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Experimental (LEED) intensity-voltage (I–V) beam profiles for clean Ni(110), Al(110) and Ag(110) have been analyzed by the convolution-transform method. Results indicate that the first layer spacing of Ni(110), Al(110) and Ag(110) is contracted by 5%, 4% and 7% of their bulk spacing, respectively. These results are in good agreement with the results obtained earlier using dynamical calculations.

The reconstruction of Ir(110)-(1 × 2) has been re-analyzed by low-energy electron diffraction. In this study, the missing-row model with paired rows in the second layer and buckled rows in the third layer, as well as the Bonzel-Ferrer (sawtooth) model have been examined. In addition, two other models, which are obtained by putting the missing-row atoms back onto the surface in other sites, have also been considered. It is found that the missing-row model with paired rows in the second layer and buckled rows in the third layer gives the best R-factor among all the models considered in this study. This missing-row model with a three-layer reconstruction is thus proposed to solve the Ir(110)-(1 × 2) structure.

The results of a quantitative LEED study of the clean, reconstructed Pt(110)-(1 × 2) surface are described, in which experimental intensity-energy spectra for ten diffracted beams have been compared, by an γ-factor analysis, to intensity spectra calculated using a dynamic theory for various structural models. Despite considerable effort in ensuring the reliability of the experimental and calculated intensities, a satisfactory level of agreement has not been achieved for any of the model structures considered although a “missing-row” model leads to the closest agreement between experimental and calculated intensities.

The first Low-Energy Electron Diffraction (LEED) intensity analysis of molecular adsorption in multiple sites is presented. A 34 monolayer CO overlayer on Rh(111) is found to involve one bridge site and two near-top sites in each unit cell, which supports the bridge and top assignment based on earlier High-Resolution Electron Energy Loss Spectroscopy (HREELS) data. The near-top site is asymmetrical, with the CO axis close to the surface normal (but possibly tilted about 5°), yielding a bent Rh-C-O species. The near-top CO molecules are forced sideways from the ideal top site by ~ 0.53 A due to bridge-bonded CO molecules located at a distance of 2.85 A. This short CO-CO separation is somewhat smaller than corresponding intermolecular separations found in metal carbonyl clusters and crystalline CO and CO2. The resulting CO overlayer has a hexagonal geometry that is distorted both parallel and perpendicular to the surface due to the strong metal-CO bonding. The Zanazzi-Jona and Pendry R-factors for the best structure are 0.25 and 0.47, respectively. The LEED analysis exhibits the use of different levels of approximation in the multiple scattering theory in order to show the value of approximate but fast calculations in preliminary structural searches. A new algorithm to obtain experimental I–V curves from digitized images of LEED patterns is described.

The tilting of carbon monoxide molecules adsorbed on a Pt(110) surface has been modeled by using the extened Huckel molecular-orbital (EHMO) method. Electronic properties, including total energies, were obtained for various adsorbate-substrate cluster systems in order to explore the qualitative features of this system. The tilting away from the surface normal is due to excessive close-packing of CO molecules along the atomic ridges of the Pt(110) surface. It is found energetically favorable for the tilting to alternate on either side of each ridge in a zig-zag fashion, as indicated by glide plane symmetry in observed LEED patterns. A polar tilt angle of ∼16o is found, in reasonable agreement with experimental values. This yields CO−CO distances which are similar to the smallest values found in organometallic clusters and small compared with values in CO and CO2 crystals. An azimuthal twist in the direction of the ridges is found to be slightly unfavorable energetically with EHMO, although it is thought to have been observed in some experiments and not in others. The optimal polar tilt angle slightly changes with the size of the metal cluster. To obtain a tilted equilibrium direction one needs at least three CO molecules on neighbouring Pt atoms, and six Pt atoms in two layers (directly beneath each other).

A review is given of recent developments in methods of surface crystallography, emphasizing Low-Energy Electron Diffraction theory, and of specific results, including trends obtained with these methods.

