Determination of Atomic Positions in theC(2×2)Oxygen Structure on a Nickel (100) Surface by a Dynamical Low-Energy Electron-Diffraction Method

The dynamical method of low-energy electron-diffraction intensity analysis has been applied to the system of $C(2\mathrm{}\times 2)$ oxygen on nickel(100). This system differs considerably both geometrically and chemically from the sodium-on-nickel system previously reported. The diffraction data are consistent with a model where oxygen sits in a layer displaced normal to the surface by $d=1.5\mathrm{}\pm 0.1$ Å from the center of the top layer of nickel atoms. Oxygen atoms sit above pyramidal hollows in the nickel layer below.