Showing papers by "Fritz Haber Institute of the Max Planck Society published in 2002"
TL;DR: The 13 Å resolution three-dimensional reconstruction determined by cryo-electron microscopy of the kirromycin-stalled codon-recognition complex provides a detailed snapshot view of an important functional state of the ribosome and suggest mechanisms of decoding and GTPase activation.
Abstract: The mRNA codon in the ribosomal A-site is recognized by aminoacyl-tRNA (aa-tRNA) in a ternary complex with elongation factor Tu (EF-Tu) and GTP. Here we report the 13 A resolution three-dimensional reconstruction determined by cryo-electron microscopy of the kirromycin-stalled codon-recognition complex. The structure of the ternary complex is distorted by binding of the tRNA anticodon arm in the decoding center. The aa-tRNA interacts with 16S rRNA, helix 69 of 23S rRNA and proteins S12 and L11, while the sarcin-ricin loop of 23S rRNA contacts domain 1 of EF-Tu near the nucleotide-binding pocket. These results provide a detailed snapshot view of an important functional state of the ribosome and suggest mechanisms of decoding and GTPase activation.
213 citations
TL;DR: One of the most fundamental reactions, the dissociative chemisorption of H2 on metal surfaces, can now be treated accurately using quantum mechanics, modeling all six molecular degrees of freedom.
Abstract: Recent theoretical progress in gas−surface reaction dynamics, a field relevant to heterogeneous catalysis, is described. One of the most fundamental reactions, the dissociative chemisorption of H2 ...
162 citations
TL;DR: The computed theta(c) are found to be very similar to those above which the bulk oxide phase becomes thermodynamically more stable, thus suggesting that the initial incorporation of O actuates the formation of a surface oxide on TM surfaces.
Abstract: We present a density-functional theory trend study addressing the incorporation of oxygen into the basal plane of the late $4d$ transition metals (TMs) from Ru to Ag. Occupation of subsurface sites is always connected with a significant distortion of the host lattice, rendering it initially less favorable than on-surface chemisorption. Penetration starts only after a critical coverage ${\ensuremath{\theta}}_{c}$, which is lower for the softer metals towards the right of the TM series. The computed ${\ensuremath{\theta}}_{c}$ are found to be very similar to those above which the bulk oxide phase becomes thermodynamically more stable, thus suggesting that the initial incorporation of O actuates the formation of a surface oxide on TM surfaces.
159 citations
TL;DR: In this article, an effective embedding potential is calculated by periodic density functional theory (DFT) and is used as a one-electron operator in subsequent cluster calculations, where the interaction of the cluster with the extended condensed phase is taken into account.
Abstract: We extend our recently reported embedding theory [J. Chem. Phys. 110, 7677 (1999)] to calculate not only improved descriptions of ground states, but now also localized excited states in a periodically infinite condensed phase. A local region of the solid is represented by a small cluster for which high quality quantum chemical calculations are performed. The interaction of the cluster with the extended condensed phase is taken into account by an effective embedding potential. This potential is calculated by periodic density functional theory (DFT) and is used as a one-electron operator in subsequent cluster calculations. Among a variety of benchmark calculations, we investigate a CO molecule adsorbed on a Pd(111) surface. By performing complete active space self-consistent field, configuration interaction (CI), and Moller–Plesset perturbation theory of order n (MP-n), we not only were able to obtain accurate adsorption energies via local corrections to DFT, but also vertical excitation energies for an int...
154 citations
TL;DR: In this paper, isothermal titration calorimetry (ITC) and surface tension (ST) and electromotive force (emf) measurements using a coated wire sodium dodecyl sulfate membrane-selective electrode to measure the mixed micellar composition of various mixtures of the triblock copolymer EO97PO69EO97, a nonionic surfactant code-named Pluronic F127, with SDS.
Abstract: We applied isothermal titration calorimetry (ITC) and surface tension (ST) and electromotive force (emf) measurements using a coated wire sodium dodecyl sulfate membrane-selective electrode to measure the mixed micellar composition of various mixtures of the triblock copolymer EO97PO69EO97, a nonionic surfactant code-named Pluronic F127, with sodium dodecyl sulfate (SDS). In the region where mixed micelles are formed, the interaction between the two surfactants showed synergistic behavior and interaction parameters β, which characterize the nonideal interaction in the mixed micelles, could be calculated over a range of mole ratios. For several compositions, the critical micelle concentrations of the mixed micelles were determined using ITC and ST measurements. In addition, small-angle neutron scattering (SANS) experiments were carried out in order to investigate the structure and provide additional information about the composition of the mixed micelles, taking advantage of contrast variation between SDS-...
