S. Andersson

On the structure of dense CO overlayers

Abstract A series of dense CO structures formed on the Ni(100), Cu(100) and Pd(100) surfaces have been investigated using high resolution electron energy loss spectroscopy (EELS). The structures were monitored by low energy electron diffraction (LEED). The simple half-monolayer structures i.e. the c(2×2) CO on Ni and Cu and the p (2 2 × 2 ) R 45° CO on Pd are fully compatible with on-top site (Ni, Cu) and bridge site (Pd) adsorption, respectively. The denser structures form in a somewhat different manner on the three surfaces, with the common factor that the CO molecules bend in order to accommodate the increasing CO-CO repulsion while still retaining simple coordinations with respect to the substrate surface atoms.

Electron structure of Na and K adsorbed on Ni (100)

Abstract Electron excitations of Na and K adsorbed on Ni (100) have been investigated by means of electron energy loss spectroscopy. The experiments were performed in conjunction with measurements of adatom distributions and work function changes. Excitations from occupied valence and core states to unoccupied valence states and a collective excitation among the valence electrons are reported. It is found that the excitations are characterized by their dependence on the surface density and the charging state of the adatoms. The excitation among the valence states decreases in energy with increasing density due to the electrostatic interaction among the charged adatoms. This observation has in conjunction with the work function data been utilized to explore the adatom valence electron structure and charging. The observed shifts of the Na2p and K3p core level excitation thresholds are consistent with these charge assignments.

Structure of CO adsorbed on Ni (100)

Abstract The c(2 × 2) configuration of CO chemisorbed on Ni(100) has been examined by the dynamical LEED method of surface structure analysis. Experimental LEED intensity spectra of the (00), ( 1 2 1 2 ) (10) and (11) LEED beams measured at 175 K are compared with the corresponding calculated spectra for two different CO potential constructions and a number of trial structures. The best agreement was found for a structure where the CO molecules sit directly above the Ni atoms with vertical spacings between the Ni and C and the C and O layers of 1.80 ± 0.10 A and 0.95 ± 0.10 A respectively. It is proposed that the CO molecule is tipped over at an angle of 34° ± 10° with respect to the surface normal so that the actual carbon-oxygen bond length is close to the figure 1.15 A found in Ni(CO) 4 .

Adsorption of water monomers on Cu(100) and Pd(100) at low temperatures

Abstract The adsorption of H 2 O on Cu(100) and Pd(100) at temperatures around 10 K has been investigated by electron-energy-loss spectroscopy (EELS). Water is found to adsorb associatively as monomers at low coverages with its molecular axis significantly tilted relative to the surface normal.

Vibrational excitations and structure of C2H4 adsorbed on Cu(100)

Abstract The adsorption of C 2 H 4 on Cu(100) at 80 K has been investigated by angle-dependent high-resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). Our observations are consistent with a model where the ethene molecule is adsorbed in a configuration parallel to the Cu(100) surface. The molecule-metal interaction is weak and presumably of π character.

Low energy electron diffraction from Na(110) and Na2O(111) surfaces

Abstract Surfaces ofNa(110) are grown, investigated and oxidised to give Na 2 O(111) surfaces. LEED spectra are taken for these surfaces and compared with theory to determine the surface composition of sodium oxide: the surface terminates the crystal in an integral number of electrically neutral NaONa sandwiches, with a bulk-like inter layer spacing. The effective Debye temperature for the Na(110) surface was found to be 107 K.

Adsorption of hydrogen on a stepped nickel surface

Abstract The adsorption of hydrogen and deuterium on a stepped nickel surface, Ni(510), has been studied using electron energy loss spectroscopy, EELS. At a surface temperature of ~100 K both dissociative and molecular chemisorption is found. For small doses hydrogen atoms adsorb in both hollow sites on the terraces and in low symmetry bridge sites at the steps with a preference for the step sites. For larger exposures, 0.5–0.9 L H 2 , a dense layer of hydrogen atoms is formed on the terraces and the coverage increases at the steps where a second kind of step site gets occupied. Finally, for exposures larger than 0.9 L H 2 , hydrogen molecules chemisorb at sites above the steps.

Soft X-ray appearance potential spectroscopy of C, N, O, and S chemisorbed on some 3d transition metals

Abstract The 1s level soft X-ray appearance potential spectra of C, N, O, and S adsorbed on polycrystalline 3d transition metal surfaces (e.g. Ti, Cr, Fe, and Ni) are presented. In the chemisorption regime of adsorption the adsorbate Is level spectra are characterized by a single peak above the excitation threshold. The peak is thought to be related to an enhanced local density of unfilled valence states on the adatoms. The peak widths correlate with the widths of the unfilled portion of substrate d-band in a way consistent with a mixing of the adsorbate valence states and the substrate d-states.

Hydrogen recombination and Σ-desorption from the Ni(100)HCO coadsorption system

Abstract The vibrational motion and structure of hydrogen and carbon monoxide, coadsorbed on Ni(100), have been investigated using HREELS and LEED. The initial structure, Ni(100)p(1×1)Hc(2×2)CO, formed at 80 K by successive adsorption of hydrogen and carbon monoxide, is metastable. At ∼150 K, i.e. prior to the so called Σ-desorption the system transforms irreversibly into a stable (2√2 × √2)R45° structure. Part of the CO molecules change adsorption sites and most notably part of the adsorbed H recombine to form chemisorbed H 2 . This molecular state seems to be stabilized by the specific CO environment. We propose that the Σ-H 2 desorption around 220 K essentially derives from the chemisorbed H 2 . We have found that the H adsorbate layer can weaken the NiCO bond appreciably and that the Σ-CO desorption appears to be an intrinsic property of the H modified Ni(100) surface. The Σ-H 2 desorption is then simply a consequence of the Σ-CO desorption.

Low-energy electron diffraction intensities from the clean copper (001) surface

Abstract Experimental absolute intensity curves of the 00, 11, 20 and 22 diffraction beams from a clean copper (001) surface obtained for incident electron energies 1–600 eV are presented. A complete band gap extending to about 2 eV above the vacuum level in the electron energy band structure for copper accounts for most intense peak in the curves: the first Bragg peak around 1 eV in the 00 beam intensity. With increasing electron energy the peak intensities decrease and the peak widths increase. In the electron energy region 30–140 eV strong multiple scattering gives rise to intense secondary Bragg peaks and unsymmetrical peaks in the curves. These effects are due to large atomic scattering cross section of the copper atoms probably related to the 3p and 3s core states.