Showing papers by "Fritz Haber Institute of the Max Planck Society published in 1990"
TL;DR: A complete atomic model for bacteriorhodopsin between amino acid residues 8 and 225 has been built and suggests that pK changes in the Schiff base must act as the means by which light energy is converted into proton pumping pressure in the channel.
2,772 citations
TL;DR: Spectroscopy and spin-Polarized STM Reference LNS-ARTICLE-1990-002 provide new insights into the determinants of spin- polarization in the Higgs boson.
Abstract: Keywords: Surface Electronic and Atomic Structure ; Spectroscopy ; Spin-Polarized STM Reference LNS-ARTICLE-1990-002doi:10.1103/PhysRevB.42.9307 Record created on 2009-04-14, modified on 2017-05-12
1,123 citations
TL;DR: Laterally varying surface concentrations associated with the oscillatory oxidation of carbon monoxide on a Pt(110) surface were imaged by photoemission electron microscopy and a large variety of spatiotemporal patterns were observed.
Abstract: Laterally varying surface concentrations associated with the oscillatory oxidation of carbon monoxide on a Pt(110) surface were imaged by photoemission electron microscopy. Depending on the applied conditions, a large variety of spatiotemporal patterns were observed that are characteristic for the nonlinear dynamics of reaction-diffusion systems.
601 citations
TL;DR: In this article, a two-dimensional precipitation from a dilute mixed fluid into a solid phase rather than a solid-solid transformation is described, which is more appropriately described to be of ''added-row'' rather than of ''missing-row''.
Abstract: Scanning tunneling microscopy investigations on nucleation and growth of the (2\ifmmode\times\else\texttimes\fi{}1)O-Cu(110) structure revealed that the new phase is formed by condensation of mobile chemisorbed O atoms with Cu adatoms evaporating from steps and diffusing across the terraces of the substrate surface. This process can be regarded as two-dimensional precipitation from a dilute mixed fluid into a solid phase rather than a solid-solid transformation. The resulting structure is more appropriately described to be of ``added-row'' rather than of ``missing-row'' type.
415 citations
TL;DR: The presence of a surface photovoltage seriously affects the determination of surface band bending by photoelectron spectroscopy.
Abstract: Photoelectron spectra from metal overlayers on GaP(110) show that the photoionization light source may induce a surface photovoltage, causing an energy shift of valence- and core-level peaks. We analyze the dependence of this surface photovoltage on metal coverage, substrate doping, and temperature. The presence of a surface photovoltage seriously affects the determination of surface band bending by photoelectron spectroscopy, a technique which is generally thought to reflect the equilibrium electronic structure of metal-semiconductor interfaces.
208 citations
TL;DR: In this article, the authors focused on the oscillatory catalytic reactions at single-crystal surfaces and explored the elementary steps underlying the complex temporal and spatial effects in fair detail, which information, in turn, could be used for theoretical modeling.
Abstract: Publisher Summary This chapter focuses on the oscillatory catalytic reactions at single-crystal surfaces. Among oscillating chemical systems, catalytic reactions at well-defined single-crystal surfaces under low-pressure conditions are conceptually particularly attractive for the following reasons: The reacting medium is strictly two-dimensional and uniform on the mezoscopic length scale, which is relevant for spatial pattern formation. In addition, the low partial pressures cause only minor heat production by the reaction, so that usually isothermal conditions can be kept to within a very good approximation. By combining kinetic measurements with surface-sensitive techniques, it became possible to explore the elementary steps underlying the complex temporal and spatial effects in fair detail, which information, in turn, then could be used for theoretical modeling. For the systems described in the chapter, the following mechanisms is found to determine the occurrence of isothermal kinetic oscillations: (1) adsorbate driven structural transformation, (2) formation and depletion of a subsurface oxygen species, and (3) autocatalytic surface reaction.
183 citations
Abstract: Despite the great importance of heterogeneous catalysis, research in this field has long been characterized by its empiricism. Now, however, thanks to the rapid development of methods in surface physics, the elementary steps can be identified at the atomic level and the underlying principles understood. Defined single crystal surfaces are employed as models, based on the analysis of the surfaces of ‘real’ catalysts. Direct images, with atomic resolution, can be obtained using scanning tunneling microscopy, while electron spectroscopic methods yield detailed information on the bonding state of adsorbed species and the influence of catalyst additives (promotors) upon them. The successful application of this approach is illustrated with reference to the elucidation of the mechanism of ammonia synthesis. The catalyst surface is usually transformed under reaction conditions, and, as the processes involved are far-removed from equilibrium, such transformations can lead to intrinsic spatial and temporal self-organization phenomena. In this case, the reaction rate may not remain constant under otherwise invariant conditions but will change periodically or exhibit chaotic behavior, with the formation of spatial patterns on the catalyst surface.