The atomic positions of the Rh (111) + ( 3 × 3 ) R 30° CO and CO2 surfaces are analyzed by dynamical LEED. The Rh (111) + ( 3 × 3 ) R 30° CO and CO2 systems produce identical I–V curves, confirming the dissociation of CO2 to CO on this surface. The adsorbed CO is found to stand perpendicular to the surface with the carbon end down at an atop site (that is, terminally bonded). The CO overlayer spacings are d RhC = 1.95 ± 0.1 A and d CO = 1.07 ± 0.1 A . This geometry yields a Zanazzi-Jona R-factor of 0.40 and a Pendry R -factor of 0.50.

Presentation des resultats et comparaison avec la structure du benzene coadsorbe avec CO sur Pt(111)

The atomic surface structures of the close-packed faces of single crystalline cobalt in the bulk hcp and bulk fcc phases have been determined by low-energy electron diffraction. The two separate structures were monitored on a single-crystal sample cycled through the surface-transformation temperature of 450 \ifmmode\pm\else\textpm\fi{} 10\ifmmode^\circ\else\textdegree\fi{}C. The structure of the low-temperature phase was determined to be an hcp (0001) surface layer on an hcp bulk crystal with a top surface-layer spacing equivalent to that of the bulk 2.05 \AA{}. The surface structure of the high-temperature phase was determined to be an fcc (111) surface layer on an fcc bulk crystal with a top surface-layer spacing equivalent to that of the bulk 2.05 \AA{}.

Intensity-voltage ( I - V ) curves for Ni(001) c(2 × 2)-CO are calculated using phase shifts obtained from self-consistent Xα potentials. The CO scattering potentials are constructed such that the distances between C-O and between Ni-C are compatible with the optimal structure determined. The calculated I - V curves are compared with recent data at normal and off-normal incidences. The accuracy of an approximation scheme is tested against a full dynamical calculation for the Ni(001) c(2 × 2)-CO system.

Elastic low-energy electron diffraction (LEED) intensity versus voltage ( I - V ) measurements for the clean Ir(111) surface have been obtained. Seven specular I - V spectra were measured from 15 to 975 eV at incident angles from 7° to 62.5° relative to the surface normal. The outermost atomic layer spacing of the unreconstructed Ir(111) surface was determined both by the convolution-transform method we have presented previously (including certain convenient modifications) and by dynamical calculations. Results from the analysis of the I - V spectra by the convolution-transform method indicate that the outermost Ir(111) layer spacing is either unrelaxed or contracts by 4% of its bulk value depending upon whether the θ=7° data or the θ = 25° data are used. In agreement with this, the dynamical calculations show that the outermost Ir(111) layer spacing contracts by 2.5± 5% and, in addition, that the registry of the first layer of the crystal surface is not shifted, maintaining the fcc structure.

We report an extensive study of the TiW/GaAs interface after rapid thermal annealing between 750 and 1050 °C. Characterization of the interface was performed by secondary ion mass‐spectroscopy (SIMS), Auger, Rutherford backscattering spectroscopy (RBS), x‐ray diffraction, transmission electron microscopy, and electrical measurements. Schottky barrier heights extracted from I‐V and C‐V data show a continuous increase with annealing temperatures up to 950 °C. The reverse I‐V measurements exhibit a transition from tunneling to avalanche breakdown. SIMS, Auger, and RBS show significant motion of the Ti, resulting in surface accumulation, as well as Ti diffusion into the GaAs substrate. Interface doping by Ti produces an artificial enhancement of the Schottky barrier height.

A dynamical LEED analysis is presented of the ( 3 × 3 ) R 30° superlattice on the (111) surface of an α-CuAl alloy single crystal with a bulk Al concentration of 16 at %. This surface ordering contrasts with the lack of long-range order in the bulk. In the ordered surface the Al atoms are found to be subtitutionally arranged within the topmost layer with the topmost layer with (√3×√3)R30° symmetry. No evidence for the ordering of Al in deeper layers is detected by this analysis. The fcc-(111) stacking sequence is maintained at the surface, and the surface atoms occupy positions within ±0.05 A of bulk Cu lattice positions, i.e. no appreciable top-layer spacing change and no mixed-layer buckling are detected.

