Search

Hamiltonians for a particle on a manifold in a magnetic field are constructed as Bochner Laplacians. We show for the case of a torus and a given magnetic field that they are in one to one correspondence with the constituents in the Bloch decomposition of the unique Hamiltonian on the universal covering.

The structural parameters of the R30° overlayer formed by N 2 adsorption on Ru(0001) have been determined by a LEED IV analysis. The N 2 molecules are coordinated to the surface at on-top sites with the interatomic axis perpendicular to the surface. The NN bond length turns out to be 1.10±0.04 A which equals the bond length of gaseous N 2 . The interaction of N 2 with Ru(0001) is characterized by a lack of pronounced substrate distortions.

A detailed low energy electron diffraction analysis has been performed to investigate the adsorption of potassium on Al(111) at 90 K and at 300 K. At each temperature a (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} structure is formed, however, the adsorbate positions are different. For absorption at 90 K the adatoms occupy on-top sites and at 300 K they occupy substitutional sites, both geometries have hitherto been regarded as very unusual. An irreversible phase transition from the former to the latter stable structure occurs on warming to 300 K. The obtained interatomic distances and surface relaxations are discussed in terms of an interplay of the electrostatic repulsion between adatoms and substrate screening as well as coordination number. An approximate scheme for including the effect of anisotropic atomic motions under the influence of thermal excitation was applied. It is found that the R factors for the on-top site are significantly improved whereas no improvement is found for the substitutional site.

Sodium adsorbed on the Ru(0001) surface below room temperature forms a number of ordered phases, commensurate as well as incommensurate. They are investigated by low-energy electron-diffraction (LEED) and work-function measurements (Δφ). The structural analyses for the p(2 × 2)-, (√3 × √3)R30°- and (3 × 3)-phases at coverages of 1 4 , 1 3 and 4 9 , respectively, reveal coverage-dependent adsorption sites. As for K adsorbed on Ru(0001), Na atoms occupy threefold fcc-sites in the p(2 × 2) phase while for the (√3 × √3)R30° phase the hcp site is favoured. A structural analysis of the (3 × 3) phase, including 4 adsorbate atoms within the unit cell, revealed that nearest-neighbour distances are maximized at the cost of occupying low symmetry sites. This conclusion is supported by comparison of relative theoretical and experimental adsorbate-induced spot intensities. For all three phases the Na hard-sphere radii are nearly the same and close to the covalent Pauling radius.

Metal oxidation is often unwanted--for example, in the case of corrosion--but under controlled conditions the oxidation of metals can be beneficial for the manufacture and performance of advanced materials9. In their Perspective, Over and Seitsonen provide a brief overview of the complex oxidation behavior of metal surfaces. They highlight the report by ThA¼rmeret al., who demonstrate the crucial role played by surface impurities in the oxidation of lead crystallites.

First principles-based kinetic Monte Carlo (kMC) simulations are performed for the CO oxidation on RuO(2) (110) under steady-state reaction conditions. The simulations include a set of elementary reaction steps with activation energies taken from three different ab initio density functional theory studies. Critical comparison of the simulation results reveals that already small variations in the activation energies lead to distinctly different reaction scenarios on the surface, even to the point where the dominating elementary reaction step is substituted by another one. For a critical assessment of the chosen energy parameters, it is not sufficient to compare kMC simulations only to experimental turnover frequency (TOF) as a function of the reactant feed ratio. More appropriate benchmarks for kMC simulations are the actual distribution of reactants on the catalyst's surface during steady-state reaction, as determined by in situ infrared spectroscopy and in situ scanning tunneling microscopy, and the temperature dependence of TOF in the from of Arrhenius plots.

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 today's 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.

Quantitative low-energy electron-diffraction (LEED) IV spectra analyses of Ag, Li, and metastable clean (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30 \ifmmode^\circ\else\textdegree\fi{} Si(111) reveal that striking similarities in the LEED spectra are due to common scattering blocks of substrate atoms while the metal atoms are adsorbed at open sites on the Si lattice. This leads to the use of LEED IV spectra as a characterization method. For the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30 \ifmmode^\circ\else\textdegree\fi{} phases of Li and Ag on Si(111), it is found that both structures can be described by a modified honeycomb-chained-trimer model. The main structural element consists of Si trimers and Si atoms in the deeper reconstructed layers. A counterexample is provided by Si(111)-(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30 \ifmmode^\circ\else\textdegree\fi{}-Au for which the IV spectra are very different from those of the other three systems because the common scattering block is absent.