165 citations
TL;DR: In this article, the active phase is a thin layer of KFeO 2 supported on a solid solution of K 2 Fe 22 O 34 in Fe 3 O 4, which acts as storage medium from which the active surface is continuously supplied with a near-monolayer coverage of potassium ions in an environment of Fe 3+ ions.
149 citations
TL;DR: In this article, low energy electron diffraction (LEED) and infrared spectroscopy are used to illustrate these phenomena in the adsorption systems Pt{111}-CO and Pd{111]-CO.
Abstract: Vibrational spectroscopy has indicated that the ordered overlayers formed by CO on metal surfaces at high coverage are not “compression” structures, or “floating” phases, but rather coincident site lattices in which the molecules remain adsorbed on high symmetry sites. In an equivalent description such layers may be regarded as a mixture of phase and anti-phase domains of a lower coverage structure separated by regularly spaced domain walls (solitons), at which the local CO density is higher. Due to the flat potential energy surface the molecules at the domain walls can adjust to the repulsive CO-CO interaction by moving off the high symmetry sites. The soliton model has the advantage of providing a convenient description of the formation of such phases and of the transitions from one phase to another. In the present paper low energy electron diffraction (LEED) and infrared spectroscopy are used to illustrate these phenomena in the adsorption systems Pt{111}-CO and Pd{111}-CO.
125 citations
TL;DR: Binding energies of excitons in a quantum-well structure are calculated including fully the effects of image charges, finite barriers, the z correlation of electrons and holes, and anisotropic hole masses.
Abstract: Binding energies of excitons in a quantum-well structure are calculated including fully the effects of image charges, finite barriers, the z correlation of electrons and holes, and anisotropic hole masses. The influence of discontinuous masses and discontinuous dielectric constants across the interfaces is evaluated in detail: While the mass difference becomes important only when the excitonic wave function penetrates into the barrier, the image charges appreciably modify the Coulomb interaction and therefore influence the exciton binding energy even at well widths larger than the exciton Bohr radius. Results for technologically important, particular material systems are presented.
99 citations
TL;DR: In this article, the adsorption of NO on Pd(111) was studied by means of HREELS, LEED, UPS and Δo measurements in the temperature range between 20 and 300 K.
TL;DR: Photosystem I preparations from the cyanobacterium Synechococcus sp.
TL;DR: In this article, in-situ STM images were used to study the changes in surface topography occurring on flame-annealed Au(1OO) electrode surfaces, for potential cycles which were limited to the double-layer region and also to potentials of surface oxidation.
TL;DR: A new class of structure models for the (113) and (115) orientations of Si and Ge is proposed, based on dimer and adatom formation as the main building blocks for the reduction of dangling bonds, which favors the models for Ge(113) which have not the minimum DB density but the minimum of strain.
Abstract: A new class of structure models for the (113) and (115) orientations of Si and Ge is proposed. They are based on dimer and adatom formation as the main building blocks for the reduction of dangling bonds (DB), on relaxation towards more ${\mathit{sp}}^{2}$- or ${\mathit{s}}^{2}$${\mathit{p}}^{3}$-like configurations, and on the minimization of strain. Four structural alternatives are discussed for (113) which are consistent with the observed 3\ifmmode\times\else\texttimes\fi{}1 and 3\ifmmode\times\else\texttimes\fi{}2 periodicities. They have either a low DB density and large strain or a higher DB density and low strain. For Si(113), a decision between them on the basis of the available experimental results is not unique. The analysis of the orientation-dependent adsorption of ${\mathrm{H}}_{2}$S and NO in terms of preferential adsorption by certain structural elements favors the models for Ge(113) which have not the minimum DB density but the minimum of strain. Steps on (001) vicinals induce strain due to bond stretching which is equilibrated over the terraces as long as they are wide enough. The switching of the twofold periodicity of the dimers along [1\ifmmode\bar\else\textasciimacron\fi{}10] on (001) and its vicinals to the threefold periodicity at (115) is explained to occur because the 3\ifmmode\times\else\texttimes\fi{}n models resolve bond stretching into bond bending by a meandering arrangement of microterraces which are separated by the energetically most favorable type-${\mathit{S}}_{\mathit{A}}$ single-layer steps.