Complete local-bonding geometries in a disordered molecular overlayer have been determined, together with the relative occupation of different coexisting adsorption sites. Diffuse LEED intensities were measured with a new, highly sensitive, and low-noise, digital-LEED detector, and calculated with an extension of the efficient beam-set-neglect theory. At one-third of a monolayer of carbon monoxide on Pt(111) at 160 K, (88 +- 5)% of the molecules are found to occupy top sites and (12 +- 5)% bridge sites, with bond lengths consistent with earlier results for ordered layers.

The chemisorption bond-length and binding location of the c(2×2) sodium overlayer on Al(001) surface are determined by a convergent perturbation calculation of the dynamical LEED approach. The use of convergent perturbation calculation allows accurate surface structures to be determined with very economical amounts of computation effort. Efficiency features specific to our computation method are outlined. Using a set of simple dynamical inputs, we determine that the sodium atom sits in a 4-fold coordinated binding site with a NaAl chemisorption bond-length of 2.08 ± 0.12 A.

The combination of HREELS (high-resolution electron energy-loss spectroscopy) and LEED (low-energy electron diffraction) has been successfully applied to study complicated coadsorption structures of benzene and CO on various transition metal surfaces. Through these studies, different benzene structures have emerged depending on the substrate metal. Recent work concerns coadsorption of benzene and CO on Pd(111), where an almost gas-phase like benzene structure has been detected.

A method is described for quantitative analysis of low-energy-electron diffraction (LEED) intensity-energy spectra by comparison of experimental spectra with spectra calculated using a dynamic theory of LEED. The method involves a minimization of the variance of the fit between experimental and calculated spectra as a function of the calculational variables, both structural and nonstructural, leading to determination of optimum parameter values. Correlation between the variables is taken into account. The method is applied to extensive, new experimental LEED data for Pt(111). It is shown quantitatively that the variance of the fit depends strongly on the value of a single structural variable, ${d}_{\ensuremath{\perp}}$, the first interlayer spacing, but weakly on the values of three nonstructural variables, ${V}_{0}$, the inner potential, ${V}_{\mathrm{im}}$, the absorption potential, and ${\ensuremath{\Theta}}_{D}$, the Debye temperature. The analysis leads to an optimum value of ${d}_{\ensuremath{\perp}}=2.29$ \AA{}, corresponding to a 1% expansion of the first layer spacing, with an estimated error of \ifmmode\pm\else\textpm\fi{}0.1 \AA{}. This result is in good agreement with the earlier studies of Kesmodel and co-workers.

An Ir(110)-c(2 × 2)O structure has been prepared by adsorbing a half-monolayer of oxygen at room temperature on an unreconstructed (1 × 1)Ir surface stabilized by a quarter-monolayer of randomly adsorbed oxygen. Results of the low energy electron diffraction structural analysis indicate that the ordered oxygen atoms are residing on the short-bridged sites on the (110) surface. The Ir-O interlayer spacing is 1.37 ± 0.05 A, and the bond length is 1.93 ± 0.07 A. The topmost substrate interlayer spacing is found to be 1.33 ± 0.07 A rather than 1.26 ± 0.07 A which is the topmost interlayer spacing of the unreconstructed (1× 1)Ir surface.

Medium-energy electrons are forward focused by surface atoms in the directions along interatomic axes. This allows the surface structure to be obtained. We analyze this method of structure determination with the help of a convergent multiple scattering formalism. The dependence on the atomic number, the interatomic distance and other factors is explored. The method is applied to experimental data for Auger electron emission from layered surface alloys.

A J dependence is revealed in the isotope shifts of the 5d2D3/2, 5/2 to 6p2P3/2 transitions in Ba ions, using collinear fast-beam laser spectroscopy. The experimental ratio of the field shift for the 5d2D3/2 to 6p2P3/2 transition to that for the 5d2D5/2 to 6p2P3/2 transition is 1.0465+or-0.0010. This result is compared with relativistic Hartree-Fock calculations.