A quantitative low-energy electron-diffraction (LEED) analysis of the Rh(110)-(2 × 1)pg-O structure revealed a model which consists of O zigzag chains with oxygen located in threefold-coordinated sites on an otherwise undistorted Rh(110) surface. Due to the strong interaction of oxygen with Rh the contraction of the first Rh interlayer spacing (observed for the clean surface) is globally removed. LEED investigations on the (2 × 2)pg-O structure prove the presence of a missing-row reconstruction of the Rh substrate.

The crystallographic aspects of the growth of ultrathin films of Fe on Au[l brace]001[r brace] are studied by means of quantitative low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The first monolayer of Fe is found to be pseudomorphic to the Au[l brace]001[r brace] substrate with an interlayer spacing of 1.825 A, and to be covered by a monolayer of Au at an interlayer distance of 1.85 A, which acts as a surfactant. Thicker films of Fe [up to 45 layer equivalents (LE)] may be pseudomorphic and hence slightly strained (the lattice misfit is 0.6%) or equilibrium bcc with surface relaxation (the present analysis cannot distinguish between the two possibilities), but contain defects and disordered steps. Gold atoms are found to be segregated in the surface region of 45-LE Fe films, but mostly in a disordered arrangement. A comparison between the results obtained with two different computer programs for the calculation of LEED intensities finds that differences of 0.025 A in the values of structural parameters may occur.

It is found that the adsorption of potassium on Al(111) at 90 K and at 300 K both result in a (√3 × √3)R0° structure. Through a detailed LEED analysis it is revealed that at 300 K the adatoms occupy substitutional sites and at 90 K the adatoms occupy on-top sites; both geometries have hitherto been considered as very unusual. The relationship between bond length and coordination is discussed with respect to the present results, and with respect to other quantitative studies of alkali-metal/metal adsorption systems.

We present the first comprehensive discussion of automatic optimization procedures for surface-structure determination by LEED. These procedures 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 today's workstations. We list over two dozen structures so determined, many as yet unpublished.

The structure of the √3 × √3 R30° overlayer formed by CO molecules adsorbed on a Ru(0001) was analyzed by low energy electron diffraction. Anisotropic atomic motions under the influence of thermal excitation were taken into account by adopting the concept of split positions. Apart from considerable improvement in the structural refinement this technique provides information about dynamic processes. In particular, the molecular axis of the CO molecules was found to be tilted on the average by (12±3° at 150 K, which is attributed to excitation of the bending mode vibration (i.e., frustrated translation).

Structure–activity experiments were performed for the HCl oxidation reaction (Deacon-like process) over RuO2 model catalysts – RuO2(1 1 0) and RuO2(1 0 0) – applying in situ surface X-ray diffraction (SXRD) combined with on-line mass spectrometry. The studied model catalysts turned out to be long-term stable under reaction conditions with gas feed ratios p(HCl):p(O2) ranging from 1:4 to 4:1 in the mbar pressure regime and temperatures as high as 685 K. Even pure HCl exposure in the mbar regime was not able to reduce RuO2 below 600 K; above 650 K chemical reduction of the oxide sets in. Under strongly oxidizing reaction conditions, the (surface) oxides grow slowly in thickness. On-line reactivity experiments of both types of model catalysts in a batch reactor yield a mean turn-over frequency (TOF) of 0.6 Cl2 molecules per second and active site for the HCl oxidation at 650 K and initial partial pressures of p(HCl) = 2 mbar and p(O2) = 0.5 mbar. The HCl oxidation over RuO2 is therefore considered to be structure insensitive.