TL;DR: In this paper, it is demonstrated that analysis based on intensity data taken only at discrete energy intervals (of up to about 20 eV) leads to no loss in accuracy if compared with the evaluation of continuous I/V-spectra.
TL;DR: This work proposes a method in which first rotation-, translation- and mirror-invariant functions are derived from the large set of input images and subsequently classified automatically using multivariate statistical classification techniques.
TL;DR: The energies of the two energetically lowest dipole-allowed electron-hole-pair states in semiconductor microcrystallites are computed variationally and significant deviations from the infinite-potential approximation are obtained.
Abstract: The energies of the two energetically lowest dipole-allowed electron-hole-pair states in semiconductor microcrystallites are computed variationally. Details of the quantum confinement conditions, such as the finite value of the quantum confinement potential and the different effective electron-hole masses inside and outside the crystallites, are considered explicitly. Significant deviations from the infinite-potential approximation are obtained.
TL;DR: This nonadiabatic process becomes possible because resonance ionization of the affinity level of the impinging O{sub 2} molecule upon crossing the Fermi level {ital E}{sub {ital F}} is efficiently suppressed due to the absence of occupied states near {italE}{sub} F}} at the surface.
Abstract: Electron emission during oxidation of thin Cs films is confined to the stage Cs{sub 2}O{sub 2}{r arrow}CsO{sub 2} and is caused by Auger deexcitation accompanying the (formal) reaction O{sub 2}{sup 2{minus}}({ital s})+O{sub 2}{r arrow}2O{sub 2}{sup {minus}}({ital s}). This nonadiabatic process becomes possible because resonance ionization of the affinity level of the impinging O{sub 2} molecule upon crossing the Fermi level {ital E}{sub {ital F}} is efficiently suppressed due to the absence of occupied states near {ital E}{sub {ital F}} at the surface.
TL;DR: In this article, the decomposition of methanol on a Pd(111) surface has been investigated under steady reaction conditions and at temperatures T > 300 K by combined static secondary ion mass spectrometry (SSIMS) and X-ray photoelectron spectroscopy (XPS) PFDMS has been applied in studies of the same reaction at 300 K on a pd field emitter surface with (111) orientation of the terraces.
TL;DR: In this article, the reconstruction of the 2 × 1)O-Ni(110) system was carried out and the results showed that the missing row structure favors the "missing row" structure over the "saw-tooth" and "buckled row" models.
TL;DR: In this article, the authors define Einkristall-Oberflachen als Modellsysteme eingesetzt, and der erfolgreiche Anwendung dieser Strategie soll anhand der Aufklarung des Mechanismus der Ammoniak-Synthese illustriert werden.
Abstract: Trotz der enormen praktischen Bedeutung der heterogenen Katalyse blieb die Forschung auf diesem Gebiet lange Zeit stark von der Empirie gepragt. Dank der rapiden Entwicklung oberflachenphysikalischer Methoden lassen sich aber nunmehr die Elementarschritte im atomaren Bereich erfassen und ihre zugrundeliegenden Prinzipien verstehen. Nach der Analyse der Oberflache eines technischen Katalysators werden definierte Einkristall-Oberflachen als Modellsysteme eingesetzt. Mit dem Rastertunnelmikroskop ist eine direkte Abbildung der Oberflachen mit atomarer Auflosung moglich; elektronenspektroskopische Methoden liefern detaillierte Informationen uber die Bindungszustande der adsorbierten Teilchen und ihre Beeinflussung durch Katalysator-Zusatze (Promotoren). Die erfolgreiche Anwendung dieser Strategie soll anhand der Aufklarung des Mechanismus der Ammoniak-Synthese illustriert werden. Katalytische Reaktionen in einem Stromungsreaktor sind offene Systeme fernab vom Gleichgewicht und konnen daher zu raumlicher und zeitlicher Selbstorganisation fuhren. In diesem Fall ist der Reaktionsumsatz trotz konstanter Bedingungen nicht notwendigerweise stationar, sondern kann sich periodisch bis hin zu chaotischem Verhalten verandern, wobei es zur Musterbildung auf der Oberflache kommt.
TL;DR: In this article, the total yield of electrons emitted by UV light focussed onto a small spot on the surface through a microscope objective while the x,y-position is continuously varied is recorded.
TL;DR: In this article, a model is discussed which explains rotational excitation on the basis of an anisotropic repulsive interaction in the excited state, resulting in the fact that for those molecules quenched later more potential energy is transferred into translational and rotational energy.
TL;DR: Determination par ellipsometrie spectroscopique: pres de la limite d'absorption fondamentale, les donnees concordent avec les calculs de structure de bande.