131 citations
TL;DR: In this paper, the interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(1 1 0) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques.
105 citations
TL;DR: The second development step of the SMART project, i.e., an energy-filtered but not yet corrected photoelectron emission microscope, operates at the undulator U49/1-PGM beamline at BESSY II.
Abstract: The second development step of the SMART project, i.e. an energy-filtered but not yet corrected photoelectron emission microscope, operates at the undulator U49/1-PGM beamline at BESSY II. It already demonstrates the variety of methods of the final version: microscopy, spectroscopy and electron diffraction. Some recent experimental results are reported for these three operation modes. In addition, the theoretical improvement of lateral resolution and transmission of PEEMs in general by using an energy filter is discussed for systems without and with aberration correction.
91 citations
TL;DR: The locus of steady-state wave size as a function of excitability defines the perturbation threshold for the initiation of spiral waves and defines the excitability boundary for spiral wave behavior in active media.
Abstract: Propagating wave segments are stabilized to a constant size and shape by applying negative feedback from the measured wave area to the excitability of the medium. The locus of steady-state wave size as a function of excitability defines the perturbation threshold for the initiation of spiral waves. This locus also defines the excitability boundary for spiral wave behavior in active media.
69 citations
01 Jan 2002
69 citations
TL;DR: In this paper, the adsorption, decomposition, and oxidation of methanol on a well-defined supported Pd model catalyst were investigated using a combination of molecular beam methods, reflection absorption IR spectroscopy (RAIRS), and temperature-programmed desorption (TPD).
Abstract: We have investigated the adsorption, decomposition and oxidation of methanol on a well-defined supported Pd model catalyst, utilizing a combination of molecular beam methods, reflection absorption IR spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The Pd model catalyst is prepared under ultrahigh-vacuum (UHV) conditions on a well-ordered Al2O3 film grown on NiAl(110). In previous studies, this model system has been characterized in detail with respect to its geometric and electronic structure. On the alumina support, two molecular adsorption states of methanol are distinguished by RAIRS and TPD. Moreover, we can differentiate between adsorption on the Pd particles and on the alumina support, enabling us to follow surface diffusion from the alumina film to the Pd particles during the adsorption process. Upon heating, methanol partially desorbs from the Pd particles and partially undergoes decomposition,
with a reaction probability that is sensitively dependent on the initial methanol coverage. At 100 K, preadsorbed CO suppresses methanol adsorption on the Pd particles, whereas preadsorbed oxygen reduces the reaction probability. As a first intermediate, methoxy species are formed, which are stable up to temperatures of 200 K. Isotope exchange experiments indicate that a fast equilibrium is established between molecular methanol and methoxy species and that both species are rapidly exchanged with the gas phase. Further decomposition of methanol proceeds via two competing reaction pathways. The dominant pathway is dehydrogenation to CO, followed by CO2 formation in the presence of oxygen. Adsorbed oxygen has a pronounced inhibiting effect on the rate of decomposition. As a second pathway, we observe slow breakage of the carbon–oxygen bond, leading to formation of carbon and hydrocarbon species.
58 citations
TL;DR: In this paper, the activation of a Mo9V3W1.2Ox catalyst was investigated in the partial oxidation of acrolein as function of reaction temperature and atmosphere.
Abstract: The activation of a Mo9V3W1.2Ox catalyst was investigated in the partial oxidation of acrolein as function of reaction temperature and atmosphere. The activity and selectivity to acrylic acid considerably increased during activation in the acrolein oxidation reaction comparable to the recently reported activation after thermal pretreatment in inert gas. The activation during the catalytic acrolein oxidation, however, proceeds at about 200 K lower temperatures as compared to the inert gas pretreatment. The initial nanocrystalline catalyst structure changed during operation in the acrolein oxidation, as shown by X -ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy and energy dis- persive X-ray (EDX) analysis. A (MoVW)5O14-type mixed oxide was found to be the main crystalline phase in the active and selective catalysts. Hence, this (MoVW)5O14 phase crystallizes already during catalysis at the low acrolein oxidation reaction temperatures. The evolution of the catalytic performance is directly related to this low temperature formation of the (MoVW)5O14 phase. It is suggested that this (MoVW)5O14 phase has to be present in a detected, specific ordering state which is vital for optimum selective oxidation properties. In addition, other minority phases were identified by transmission electron microscopy (TEM) in the operating catalyst. The so-called bundle-type, and a new corona-type texture was detected, which show ordering in only one or two dimensions, respectively. These disordered structures are also relevant candidates for active catalyst phases as they are detected during the activation period of the MoVW mixed oxide.