We use density functional theory (DFT) calculations to study a novel Deacon-like process over pure RuO2(110) and TiO2-supported RuO2(110), a commercial process which was recently introduced by Sumitomo Chemical. During the HCl oxidation reaction the surface becomes chlorinated, which makes this catalyst stable under the harsh reaction conditions. The reaction mechanism for the chlorination of unsupported RuO2(110) proceeds via water formation in the bridging position and the final replacement of this water species by chlorine. The actual HCl oxidation reaction takes place via a one-dimensional Langmuir-Hinshelwood mechanism. The recombination of on-top chlorine atoms to form the desired product Cl2 is identified as the rate-determining step with an activation energy E a of 114 kJ/mol. DFT calculations predict that one monolayer of RuO2(110) supported on TiO2(110) reveals catalytic activity which is almost as high as that on the unsupported RuO2(110) (E a = 124 kJ/mol). Further reduction of the Ru loading to half of a monolayer still keeps the TiO2(110)-RuO2(110) as a good catalyst for the Deacon process, with an activation barrier of the rate-determining step of 29 kJ/mol higher than at pure RuO2(110).

The two ordered phases p(2 × 2) at a coverage θ = 0.25 and (√3 × √3)R30° at θ = 0.33 of potassium adsorbed on Ru(0001) were analyzed by use of low-energy electron-diffraction (LEED). In the (√3 × √3)R30° phase, the K atoms occupy threefold hcp sites, while in the p(2 × 2) phase the fcc site is favoured. In both phases, the K hard-sphere radii are nearly the same and close to the covalent Pauling radius.

The adsorption of potassium on Al(111) at 90 K and at 300 K has been investigated by low-energy electron diffraction (LEED). Although a (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} structure is formed at each temperature, a detailed LEED analysis has revealed that the adsorbate positions are quite different and unusual in each case. At 90 K the adatoms occupy on-top sites and at 300 K they occupy substitutional sites. An irreversible phase transformation from the former to the latter structure occurs on warming to 300 K. These results are discussed in the light of recent density-functional-theory calculations.

The structure of RuO 2 (110) and the mechanism for catalytic carbon monoxide oxidation on this surface were studied by low-energy electron diffraction, scanning tunneling microscopy, and density-functional calculations. The RuO 2 (110) surface exposes bridging oxygen atoms and ruthenium atoms not capped by oxygen. The latter act as coordinatively unsaturated sites—a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces—onto which carbon monoxide can chemisorb and from where it can react with neighboring lattice-oxygen to carbon dioxide. Under steady-state conditions, the consumed lattice-oxygen is continuously restored by oxygen uptake from the gas phase. The results provide atomic-scale verification of a general mechanism originally proposed by Mars and van Krevelen in 1954 and are likely to be of general relevance for the mechanism of catalytic reactions at oxide surfaces.

The electric field governing the dynamics of space charge produced by high intensity femtosecond laser pulses focused on a copper surface is investigated by time-resolved low-energy-electron-scattering. The pump-probe experiment has a measured temporal resolution of better than 35 ps at 55 eV probe electron energy. The probe electron acceleration due to space charge is reproduced within a 3-dimensional non-relativistic model, which determines an effective number of electrons in the space charge electron cloud and its initial diameter. Comparison of the simulations with the experiments indicates a Coulomb explosion, which is consistent with transients in the order of 1 ns, the terminal kinetic energy of the cloud and the thermoemission currents predicted by the Richardson-Dushman formula., 10 pages, 4 figures

The structure of the clean Co(1010) surface has been analysed by LEED. Application of a recently developed computational scheme reveals the prevalence of the termination A in which the two topmost layers exhibit a narrow spacing of 0.62 A, corresponding to a 12.8(±0.5)% contraction with respect to the bulk value, while the spacing between the second and third layer is slightly expanded by 0.8(±0.2)%.

Experiments are discussed wherein scattering of excitonic polaritons with acoustic phonons between different polariton modes can be observed. Information about this dissipation process is given by the luminescence of deep impurities acting as microscopic detectors within the crystal which are sensitive to the distribution of energy to the different polariton branches. The calculated intensity for an ensemble of ideal micro-detectors containing constant quantum yield and absorption probability agrees excellently with measurements. Es werden Experimente vorgestellt, in denen die fur die Dissipation verantwortliche Streuung von exzitonischen Polaritonen an akustischen Phononen beobachtet wird. Die Detektion des gestreuten Lichtes erfolgt anhand der Lumineszenz tiefer Storstellen. Dabei stellen die betracteten Zentren mikrosk opische Detektoren der, die empfindlich die Verteilung der Energie auf die Verschiedenen Polaritonenmoden registrieren. Die fur ein Ensemble von idealen Mikro-Detektoren, unter Berucksichtigung einer konstanten Quantenausbeute und einer konstanten Absorptionswahrscheinlichkeit, berechnete Intensitatsverteilung stimmt hervorragend mit Messungen uberein.