Abstract: The dielectric function of ${\mathrm{CaF}}_{2}$ over the photon energy region 10--35 eV has been determined by spectroscopic ellipsometry. Close to the fundamental absorption edge the data agree fairly well with band-structure calculations. A sharp exciton at the Ca 3p threshold (27.9 eV) is immediately followed by structures which can be attributed to variations in the conduction-band density of states. The strong transitions observed at higher energies cannot be explained in the one-electron band-structure model. Detailed theoretical calculations are required.
TL;DR: In this article, angle resolved photoemission spectroscopy using synchrotron radiation has been applied to study a (3 × 3) overlayer of benzene adsorbed on Ag(111).
TL;DR: This work extends automatic multivariate statistical classification techniques to find classes of pixels or features in the images (or other n-dimensional signals) that exhibit a homogeneous statistical behavior throughout the data set.
Abstract: Over the past few years, automatic multivariate statistical classification techniques have successfully been used for analyzing large and noisy electron-microscopic data sets. After the raw data are compressed with eigenvector–eigenvalue procedures, classes of images are formed, using unsupervised clustering procedures. The classes elucidate even subtle differences existing within the data set. Here we extend these methods to find classes of pixels or features in the images (or other n-dimensional signals) that exhibit a homogeneous statistical behavior throughout the data set. This feature extraction—itself a form of data compression—is mathematically entirely symmetric to the determination of the image classes and also serves the purpose of revealing the information present in the set of input images. The properties of simultaneous representations of the image-space and feature-space data onto the same two-dimensional map are discussed in relation to the metrics used in both spaces. Model data are used to illustrate the basic ideas.
TL;DR: In this paper, the authors studied the catalytic oxidation of CO on Pt{100] using photoemission electron microscopy (PEEM) and showed that the difference in work function between CO-covered regions of the surface and O2-coverage regions provides image contrast in PEEM using a deuterium lamp.
TL;DR: The application of core and valence level photoelectron spectroscopy to the study of semiconductor heterojunctions and metal-semiconductor interfaces (Schottky barriers) is outlined, with an emphasis on recent results and their explanation in terms of current theories.
Abstract: The application of core and valence level photoelectron spectroscopy to the study of semiconductor heterojunctions and metal-semiconductor interfaces (Schottky barriers) is outlined, with an emphasis on recent results and their explanation in terms of current theories. While the determination of transport barriers (valence band offsets and Schottky barriers) is stressed, the identification of chemical reactions at the interface is also discussed using several examples. Photoemission can precisely determine many important quantities in these junctions; also demonstrated, however, is the disturbing influence of the photoemission process itself through the creation of a surface photovoltage in metal-semiconductor interfaces, and its possible consequences for recent investigations of Schottky barrier heights in metal overlayers on low temperature substrates.
TL;DR: In this article, a comparison of the catalysts derived from the metal alloys was carried out in a differential fixed bed reactor in the temperature range 300-500 K. The comparison was based on turnover frequencies calculated on the basis of accessible palladium sites determined by CO chemisorption.
TL;DR: A complete atomic model for bacteriorhodopsin between amino acids 8 and 225 has been built, using bulky aromatic side-chains of phenylalanine, tyrosine and tryptophan as guide points as well as a very dense feature which is the Bionone ring of the retinal chromophore.
Abstract: A map of the structure o f bacteriorhodopsin has been obtained using electron microscopy and diffraction [ 11. It shows a resolution of 3.5 A in a direction parallel to the membrane plane, but poorer than this perpendicular. The map shows many features well resolved from the main density of the seven a-helices, which we interpret as the bulky aromatic side-chains of phenylalanine, tyrosine and tryptophan as well as a very dense feature which is the Bionone ring of the retinal chromophore. Using these bulky side-chains as guide points and taking account of bulges in the helices that indicate smaller side-chains such as leucine, a complete atomic model for bacteriorhodopsin between amino acids 8 and 225 has been built. Twenty-one amino acids contributed by all seven helices surround the retinal, and 26 amino acids contributed by five helices form the proton channel. Ten of the amino acids in the middle of the Fig. 1. Artistic impression showing the relutionship between the key residiies Asp-tIS, Asp-%, Asp-212, Lys-216 and A%82 and the retinul binding site, the proton chunnel unrl overall molecrrlur hoiindury for the ground stute of hucteriorhodopsiri . _ proton channel are also part of the retinal-building site. The model also Drovides a useful basis for consideration of the The cytoplasm is at the top of the diab Tram.