TL;DR: Experimental and theoretical studies of the excitability boundary for spiral wave behavior are presented and a kinematic description of the constant-size, constant-shape wave segments is presented.
Abstract: Experimental and theoretical studies of the excitability boundary for spiral wave behavior are presented. The boundary is defined by unstable wave segments, which are stabilized by using a negative-feedback control algorithm. A kinematic description of the constant-size, constant-shape wave segments is presented.
TL;DR: In this article, a Vielzahl verwandter materialien wie Riesenfullerene, Nanorˆhren, Nanokugeln, Nanokegel, Nanob ndel oder zwiebelfˆrmige Kohlenstoffe (OLCs, onion-like carbons) wurde synthetisiert.
Abstract: Seit der Entdeckung der Fullerene 1985[1] erf‰hrt die Chemie sp2-hybridisierter nanostrukturierter Kohlenstoffe sowohl aus grunds‰tzlichen ‹berlegungen als auch wegen potentieller Anwendungen zunehmendes Interesse. Eine Vielzahl verwandter Materialien wie Riesenfullerene, Nanorˆhren, Nanokugeln, Nanokegel, Nanob ndel oder zwiebelfˆrmige Kohlenstoffe (OLCs, onion-like carbons) wurde synthetisiert.[2] Ihre einzigartigen chemischen und physikalischen Eigenschaften sollten neue Anwendungen ermˆglichen, etwa in den Bereichen Nano-Engineering und -elektronik, f r optoelektronische Sensoren, dreidimensionale Kompositmaterialien, Mikrofilter, magnetische Materialien und in der Katalyse.[3] Die Forschung zu OLCs beschr‰nkt sich zurzeit auf eine Weiterentwicklung der Synthesemethoden und Untersuchung ihrer physikalischen und chemischen Eigenschaften.[4] Wegen ihrer nahezu perfekten graphitischen und trotzdem gespannten Strukturen kˆnnten diese Materialien aus geschlossenen kugelfˆrmigen Kohlenstoffschalen katalytische Eigenschaften aufweisen. folgender Adresse in Gro britannien angefordert werden: Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB21EZ; Fax: ( 44)1223-336-033; oder deposit@ccdc.cam.ac.uk).
TL;DR: In this paper, the authors present trends in the multilayer relaxations of several vicinals of Cu(100) and Cu(111) of varying terrace widths and geometries.
Abstract: We present trends in the multilayer relaxations of several vicinals of Cu(100) and Cu(111) of varying terrace widths and geometries. The electronic structure calculations are based on density-functional theory in the local-density approximation with norm-conserving, nonlocal pseudopotentials in the mixed basis representation. While relaxations continue for several layers, the major effect is concentrated near the step and corner atoms. On all surfaces the step atoms contract inward, in agreement with experimental findings. Additionally, the corner atoms move outward and the atoms in the adjacent chain undergo a large inward relaxation. Correspondingly, the largest contraction (4%) is in the bond length between the step atom and its bulk nearest neighbor (BNN), while that between the corner atom and the BNN is somewhat enlarged. The surface atoms also display changes in registry of up to 1.5%. Our results are in general in good agreement with low-energy electron-diffraction data including the controversial case of Cu(511). Subtle differences are found with results obtained from semiempirical potentials.
TL;DR: In this article, photoemission electron microscopy (PEEM) images of the initial oxidation stages exhibit a distribution of bright grains appearing on a dark background, which are assigned to a Ru x O y region considered as a precursor of an extended RuO 2 phase.
TL;DR: In this paper, the authors used resonance enhanced multiphoton ionisation (REMPI) to detect desorbing molecules from nanostructured palladium aggregates on an epitaxial alumina support.
Abstract: UV-laser induced desorption of NO from nanostructured palladium aggregates on an epitaxial alumina support has been studied by means of resonance enhanced multiphoton ionisation (REMPI) to detect desorbing molecules quantum state resolved by Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS), X-ray photoemission spectroscopy (XPS) and thermal desorption spectroscopy (TPD). The size of the Pd-aggregates was systematically varied between 5 A and 80 A. Different morphologies were chosen depending on the growth conditions of the aggregates by Pd-atom deposition on the support. The aggregates were either amorphous (deposition at 100 K) or ordered (deposition at 300 K) with the aggregates having a cubooctahedral shape with dominating (111) terraces and a minority of (100) terraces. Adsorption is only similar to single crystal data for aggregate sizes beyond 75 A.