Emploi du pompage optique en spectroscopie par faisceau laser, colineaire du faisceau d'ions rapides pour l'etude de l'evolution temporelle des populations d'etats metastables orientes dans un champ magnetique externe. Description theorique de l'effet Hanle differentiel dans le temps, a l'etat fondamental. Application a la mesure des facteurs g d'ions a l'etat fondamental ou metastable; exemple pour le rapport des facteurs g des etats 5d 2 D 3/2 et 5d 2 D 5/2 de Ba +

The energy levels of the nuclei /sup 159/Tm and /sup 157/Er have been studied up to high spins (/sup 61///sub 2/ and /sup 53///sub 2/, respectively) and rotational frequencies (480 and 417 keV, respectively) by in-beam spectroscopic methods. On the basis of blocking arguments, the results show that the second backbending in the yrast band of even-even nuclei near /b N/=90 is due to pi /b h//sub 11/2/ protons.

The (1 × 2) missing-row reconstruction of clean Pt(110) is studied with a new low-energy electron diffraction (LEED) intensity analysis. In order to obtain theoretical intensity-voltage curves in good agreement with experiment, it is necessary to consider distortions from the basic missing-row model in the theoretical treatment. In our calculations, alternating row-pairing and vertical buckling distortions down to the fourth layer are allowed for the first time. Large contractions are seen in the first three layer spacings, and significant distortions are found even in the fourth layer. The distortions are similar to those seen in Ir and Au LEED results as well as those predicted by the embedded atom method (EAM) for Pt. In the optimum geometry, the top-layer spacing contracts by 18%. The atoms in the second layer pair towards the missing rows, in agreement with Ir and Au LEED but in the direction opposite to that predicted by EAM. The R-factors are especially sensitive to the third-layer buckling. The fourth-layer pairing is also towards the missing rows, in agreement with the EAM prediction. It is shown that full dynamical LEED calculations of this kind can be realistically run on a personal computer with suitable coprocessor board.

By means of an efficient dynamical LEED method, the surface structures of W(110) and W(100) faces are examined. It is found that the W(110) surface maintains the bulk structure, despite the possibility for the top tungsten atoms to settle into sites of higher coordination number. The W(100) face exhibits a possible contraction of the top atomic layer by 0.10 ± 0.10 A. These results fit into a trend correlating a top-layer contraction with the absence of close-packing in the top layer, i.e. with the roughness of the surface.

A tabulation is presented of the ordering characteristics of clean and adsorbate-covered single crystal surfaces based on diffraction patterns observed with LEED (Low Energy Electron Diffraction). Over 3000 structures are classified by rotational symmetry of the substrate surfaces, and by important sub-classes which reflect recent directions of LEED studies. These include metallic monolayers, alloy surfaces, organic overlayers, coadsorbed overlayers, physisorbed overlayers, and high-Miller-index (stepped) surfaces. We review the important characteristics of each sub-class, and propose future directions of LEED investigations.

C3 and C4 hydrocarbon (methylacetylene, propylene, and the 2-butenes) adsorption on the Pt(111) face was studied by observing the LEED patterns that formed and by measuring the intensity versus voltage spectra for each structure. Two phases exist for each of these molecules adsorbed on the Pt(111) surface. At low temperatures, the unsaturated C-C group forms a di-σ bond to two Pt atoms. Upon warming to about room temperature, and in the presence of hydrogen for the alkynes, a conversion takes place to an alkylidyne species that is bonded to three Pt atoms and has its C-C bond nearest to the metal substrate oriented perpendicularly to the surface. The butylidyne species is shown to order its ethyl group into an (8×8) or ( 2 3 2 3 )R30° superlattice when the hydrocarbon exposure is increased; this ordering is probably a natural consequence of the steric hindrance among neighboring ethyl groups as the hydrocarbon coverage increases slightly with larger exposures.