The An=1-exciton in CdS can hardly be excited by light of polarisation E ∥ c. By simple rotating the sample connected with an increasing of the transition probability the energy transfer between excitons and impurities can widely be changed. The measured excitation spectra of the integrated luminescence (720 nm < λ < 900 nm) are compared with the absorption, thereby unintentionally doped and thin crystals are used. Das An=1-Exziton in CdS ist fur Licht der Polarisation E ∥ c nur schwach anregbar. Durch einfaches Drehen der Probe, verbunden mit einer Vergroserung der Ubergangswahrscheinlichkeit, kann der Energieubertrag des Exzitons an die Storstellen in weiten Grenzen variiert werden. Die gemessenen Anregungsspektren der integralen Lumineszenz zwischen 720 und 900 nm an nicht vorsatzlich dotierten, dunnen Kristallen werden quantitativ mit der Absorption verglichen.

The decomposition of borazine (HBNH)3 precursor molecules on the hot rhodium (111) surface leads to the spontaneous formation of a highly regular hexagonal structure with a periodicity of 3.2 nm and with the appearance of a mesh with pores of about 2 nm in diameter: the so-called nanomesh [1]. This nanomesh is identified as a single, complete monolayer [2,3]. The 2 nm sized pores are formed by regions where the layer binds strongly to the underlying metal, while the regular network of mesh wires represents regions where the layer is not bonded to the substrate, resulting in a corrugation of about 0.5A in the layer [2]. The nanomesh exhibits a remarkable thermal stability and chemical inertness: it is robust against immersion in water and electrolyte solution [3]. Moreover, it shows the functionality of a regular array of trapping potentials as it is demonstrated by the stable and site selective adsorption of different kinds of planar molecules as observed in scanning tunneling microscopy and ultraviolet photoelectron spectroscopy [3]. Depending on the size of the molecules and the molecular coverage, individual molecules or molecular assemblies are trapped in the pores.

Buckling nanopatterns of monoatomic layer 2D materials on metal substrates attract significant attention due to their rich interface morphology affecting electronic applications. An experimental–theoretical study of a 2D boron–nitrogen–carbon (B x /2N x /2C1−x ) alloy on a Ru(0001) surface is conducted and a profound relation between the composition x and the degree of buckling is discovered. Experimentally, real carbon–boron–nitrogen alloys on the Ru(0001) surface are demonstrated and various morphologies of pure and mixed compounds are shown. Density functional theory calculations are further carried out using the supercells of graphene, hexagonal boron nitride (h-BN), and random BNC on Ru(0001), as well as Monte Carlo simulations for elucidating the kinetics of their growth. The results show that unlike pure compounds (h-BN or C), the carbon–boron–nitrogen mix on Ru(0001) mostly exists in an uncorrugated form, thus greatly improving the interface contact. The likely cause of the diminished corrugation is a softening of bond angular interactions in the alloy relative to the pure phases.

The reduction mechanism of the RuO(2)(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO(2)(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (O(br)-H) species and subsequently part of these O(br)-H groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.

Rhodium grows on single-crystal Ru(0001) surface initially layer by layer, adapting the in-plane lattice parameters of Ru(0001) (pseudomorphic growth). Using quantitative low-energy electron diffraction (LEED) and density functional theory (DFT) calculations, we determined the atomic structures of 1-monolayer (ML), 2-ML, and 3-ML Rh films on Ru(0001). The 1-ML Rh film continues the hexagonal-close-packed (hcp) stacking sequence of Ru(0001) (...ABAb), while the 2-ML Rh film already adopts its face-centered-cubic (fcc) stacking sequence (...ABAbc) if the topmost Ru layer is included into the stacking consideration. Three MLs of Rh form a fcc stacking sequence of ...ABAbac, requiring a massive restructuring of the second Rh layer when adding one ML of Rh onto the 2-ML Rh film.