For smaller aggregates NO is bound on on-top sites of palladium. On small amorphous aggregates a substantial amount of NO is weakly bound, which has only been observed for stepped single Pd-crystals. Dissociation of NO occurs at elevated temperatures above 350 K. The system was excited with nanosecond laser pulses at 6.4 eV. In contrast to single crystals, desorption of intact NO molecules has been observed for small aggregates with increasing efficiencies with decreasing aggregate size for aggregate sizes of 80 A and below. Desorption cross sections vary by at least one order of magnitude. Dominantly the weakly bound species desorbs. REMPI data do not show a strong size dependence. Different models are discussed to explain the data, including the role of local effects of the adsorption site, spill-over to the alumina support or formation of oscillations in electron densities.
TL;DR: In this article, multiple scattering in non-spherical potentials (MSNSP) is used to calculate the angular distributions of electrons photoemitted from the 1s-shells of CO and N2 gas phase molecules with fixed-in-space orientations.
Abstract: We use multiple scattering in non-spherical potentials (MSNSP) to calculate the angular distributions of electrons photoemitted from the 1s-shells of CO and N2 gas-phase molecules with fixed-in-space orientations. For low photoelectron kinetic energies (E<50 eV), as appropriate to certain shape-resonances, the electron scattering must be represented by non-spherical scattering potentials, which are naturally included in our formalism. Our calculations accurately reproduce the experimental angular patterns recently measured by several groups, including those at the shape-resonance energies. The MSNSP theory thus enhances the sensitivity to spatial electronic distribution and dynamics, paving the way toward their determination from experiment.
TL;DR: In this paper, both heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB) and TRIMEB were investigated to shed light on their solubility behavior in water where they exhibit a negative temperature coefficient.
Abstract: Methylated cyclodextrins were investigated to shed light on their solubility behavior in water where they exhibit a negative temperature coefficient, contrasting the positive temperature coefficient of unmodified cyclodextrins. Both heptakis(2,6-di-O-methyl)-β-cyclodextrin (DIMEB) and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TRIMEB) show two different conditions of crystallization. At low temperatures around 18 °C, highly hydrated clathrates are formed, whereas, at high temperatures around 60−70 °C, DIMEB crystallizes as an anhydrate and TRIMEB as a monohydrate. The crystallization at high temperature is driven by entropy gain of water to compensate for the positive enthalpy change associated with this crystallization process, as could be shown by differential scanning calorimetry experiments in H2O and D2O.
TL;DR: In this article, the authors demonstrate the applicability of this instability detection method by real-time, computer-assisted detection of period doubling bifurcations of an electronic circuit; the circuit implements an analog realization of the Roessler system.
Abstract: We present an example of the practical implementation of a protocol for experimental bifurcation detection based on on-line identification and feedback control ideas. The idea is to couple the experiment with an on-line computer-assisted identification/feedback protocol so that the closed-loop system will converge to the open-loop bifurcation points. We demonstrate the applicability of this instability detection method by real-time, computer-assisted detection of period doubling bifurcations of an electronic circuit; the circuit implements an analog realization of the Roessler system. The method succeeds in locating the bifurcation points even in the presence of modest experimental uncertainties, noise and limited resolution. The results presented here include bifurcation detection experiments that rely on measurements of a single state variable and delay-based phase space reconstruction, as well as an example of tracing entire segments of a codimension-1 bifurcation boundary in two parameter space.
TL;DR: In this paper, the interaction between inversion domain boundaries (IDBs) and intrinsic basal stacking faults (SFs) in wurtzite-structured GaN grown on (0001) sapphire was studied using high-resolution electron microscopy (HREM) observations and empirical potential calculations.
TL;DR: In this paper, the local structure around the adsorbed OH species resulting from the interaction of H2O with a Si(100)(231) has been determined, by a combination of direct data inversion using a "projection" method and multiple-scattering simulations.
Abstract: Using scanned-energy mode photoelectron diffraction from the O 1s level, the local structure around the adsorbed OH species resulting from the interaction of H2O with a Si(100)(231) has been determined, by a combination of direct data inversion using a ‘‘projection’’ method and multiple-scattering simulations. The O atom is bonded to a surface Si atom with a Si-O bond length of 1.6760.03 A, the Si-O bond being tilted away from the surface normal by 1964°. This bonding Si atom is at one end of a surface dimer, which lies parallel to the surface to within 69°, but there appears to be a lateral offset of the dimer along the dimer direction away from the fully symmetric position by approximately 0.3 A, possibly reflecting a residual asymmetry associated with the adsorbate bonding. The main structural parameters are in excellent agreement with the results of a previously published density-functional theory slab calculation.
