M.A. Van Hove

Chemisorption geometry of hydrogen on Ni/111/ - Order and disorder

The location of a half monolayer of ordered hydrogen adatoms on Ni(111) has been analyzed by Low‐Energy Electron Diffraction (LEED), Thermal Desorption Spectroscopy (TDS), and Work Function (Δφ) measurements. It is found that the hydrogen atoms are arranged in an overlayer of graphitic structure with a (2×2) unit cell with respect to the substrate unit cell. In the ordered regions, the hydrogen adatoms occupy both types of three fold hollow sites without a detectable difference in the Ni–H bond lengths between the two sites. The Ni–H bond length is found to be 1.84±0.06 A, corresponding to an overlayer‐substrate spacing of 1.15±0.1 A. The relation between this structure and its observed order–disorder phase diagram as a function of temperature and hydrogen coverage is discussed. The disorder is discussed in detail, and a novel ’’atomic band structure’’ interpretation is given.

Adsorption of CO on Pd(100)

Adsorption of CO on a Pd(100) surface was studied in detail mainly by LEED, UPS, work function and thermal desorption measurements. Analysis of the ordered c(2√2×√2) R 45° structure occurring at Θ=0.5 revealed that in this phase each CO molecule is bridge bonded to 2 Pd atoms with Pd–C distances of 1.93±0.07 A and a C–O bond length of 1.15±0.1 A, the molecular axis being oriented normal to the surface. The mutual configuration of the adsorbed molecules is explained in terms of a short‐range repulsive interaction model, which is supported by the observation that the isosteric heat of adsorption (Ead=38.5 kcal/mole) is constant up to a coverage of Θ?0.45. The photoelectron spectra exhibit two maxima at 7.9 (5σ+1π level) and 10.8 eV (4σ level) below the Fermi level which are in agreement with the observations with other Pd planes. This also holds for an electronic excitation associated with an energy of 13.5 eV as observed by electron energy loss spectroscopy. The variation of the sticking coefficient with c...

ADSORBATE-INDUCED RESTRUCTURING OF SURFACES

Abstract Adsorbed atoms and molecules frequently cause restructuring of single-crystal surfaces, ranging from small atomic relaxations and reconstructions to macroscopic shape modifications. The occurrence of such adsorbate-induced restructuring is reviewed, and the mechanisms and dynamic time scales are discussed. The importance of adsorbate-induced restructuring in a variety of surface processes is stressed. It is proposed that such restructuring can explain the observed “structure insensitivity” of a class of catalytic reactions, and that it could play a major role in most forms of reactivity.

Automated determination of complex surface structures by LEED

Conventional surface crystallography by low-energy electron diffraction (LEED) employs a trial-and-error search controlled at each step by human effort. This trial-and-error approach becomes very cumbersome and unreliable to solve complex surfaces with a large number of unknown structural parameters. We discuss automatic optimization procedures for LEED, which combine numerical search algorithms with efficient methods of determining the diffracted intensities for varying structures. Such approaches can reduce the computer time required for an entire structure determination by many orders of magnitude, while fitting many times more unknown structural parameters. Thereby, relatively complex structures, with typically 10 adjustable atoms (or 30 adjustable coordinates), can be readily determined on todays workstations. These include non-symmetrically relaxed structures, surface reconstructions and adsorbate-induced substrate distortions. We also address the theoretical and experimental requirements for an accurate structural determination.

Magnetite Fe3O4(111): surface structure by LEED crystallography and energetics

Abstract The atomic structure of the Fe3O4(111) surface was determined by means of dynamical low-energy electron diffraction (LEED) after being prepared in two different ways. In a first experiment up to 10 monolayers of well-ordered iron oxide films were grown epitaxially onto Pt(111) substrates. A 1 monolayer thick film forms a hexagonal lattice with a lateral repeat distance of 3.2 A, 15% larger than the lateral periodicity of Pt(111). Above 1 monolayer coverage the LEED pattern reveals a lateral repeat distance of 3.0 A, indicating a contraction of the oxide lattice with respect to the first monolayer. This new LEED pattern shows half-order spots and is compatible with (2 × 2) reconstructed FeO(111) and bulk terminated Fe3O4(111) surfaces. By applying automated tensor LEED to many possible surface structures of these two iron oxides, 8 monolayer thick films were identified to be magnetite, Fe3O4. Auger electron spectroscopy (AES) measurements on these films also reveal a stoichiometry close to that of Fe3O4. In a second experiment the (111) surface of an α-Fe2O3 single crystal was prepared by Ar+ ion bombardment and subsequent annealing. Brief annealing to 900, 1000 and 1200 K in 10−10 and 10−6 mbar oxygen creates three different LEED patterns indicating structural transformations occurring in the surface region of this crystal. Prolonged annealing to temperatures between 900 and 1200 K stabilizes the same LEED pattern and gives identical intensity-voltage curves as obtained on the 8 monolayer thick films. Therefore the crystal surface region has been reduced to Fe3O4 and has the same surface structure as the epitaxially grown films. X-ray photoelectron spectroscopy (XPS) measurements on this surface also reveal a stoichiometry dose to that of Fe3O4. The best fit structure for both preparations corresponds to an unreconstructed, but strongly relaxed, polar (111) surface termination of magnetite that exposes 1 4 monolayer of Fe ions over a distorted hexagonal close-packed oxygen layer and minimizes the number of dangling bonds. The surface relaxations are probably driven by electrostatic forces. Our results indicate that minimization of both the number of dangling bonds and the electrostatic surface energy are important in determining the termination and relaxations of this polar metal oxide surface. The electrostatic surface energetics is qualitatively discussed within general, simple concepts applicable to all ionic crystals.