RuO2(110) exposes two kinds of active surface species (acidic and basic centers) that govern the interaction of the gas phase in contact with the catalyst's surface. Here we will elucidate the cooperative interplay of these two active surface sites for a simple model reaction, namely the water formation over RuO2 catalysts when supplying hydrogen and oxygen from the gas phase. The bridging O atoms harvest the hydrogen from the gas phase, while the on-top O atoms pick up those adsorbed hydrogen atoms from the bridging O atoms to form water. This mechanism of hydrogen transfer is mediated by a strong hydrogen bond. Hydrogen transfer is expected to play a vital role for the whole class of catalyzed hydrogenation and dehydrogenation reactions of hydrocarbons over RuO2.

The visualization of surface reactions on the atomic scale provides direct insight into the microscopic reaction steps taking place in a catalytic reaction at a (model) catalyst's surface. Employing the technique of scanning tunneling microscopy (STM), we investigated the CO oxidation reaction over the RuO2(110) and RuO2(100) surfaces. For both surfaces the protruding bridging O atoms are imaged in STM as bright features. The reaction mechanism is identical on both orientations of RuO2. CO molecules adsorb on the undercoordinated surface Ru atoms from where they recombine with undercoordinated O atoms to form CO2 at the oxide surface. In contrast to the RuO2(110) surface, the RuO2(100) surface stabilizes also a catalytically inactive c(2 x 2) surface phase onto which CO is not able to adsorb above 100 K. We argue that this inactive RuO2(100)-c(2 x 2) phase may play an important role in the deactivation of RuO2 catalysts in the electrochemical Cl2 evolution and other heterogeneous reactions.

RuO(2)-based catalysts are much more active in the oxidation of CO than related metallic Ru catalysts. This high catalytic activity (or low activation barrier) is attributed to the weak oxygen surface bonding of bridging O atoms on RuO(2)(110) in comparison with the strongly chemisorbed oxygen on Ru(0001). Since the RuO(2)(110) surface is able to stabilize an even more weakly bound on-top oxygen species, one would anticipate that the catalytic activity will increase further under oxidizing conditions. We will show that this view is far too simple to explain our temperature-programmed reaction experiments, employing isotope labeling of the potentially active surface oxygen species on RuO(2)(110). Rather, both surface O species on RuO(2)(110) reveal similar activities in oxidizing CO.

The Council for Competitiveness in Washington D.C. classifies catalysis as one of the technologies that is critical to international competitiveness of the U.S. economy. Due to the strong chemical industry in Germany this statement is equally valid for the German economy. So far, efficient catalysts are designed by chemical and engineer’s intuition [1]. However, the development of future and more efficient catalysts is considered to rely on atomic-scale tailored materials. To accomplish this goal a close cooperation between experimentalist, theorists as well as engineers is mandatory. It requires first of all a detailed microscopic description of catalytic reactions on the atomic scale as it is accessible by large-scale density-functional [2,3,4] calculations. During the past few years, this concept has been pursued by the Topsoe company in collaboration with the universities in Lyngby and Aarhus [5].

The adsorption sites of CO on the reduced ${\mathrm{RuO}}_{2}(110)$ surface were determined by employing the techniques of density-functional theory calculations and quantitative low-energy electron diffraction. On the mildly reduced ${\mathrm{RuO}}_{2}(110)$ surface, where the bridging oxygens are removed, twofold (2f) undercoordinated Ru atoms [2f- coordinatively unsaturated site (cus) Ru] are exposed. On this surface, the CO molecules initially adsorb above these 2f-cus Ru atoms in symmetric bridge positions. The CO coordinated 2f-cus Ru atoms are drawn towards the CO molecule by 0.2 \AA{}. With increasing CO coverage, the CO adsorption site changes from the symmetric to the asymmetric bridge position. When all 2f-cus Ru atoms have been capped by bridging CO molecules, the 1f-cus Ru atoms are also progressively occupied by on-top CO. For comparison, on the stoichiometric ${\mathrm{RuO}}_{2}(110)$ surface, CO molecules adsorb exclusively on top of the onefold coordinatively unsaturated Ru atoms (1f-cus Ru) at temperatures below 200 K. The energetics of the CO adsorption on the reduced ${\mathrm{RuO}}_{2}(110)$ surface compares favorably with the corresponding thermal-desorption spectrum.