Recent studies of the molecular structure and chemical bonding of organic monolayers on metal surfaces are reviewed. Most of the research is concentrated on alkenes and alkynes adsorbed on platinum and rhodium. The applications of low-energy electron diffraction, high-resolution electron energy loss spectroscopy, and thermal desorption spectroscopy provided most of the structural and bonding information in studies that utilized single-crystal metal surfaces. Hydrocarbon monolayers exhibit a rich and diverse structural chemistry that may be correlated to analogous organometallic clusters. Alkylidyne (==C(CH/sub 2/)/sub n/CH/sub 3/) bonding predominates near 300 K on Pt and Rh although a variety of different molecular conformations is found. Perhaps the most pronounced characteristic of the organic monolayers is the temperature-dependent variation of their structure and bonding. The bonding also varies markedly across the periodic table; much stronger for the early transition metals (W, Fe) and much weaker on Ag and Cu than on Pt and Rh. These temperature- and metal-dependent trends are altered to some extent when considering different crystallographic planes for the metal surface. Finally the possible directions that this field of research may take are discussed.

A LEED analysis predicts that hydrogen chemisorbed in a half monolayer on Ni(111) arranges in an overlayer with graphite-layer geometry of (2×2) unit cell, adsorbing in both types of threefold hollow sites with indistinguishable Ni-H bond lengths of 1.84+-0.06 A, corresponding to an overlayer-substrate spacing of 1.15+-0.1 A.

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 coverage is described in terms of a second‐order precursor state model with repulsive interactions. The pre‐exponential factor for desorption (3×1016 sec−1) varies only little with coverage. The dipole moment of the adsorbate remains constant up to Θ?0.35 (0.17 D) where the overlayer starts to order, and exhibits an appreciable higher value near Θ=0.5. The maximum work function increase is Δφmax=0.93 V at Θ=0.5. The differential entropy of the adsorbed layer around 450 K was derived from the experimental adsorption isotherms. Up to Θ=0.35 the data fall well between the limits of two theoretical models for localised and delocalised adsorption of noninteracting particles. Deviations at higher coverages reflect again the onset of ordering. The energetic and work function data are in some variance to results reported earlier which is ascribed to the fact that these quantities may be sensitively influenced by spurious amounts of carbon impurities on the surface.

Thin plates with a thicknessd smaller than the phonon mean-free path can be used to increase the heat exchange in 3 He- 4 He refrigerators at very low temperatures. The resistanceR for the heat current density of these plates is theoretically investigated. For plates withd still larger than the dominant thermal-phonon wavelength λ T in the plate,R is given by twice the Kapitza resistance. Ford ≈ λ T the resistanceR decreases due to phonon tunneling. Ford ≪ λ T there exists a region whereR varies asT −1, in contrast to theT −3 dependence for thicker plates. Numerical calculations are carried out for copper plates immersed in pure 4 He. The results show that one obtains a large increase of the heat exchange due to phonon tunneling for plates with a thickness of a few microns at temperatures in the millidegree range.

A general formalism is developed by means of which the radiative heat transfer between macroscopic bodies of arbitrary dispersive and absorptive dielectric properties can be evaluated. The general formalism is applied to the heat transfer across a vacuum gap between two identical semi-infinite bodies at different temperatures. The peculiarities arising when the gap width is of the order of, or smaller than, the dominant thermal radiation wavelengths are studied and quantitatively evaluated for the case of two metal bodies. The predicted strong increase with diminishing gap width is in qualitative agreement with experimental results.

The lifetime of the Xe II 5p46p 4P5/20 level is measured by selective laser excitation of a fast beam of metastable 5p45d 4D7/2 xenon ions and observations of the fluorescent light intensity as a function of flight time. Comparison of the obtained mean lifetime tau =7.53+or-0.15 ns with earlier measurements confirms the results of the photon-photon delayed-coincidence technique.

Gold was deposited on Pt(100) and platinum was deposited on Au(100) single-crystal surfaces. Cyclohexene dehydrogenation rates were enhanced about fivefold relative to clean Pt(100) by just one monolayer of gold on Pt(100) and also by ordered platinum layers on inactive Au(100).

The structure of the reconstructed Ir(110)-(1 x 2) surface has been analyzed by low-energy electron diffraction. Three models proposed for the reconstructed Ir(110)-(1 x 2) surface — the missing row model, the paired row model and the buckled surface model — were tested. Based on a comparison between experimental data consisting of intensity-voltage profiles for ten half-order beams and eight integral-order beams and the calculated curves, the missing row model with a topmost interlayer spacing of 1.22 ± 0.07 A is the preferred structure.

