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 coad sorbed with O and benzene, respectively, on Ni(111). Pure CO layers at 80 K were characterised by low energy electron diffraction and temper ature programmed desorption and studied by X-ray photoelectron spectro scopy 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 hol low 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 pr eference for high symmetry adsorption sites. At room temperature the a dditional occupation of on-top and bridge sites is even observed at lo w coverages. In the p(2 root 3 x 2 root 3)R30 degrees (benzene+2CO) st ructure the CO resides most likely on threefold hollow sites with benz ene molecules occupying the same adsorption sites. After the adsorptio n onto a p(2 x 2) oxygen layer at about 100 K the CO occupies two site s: 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 Sc ience B.V.