstrom above the first Ni plane, in excellent agreement with the most recent LEED determination. There is also strong evidence from the XPD results that oxide nucleation occurs very early in the chemisorption region (much earlier than observable by LEED) and this effect could explain the proposals for in-plane bonding or pseudobridge bonding previously reported in the literature. The saturated oxide that forms at ambient temperatures is found from LEED and XPD to form a strained superlattice that is expanded by 1/6 with respect to the underlying Ni(001). We also find that the saturation oxide coverage is much larger than that previously reported in the literature of 2-3 monolayers (ML), and that it is in fact 4-5 ML. Other conclusions concerning the nature of the strain in the oxide are also possible, including a vertical expansion in lattice constant from the first to second plane of 0.1-0.2 angstrom.