Molecular adsorbate structures can be determined by combining Low-Energy Electron Diffraction (LEED) with High-Resolution Electron Energy Loss Spectroscopy (HREELS). The HREEL vibrational spectroscopy identifies the molecular species, whose bond lengths and angles are obtained by LEED. Appropriate calculational methods are required in LEED to solve complex molecular structures: such methods will be discussed for large molecules, for large unit cells and for disordered adsorbates. Results have been obtained for the following molecular species adsorbed on several low-Mi11er-index metal surfaces: CO, C2H2, → CCH3, C3H4, → CCH2CH3 and C6H6. The presently available methods should be capable of solving a multitude of molecular adsorbate structures, including large molecules, coadsorbates and disordered species.

It is shown that the diffuse low-energy electron diffraction (LEED) that occurs between sharp LEED beams can be used to determine the local bonding configuration near disordered surface atoms. Two approaches to the calculation of diffuse LEED intensities are presented for the case of lattice-gas disorder of an adsorbate on a crystalline substrate. The capabilities of this technique are most similar to those of near-edge extended x-ray absorption fine structure, but avoid the restrictions due to the use of photons.

The history of surface structure determination with low-energy electron diffraction (LEED) will be briefly reviewed, setting the stage for a discussion of recent and future developments. The aim of these developments is to solve complex and disordered surface structures. Some efficient solutions to the theoretical and experimental problems will be presented. Since the theoretical problems dominate, the emphasis will be on theoretical approaches to the calculation of the multiple scattering of electrons through complex and disordered surfaces.

The adsorption of carbon monoxide on low index metal surfaces has been the object of intense research, presenting some apparently conflicting results. Thus, for three systems, CO on Cu(100), on Cu(111) and on Ru(0001), HREELS or IRS show only one C-O stretching frequency while an interpretation of the LEED diagram with a compact monolayer of CO gives at least two adsorption sites. A reexamination of these systems with a model based on a finite coincidence unit cell and one adsorption site, allows a better interpretation of all the data. Laser simulation of LEED is used to test this interpretation and to determine molecular positions within the larger coincidence unit cells.

Multiple-scattering effects of electrons traveling along atomic chains are shown to be important and useful for surface structure determination, especially in the medium-energy range (200\char21{}1000 eV). This applies to techniques based on diffraction of elastic electrons, such as medium-energy electron diffraction, as well as to techniques based on the angle-resolved detection of ``secondary'' electrons such as photoelectrons, Auger electrons, and other electrons. Two new methods for computing multiple-scattering amplitudes for chains are derived and used to show how multiple scattering along chains of atoms produces focusing, defocusing, amplification, and layer-dependent enhancements that can be put to effective use in structure determination. Strong forward-focusing peaks along internuclear axes are dominant under many conditions: this is a single-scattering effect. However, multiple scattering can totally defocus such forward-focused peaks, while other interference effects produce additional scattering peaks that do not have the direction of an internuclear axis. In addition, it follows that surface composition analysis by conventional Auger-electron spectroscopy can be quite sensitive to the incident direction of the electron beam used to produce Auger emission.

An Ir(110) – (1×1) surface has been prepared by adsorbing 1/4 monolayer of oxygen at 850 K on a clean, reconstructed (1×2) surface. Good agreement was achieved between theoretical and experimental intensity–voltage (I–V) spectra for a surface model in which the oxygen is randomly and uniformly distributed over the cyrstal surface, and the (1×1) structure is the same as for a clean unreconstructed (1×1) surface with a topmost interlayer spacing of 1.26±0.05 A. Three different models have been tested for the reconstructed Ir(110) – (1×2) surface, namely, the missing‐row model, the paired rows model, and the buckled surface model. Based on the comparison between experimental data (consisting of ten half‐order beams and eight integral‐order beams) and the theoretical calculations, the mising‐row model with a topmost interlayer spacing of 1.22±0.07 A is the most satisfactory model tested.