The local adsorption structure of oxygen on Cu(1 0 0) has been studied usin
g O Is scanned-energy mode photoelectron diffraction. A detailed quantitati
ve determination of the structure of the 0.5 ML (root2 x 2 root2)R45 degree
s -O ordered phase confirms the missing-row character of this reconstructio
n and agrees well with earlier structural determinations of this phase by o
ther methods, the adsorbed O atoms lying only approximately 0.1 Angstrom ab
ove the outermost Cu layer. At much lower coverages, the results indicate t
hat the O atoms adopt unreconstructed hollow sites at a significantly large
r 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 lay
er spacings of 0.41 and 0.70 Angstrom in hollow sites: these two sites (als
o implied by an earlier electron-energy-loss study) are proposed to be asso
ciated with edge and centre positions in very small c(2 x 2) domains as see
n in a recent scanning tunnelling microscopy investigation. (C) 2001 Elsevi
er Science B.V. All rights reserved.