The Rh(110)-c(232)-CN phase has been examined by means of scanning tunneling microscopy ~STM! and full dynamical low-energy electron diffraction ~LEED!. From STM large c(232) domains are observed. The detailed LEED-IV structural analysis indicates that CN is located in the grooves of the ~110! surface, approximately atop second layer rhodium atoms. The CN molecules lie almost flat with their bond axes oriented perpendicular to the rhodium troughs. An outward relaxation of the first substrate interlayer distance and a strong buckling of the second Rh layer are induced by CN adsorption. Calculated and experimental intensity curves are in good agreement. An exhaustive set of other possible adsorption sites and configurations was tested and excluded on the basis of reliability-factor analysis.

Exposing water to a (2 x 2)-O precovered Pt(111) surface at 100 K and subsequently annealing at 155 K led to the formation of a well-ordered (square root 3 x square root 3)R30 degrees overlayer. The structure of this overlayer is determined by DFT and full dynamical LEED calculations. There are two O containing groups per (square root 3 x square root 3)R30 degrees unit cell and both occupy near on-top positions with a Pt-O bond length of (2.11 +/- 0.04) A. DFT calculations determined the hydrogen positions of the OH species and clearly indicate hydrogen bonds between the neighboring adsorbed OH groups whose interaction is mainly of electrostatic nature. A theoretical comparison with H(2)O shows the hybridization of OH on Pt(111) to be sp(3).

Low-energy electron diffraction and density-functional theory calculations are used to examine the adsorption properties of CO and ${\mathrm{N}}_{2}$ on ${\mathrm{RuO}}_{2}(110).$ Both molecules adsorb over the coordinatively unsaturated Ru sites (cus-Ru atoms) with their molecular axes normal to the surface plane. The chemisorption mechanism is well described within a donor-acceptor model, i.e., the Blyholder model. Since ${\mathrm{N}}_{2}$ is not reacting with lattice oxygen of ${\mathrm{RuO}}_{2},$ quite in contrast to CO, ${\mathrm{N}}_{2}$ may serve as a chemical, nondestructive probe to titrate but also to selectively block the cus-Ru atoms; recently, the cus-Ru atoms were shown to be the active centers for the chemisorption of molecules.

The electrochemical uptake of oxygen on a Ru(0001) electrode was investigated by electron diffraction, Auger spectroscopy, and cyclic voltammetry. An ordered (2 × 2)-O overlayer forms at a potential close to the hydrogen region. At +0.42 and +1.12 V vs Ag/AgCl, a (3 × 1) phase and a (1 × 1)-O phase, respectively, emerge. When the Ru electrode potential is maintained at +1.12 V for 2 min, RuO2 grows epitaxially with its (100) plane parallel to the Ru(0001) surface. In contrast to the RuO2 domains, the non-oxidized regions of the Ru electrode surface are flat. If, however, the electrode potential is increased to +1.98 V for 2 min, the remaining non-oxidized Ru area also becomes rough. These findings are compared with O overlayers and oxides on the Ru(0001) and Ru(1010) surfaces created by exposure to gaseous O2 under UHV conditions. On the other hand, gas-phase oxidation of the Ru(1010) surface leads to the formation of RuO2 with a (100) orientation. It is concluded that the difference in surface energy b...

We have studied structural, electronic and energetical aspects of ordered overlayers which were prepared by depositing Cs and CO on the (2 x 2)-O-precovered Ru(0001) surface. This ternary system may serve as a model system to study the Cs promoted CO oxidation reaction over Ru(0001), The electronic properties of the Cs subsystem, in particular the delocalized character of the Cs-6 s state, are only little affected by the (2 x 2)-O overlayer, while the addition of CO leads to a demetallization of the Ca overlayer. The ternary system reveals two metastable configurations (besides a stable one) which are produced at low sample temperatures, In the first one, Cs and CO have changed their adsorption sites if compared to the stable ternary (2 x 2)-O+Cs+CO phase, i.e. Cs sits in hcp and CO in on-top position. Our studies emphasize the importance of the threefold hollow position of CO to strengthen the back bonding of CO. The surface structure of the second metastable (ternary) arrangement is still elusive. (C) 2000 Elsevier Science Ltd. All rights reserved.

