The structure of oxygen on Cu(100) at low and high coverages

The local adsorption structure of oxygen on Cu(1 0 0) has been studied using O Is scanned-energy mode photoelectron diffraction. A detailed quantitative determination of the structure of the 0.5 ML (root2 x 2 root2)R45 degrees -O ordered phase confirms the missing-row character of this reconstruction and agrees well with earlier structural determinations of this phase by other methods, the adsorbed O atoms lying only approximately 0.1 Angstrom above the outermost Cu layer. At much lower coverages, the results indicate that the O atoms adopt unreconstructed hollow sites at a significantly larger O-Cu layer spacing, but with some form of local disorder. The best fit to these data is achieved with a two-site model involving O atoms at Cu-O layer spacings of 0.41 and 0.70 Angstrom in hollow sites: these two sites (also implied by an earlier electron-energy-loss study) are proposed to be associated with edge and centre positions in very small c(2 x 2) domains as seen in a recent scanning tunnelling microscopy investigation. (C) 2001 Elsevier Science B.V. All rights reserved