Alll Surface Science 279 (1992) 287-296 North-Holland Adsorption of CO at Ni(100) surfaces: a FTIRAS - TDS J. Lauterbach, M. Wittmann and J. Kiippers Experimentalphysik 111, Universitiit Bayreuth, 8580 Bayreuth, Germany Received 24 June 1992; accepted for publication 22 September 1992 study The adsorption of CO at Ni(100) surfaces was studied in the temperature range 125-350 K using TDS, FTIRAS and LEED. With TDS, three desorption states, f12,/31, and a were observed at 450, 350 and 280 K corresponding to completion of coverages 0 = 0.25, 0 = 0.5, and 0 = 0.67 (+0.03), respectively. At coverages below 0 = 0.5 CO adsorbs at terminal and bridge sites with coverage dependent C-O stretch frequencies of 2016 (low 0) to 2048 cm -1 (high 0) and 1885 to 1924 cm -1, respectively. The population of the sites is temperature dependent, bridge sites are favored over top sites at low temperature and vice versa. At 0 = 0.5 both, terminal and bridge sites are occupied, top site occupation exceeding bridge site occupation. Above 0 = 0.5 FTIRA spectra exhibit in the terminal region a broad band from 2000 to 2100 cm -1, which contains at least 3 components and suggests that a distinct terminal site does not exist at these coverages. On the contrary, in the bridge site region a single peak at 1968 cm- 1, with a width of 25 cm- 1, signals that bridge type or almost bridge type adsorption sites are still present at the CO covered surface. The results are in accordance with the assumption that CO molecules are arranged in incommensurate or weakly incommensurate adsorbate layers above half monolayer coverage. Surface Science 279 (1992) 297-304 North-Holland SISS-92: a computer code for the simulation of ion-surface scattering C.A. Severijns a, G. Verbist b,1 and H.H. Brongersma a,. a Eindhoven University of Technology, Department of Physics, P.O. Box 513, 5600 MB Eindhoven, Netherlands b University of Antwerp (UIA), Department of Physics, Universiteitsplein 1, 2610 Antwerp, Belgium Received 14 April 1992; accepted for publication 10 September 1992 A new and efficient computer code for the simulation of ion scattering experiments is presented. Results of simulations with this code are compared to experimental data as well as existing simulation results from the literature. Within a simulation, it is possible to investigate separately the influence of thermal vibrations and the divergence of the primary ion beam. It is shown, that the broadening of the peaks in an angular-dependent intensity profile is mostly due to imperfections in the ion source and detector. Surface Science 279 (1992) 305-318 North-Holland Test of high-angular-resolution X-ray photoelectron diffraction and holographic imaging for c(2 x 2)S on Ni(001) R.S. Saiki 1, A.P. Kaduwela 2, y.j. Kim 2, D.J. Friedman 3, j. Osterwalder 4, S. Thevuthasan 5 and C.S. Fadley 2,5 Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA Received 12 December 1991; accepted for publication 18 March 1992 We have obtained azimuthal X-ray photoelectron diffraction (XPD) data with a high .angular resolution of + 1.5° for S 2p emission from the well-defined surface structure of c(2 x 2)S on Ni(001). The relatively high position of the adsorbate with respect to the substrate makes this a stringent test case of the structural sensitivity of forward-scattering-dominated XPD. With this higher resolution, the data are nonetheless found to be sensitive to atomic structure, including in particular both the vertical height of S above Ni (z) and the first-to-second layer Ni interplanar sp.acing (d12). A single scattering cluster (SSC) theoretical analysis using R-factors to judge goodness of fit yields z = 1.39 + 0.05 A and d12 = 1.86 5:0.05 ,~, in excellent agreement with other recent experimental and theoretical studies. This analysis also indicates that clusters of up to at least 25 .~ in radius (200-250 atoms) are