Determination of adsorption sites of pure and coadsorbed CO on Ni(111) by high resolution X-ray photoelectron spectroscopy

O 1s and C 1s peak positions from high resolution X-ray photoelectron spectroscopy were used to determine adsorption sites of CO and CO coadsorbed with O and benzene, respectively, on Ni(111). Pure CO layers at 80 K were characterised by low energy electron diffraction and temperature programmed desorption and studied by X-ray photoelectron spectroscopy in a wide coverage range. For low coverages up to the well-known c(4 x 2) structure (theta(CO)=0.5 ML) the CO resides on threefold hollow sites. In the p(root 7 x root 7)R19 degrees structure (theta(CO)=0.57 ML) the on-top and bridge sites are occupied; for the compressed c(2 root 3 x 4) rect structure (theta(CO)=0.62 ML) there is no clear preference for high symmetry adsorption sites. At room temperature the additional occupation of on-top and bridge sites is even observed at low coverages. In the p(2 root 3 x 2 root 3)R30 degrees (benzene+2CO) structure the CO resides most likely on threefold hollow sites with benzene molecules occupying the same adsorption sites. After the adsorption onto a p(2 x 2) oxygen layer at about 100 K the CO occupies two sites: on-top and bridge or hollow sites. If this layer is annealed to 260 K only the on-top sites are left on the surface. (C) 1998 Elsevier Science B.V.