Energy transfer processes from excitons to deep impurities are investigated by luminescence excitation and absorption spectroscopy. The measured dips and peaks in the excitation spectra indicate a strong competition between different deep centre transitions and are connected with the annihilation mechanism of excitonic polaritons. They can be captured directly by deep impurities from the photonic or excitonic part of the polariton branches, respectively. In addition excitonic polaritons can be absorbed by deep centres after having undergone a multistep process: a polariton can be scattered to another branch and thereby converted from a photon-like to an exciton-like polariton via acoustic phonons. This multistep process is verified by comparing the calculated excitations spectra using the dielectric function and the damping dispersion of excitonic polaritons with our experiments.

The variation of the Cs 6s and the Cs 5p emission in He* and Ne* metastable deexcitation spectroscopy (MDS) as a function of the CO exposure indicates a demetallization of the Ru(0001)-(2x2)-Cs and the Ru(0001)-(root 3x root 3)R30 degrees-Cs surfaces upon CO coadsorption. This observation corroborates a (substrate-mediated) charge transfer from the Cs atom to the 2 pi* orbital of CO. With the Ru(0001)-(2x2)-Cs system even at CO saturation, MD spectra show emission associated with the Cs 6s state, indicating that the Cs atoms are not completely ionized, Exposing the (root 3x root 3)R30 degrees-Cs-pre-covered Ru(0001) to CO, surplus Cs of the first layer is displaced into a second layer In this way, CO molecules are able to be accommodated into the first layer, Desorbing this second layer Cs by heating the sample to 600 K produces a (2x2) structure with one Cs and CO in the unit cell as evidenced by MDS and low energy electron diffraction.

Physical review letters 80(18), 3980 - 3983 (1998). doi:10.1103/PhysRevLett.80.3980, The structure of the (3×1) reconstructions of the Si(111) and Ge(111) surfaces induced by adsorption of alkali metals has been determined on the basis of surface x-ray diffraction and low-energy electron diffraction measurements and density functional theory. The (3×1) surface results primarily from the substrate reconstruction and shows a new bonding configuration consisting of consecutive fivefold and sixfold Si (Ge) rings in ⟨1¯10⟩ projection separated by channels containing the alkali metal atoms., Published by APS, College Park, Md.

The reconstruction of the Si(001)-2×1 surface consists of asymmetric and buckled Si dimers. The vertical separation between the up and the down atom within the dimer is about 0.72±0.05 A and the dimer bond length of 2.24±0.08 A has been found to be slightly smaller than the Si-Si distance in the bulk. The tilt of the dimer is 19±2°. The formation of Si dimers induces pronounced distortions in the substrate that were detectable down to the fifth Si layer. The structure determination is based on two independent low-energy electron-diffraction data sets taken in two different laboratories. The structural results agree well within the error limits, though noticeable differences occur between the experimental data sets. These differences in the experimental data can possibly be attributed to different preparation procedures.

The order-disorder phase transition of the Rh(lll)-(root 3 x root 3)R30 degrees-CO phase was investigated by He scattering. The critical temperature turned out to be 330+/-5 K, while the critical exponents are consistent with a phase transition belonging to the 3-state Potts universality class. The temperature dependence below the critical temperature of both CO phases was studied using elastic He scattering and LEED. These measurements demonstrate that a simple Debye-Waller evaluation of the diffraction data is not appropriate to provide information on the vibrational properties of the CO molecules. From LEED measurements at low temperatures (70 K) and various CO partial pressures there is evidence of the formation of a disordered second CO layer in equilibrium with the gas phase. The isosteric heat of CO adsorption in the second layer is about 6 kJ mol(-1).

The local adsorption geometries of the (2x2)-N and the (sqrt(3)x sqrt(3))R30^o -N phases on the Ru(0001) surface are determined by analyzing low-energy electron diffraction (LEED) intensity data. For both phases, nitrogen occupies the threefold hcp site. The nitrogen sinks deeply into the top Ru layer resulting in a N-Ru interlayer distance of 1.05 AA and 1.10 AA in the (2x2) and the (sqrt(3)x sqrt(3))R30^o unit cell, respectively. This result is attributed to a strong N binding to the Ru surface (Ru--N bond length = 1.93 AA) in both phases as also evidenced by ab-initio calculations which revealed binding energies of 5.82 eV and 5.59 eV, respectively., 17 pages, 5 figures. Submitted to Chem. Phys. Lett. (October 10, 1996)

The asymmetry of the Ge dimer in the (2\ifmmode\times\else\texttimes\fi{}1) reconstruction of Ge(100) is removed upon adsorption of deuterium D. The $R$-factor analysis indicates a slight remaining asymmetry which is attributed to the coexistence of bare and D-covered dimers. The Ge-Ge bond length of 2.4(2) \AA{} in the dimer does not change within the error limits when compared to the clean surface. The D atoms bond on top of the Ge atoms, exhibiting a Ge-D bond length of 1.6(2) \AA{}.