The surface reconstructions of the (100) crystal faces of iridium, platinum and gold : II. Structural determination by LEED intensity analysis

Abstract The investigation, in a companion paper, of the reconstructions of the Ir(100), Pt(100), and Au(100) crystal surfaces is completed here with an extensive analysis of low energy electron diffraction (LEED) intensities, using dynamical (multiple scattering) calculations. It is found that a hexagonal rearrangement of the top monolayer is a likely explanation of the surface reconstruction. At least for Ir and Pt (no calculations were made for Au), this hexagonal layer would have a registry involving bridge sites on the next square unit cell metal layer and it is contracted and buckled. Bond length contractions parallel and perpendicular to the surface occur; the Pt top layer is rotated by a small angle (0.7°) with respect to the substrate. A second model that cannot be ruled out by the LEED analysis, but disagrees with ion-scattering data, involves shifted close-packed rows of top-layer atoms and requires domain structures in the case of Pt and Au. Charge-density-wave and missing-row models are ruled out by our structure analysis. A correlation is found between the occurrence of surface reconstructions on metals and a small ratio of their Debye temperature to their melting point. This correlation singles out mainly the 5d metals as having a propensity to surface reconstruction. The effects of adsorbates on the reconstructions are also discussed.

LEED intensity analysis of the structures of clean Pt(111) and of CO adsorbed on Pt(111) in the c(4×2) arrangement

Abstract The surface structures of clean Pt(111) and CO adsorbed on Pt(111) with c(4 × 2) periodicity at one-half monolayer coverage were investigated by a dynamical LEED intensity analysis of measured I-V curves. The clean Pt(111) structure is confirmed to be nearly indistinguishable from the truncated bulk structure (within ≈ 0.025 A). The c(4 × 2) unit cell contains one CO molecule adsorbed on a top site and one CO molecule adsorbed on a bridge site, both molecules being perpendicular to the surface. This is only the second LEED structure determination of multi-site molecular adsorption. The CO molecules have metal-carbon bond lengths of 1.85 ± 0.1 and 2.08 ± 0.07 A for the top and bridge sites, respectively, and the same carbon-oxygen bond length of 1.15 ± 0.05 A. Trends relating bond lengths and vibration frequencies for CO adsorption on various metal surfaces are presented. In particular, the CO stretch frequency decreases as the metal-carbon bond length increases, which in turn increases as the carbon-metal coordination number increases.

A NEW MICROFACET NOTATION FOR HIGH-MILLER-INDEX SURFACES OF CUBIC MATERIALS WITH TERRACE, STEP AND KINK STRUCTURES

Abstract The ideal (i.e. unreconstructed and unrelaxed) strcuture of arbitrary high Miller index surfaces of cubic crystals is analyzed in terms of terraces, steps and kinks, and a new, simple and useful microfacet notation is proposed. The cubic crystals include the simple cubic, facecentered cubic, body-centered cybic, zincblende, NaCl and CsCl crystals.

Chemisorption bond lengths of chalcogen overlayers at a low coverage by convergent perturbation methods

The bond lengths and binding locations of the p (2×2) chalcogen (O, S, Se, and Te) overlayers on Ni(001) are investigated in detail by microscopic low−energy electron diffraction (LEED) calculations. Two convergent perturbation methods are used which are as accurate as equivalent exact calculations but which minimize the use of computation core storage and time. Normal and off−normal incidences are considered and eight partial waves are used in the calculations. The results show the p (2×2) O and S bond lengths are identical to those of the c (2×2) structures determined by Demuth et al. The larger chalcogens Se and Te form slightly different bond lengths (difference less than ±0.1 A) from those determined for the c (2×2) overlayer structures.

Reliability of detailed LEED structural analyses: Pt(111) and Pt(111)-p(2×2)-O

As surface structures are being examined in more detail than ever before, the reliability of structural details becomes an important issue. To discuss necessary components of a high quality, reliable dynamical LEED studies, detailed dynamical LEED analyses of the clean Pt(111) and the Pt(111)-p(2×2)-O structures have been carried out utilizing different computer programs and search methods applied to a common set of experimental LEED I-V data. We have investigated the effects of various non-structural parameters, in particular those involved in the construction of the phase shifts, on the resulting structures of the clean Pt(111) and the Pt(111)-p(2×2)-O surfaces. The use of relativistic Pt phase shifts is found to be important in order to determine the adsorption structure accurately. For clean Pt(111), we find the top interlayer spacing is noticeably expanded by 0.025 ± 0.01 A with a low Rp-factor value of 0.15. For the Pt(111)-p(2×2)-O system we find an Rp-factor of 0.18. O atoms in the p(2×2)-O overlayer are found to adsorb in fcc-hollow sites and induce buckling in both the first and second metal layers. In addition, there is an expansion of the first metal-metal interlayer spacing and a small contraction of the second.