We use a multilayer lattice gas model for adsorption and desorption to analyze and simulate desorption data for Li and Cs on Ru(0001) extracting surface binding energies and lateral interactions. The latter are repulsive for the first layer and attractive for subsequent ones. \textcopyright{} 1996 The American Physical Society.

The epitaxial growth of incommensurate Mg layers on a Ru(0001) surface is investigated in the coverage range from submonolayers to 3 ML by analyzing low-energy electron-diffraction LEED I(V) data. For the analysis of the 2-ML Mg film, we developed an efficient approximation scheme that allows the determination of mean interlayer spacings without employing the full unit cell. The Mg-Ru spacing is found to be 2.32\ifmmode\pm\else\textpm\fi{}0.05 \AA{}, regardless of the presence of further Mg layers above. The Mg-Mg layer spacing in the Mg bilayer is 5%, expanded with respect to the value of the bulk material, while this layer spacing is expanded by only 2.5% after completion of the third Mg layer. The ABAB... stacking sequence is established from the beginning of the film growth. \textcopyright{} 1996 The American Physical Society.

We present a detailed account of the concept of ``split positions'' in low-energy electron diffraction (LEED). This allows us to study the dynamics of adsorbate particles on surfaces in addition to structural information usually provided by LEED. Enhanced lateral movements of adsorbed atoms and molecules are simulated by displacing them from their equilibrium position by an amount respresenting the mean-square deviation. Within the harmonic-oscillator approximation, the mean-square deviation is closely related to the eigenfrequencies of the vibrational modes which can be compared to values obtained by (standard) spectroscopic techniques. Several examples of adsorbate overlayers on metal and semiconductor surfaces will demonstrate the merits and potentiality of this approach.

The adsorption geometries of the p(2\ifmmode\times\else\texttimes\fi{}2) and (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} Rb/Ru(0001) phases at coverages of 0.25 and 0.33, respectively, were investigated using low-energy electron diffraction. The structure analyses revealed that Rb resides in the threefold fcc site in the p(2\ifmmode\times\else\texttimes\fi{}2) phase while for the (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} phase the hcp site is favored. The effective radius of Rb does not change with the Rb coverage within the error bars of this analysis. The concept of ``split positions'' has been used in order to take the enhanced motion of the adsorbate atoms parallel to the surface properly into account.

Although the number of dynamical low-energy-electron-diffraction (LEED) calculations that need to be performed in the linear LEED technique scales linearly with the number N of structural parameters, the number of combinations of the calculated intensities that need to be compared with experiment scales exponentially with N, and could be a serious limitation when N is large. In this paper, we suggest an algorithm for overcoming this residual exponential scaling, thus allowing linear LEED to achieve its true potential for linear scaling. We demonstrate our algorithm with model calculations on the Si(111)(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}-Al surface.

The dynamic behavior of surface accommodated chlorine atoms on RuO(2)(110) was studied by a variety of experimental methods including high resolution core level shift, thermal desorption-, and in situ infrared spectroscopy as well as in situ surface X-ray diffraction in combination with state-of-the-art density functional theory calculations. On the chlorinated RuO(2)(110) surface the undercoordinated oxygen atoms have been selectively replaced by chlorine. These strongly bound surface chlorine atoms shift from bridging to on-top sites when the sample is annealed in oxygen, while the reverse shift of Cl from on-top into bridge positions is observed during CO exposure; the vacant bridge position is then occupied by either chlorine or CO. For the CO oxidation reaction over chlorinated RuO(2)(110), the reactant induced site switching of chlorine causes a site-blocking of the catalytically active one-fold coordinatively unsaturated (1f-cus) Ru sites. This site blocking reduces the number of active sites and, even more important, on-top Cl blocks the free migration of the adsorbed reactants along the one-dimensional 1f-cus Ru rows, thus leading to a loss of catalytic activity.