TL;DR: In this paper, the electron-beam-induced changes in V2O5 crystals were investigated by means of electron microscopy at liquid helium temperature (4.2 K) and the average oxidation state of the amorphous phase was estimated to be about 4+.
Abstract: The electron-beam-induced changes in V2O5 crystals were investigated by means of electron microscopy at liquid helium temperature (4.2 K). We obtained high-resolution images of this system, but observed an amorphization process during a prolonged exposure to the electron beam. The average oxidation state of the amorphous phase was estimated to be about 4+. This phase was stable at room temperature, but a partial recrystallization occurs by further irradiation at room temperature and it can be reduced to VO. These observations are discussed with respect to the reduced diffusion rate of oxygen and lattice collapses at this very low temperature.
TL;DR: In this article, multiple scattering photoelectron diffraction (MSPD) theory was used to calculate the angular patterns of electrons photoemitted from the K-shell of CO and N2 gas-phase oriented molecules.
Abstract: We use multiple scattering photoelectron diffraction (MSPD) theory to calculate the angular patterns of electrons photoemitted from the K-shell of CO and N2 gas-phase oriented molecules, as recently measured by several groups. For low (E < 50 eV) kinetic energies of the photoemitted electron, the electron scattering cannot be adequately represented by spherically symmetric potentials. We thus include nonspherical scattering potentials in our formalism through nondiagonal scattering matrices. We show that intramolecular scattering and interference are responsible for the experimentally measured patterns. This MSPD approach represents a more accurate and versatile method for dealing with such angular distributions as compared to prior calculations of these effects.
TL;DR: In this article, the main aspects of the quadratic logistic equation (QLE) and its application in physicochemical systems are described and some recent applications with emphasis on electrochemical systems are discussed.
Abstract: The concepts of dissipation and feedback are contained in the behavior of many natural dynamical systems. They have been used to predict the evolution of populations leading to the formulation of the quadratic logistic equation (QLE). More recently, the QLE has been used to provide a better understanding of physicochemical systems with promising results. Many physical, chemical and biological dynamic phenomena can be understood on the basis of the QLE and this work describes the main aspects of this equation and some recent applications, with emphasis on electrochemical systems. Also, it is illustrated the concept of potential energy as a convenient way of describing the stability of the fixed points of the QLE.
17 Oct 2002
TL;DR: In this paper, the authors showed that it is possible to grow thin V2O3(0001) films on Au(111) and W(110) substrates by using an evaporation of metallic vanadium in an oxygen atmosphere, followed by an annealing at 700 K in 5.10-8 mbar of oxygen.
Abstract: In this work, we firstly showed that it is possible to grow thin V2O3(0001) films on Au(111) and W(110). The preparation process consists of an evaporation of metallic vanadium in an oxygen atmosphere, followed by an annealing at 700 K in 5.10-8 mbar of oxygen. The low energy electron diffraction (LEED) patterns obtained for both substrates exhibit sharp spots, indicating a well-defined surface structure. The stoichiometry of the film has been characterized by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). The XP spectrum in the binding energy range 500-540 eV shows three features corresponding to the V 2p3/2, V 2p1/2 and O 1s lines, respectively. Relevant parameters for the determination of the stoichiometry of the oxide are the distance between the O 1s and V 2p signals, the Full Width at Half Maximum (FWHM) and the shape of the spectra. Our spectra show good agreement with those found in the literature for V2O3 single crystals. V L-edge NEXAFS spectra present noticeable chemical shifts characteristic of the different vanadium valencies and their shape depends implicitly on the local symmetry of the vanadium cation. Each vanadium oxide type therefore displays a typical spectrum. A comparison of our spectrum to reference spectra permits the identification of our vanadium oxide thin film to V2O3. We proved with infrared absorption spectroscopy (IRAS) the existence of two possible terminations of the V2O3 (0001) surface. These two terminations differ only by the presence or not of oxygen atoms on the top of the surface, forming vanadyl groups with the surface vanadium atoms. The first termination, called -V=O termination, is obtained after the preparation process. The second termination - the -V termination - is obtained by heating the -V=O surface up to 600 K with electron bombardment. We studied with UV photoelectron spectroscopies (UPS), XPS and NEXAFS the electronic structure of our V2O3 (0001) thin films. The UP spectra of the -V=O terminated surface clearly show a gap for the -V=O terminated surface. These data therefore evidence a metal to insulator transition induced by the formation of the vanadyl groups on the surface. This result is confirmed by our NEXAFS O K edge and XPS results. The NEXAF O K edge spectra consist of two features. The first one is attributed to the tansition to the unoccupied V 3d egΠ and a1g (t2g) states with O 2p character and the second one to the unoccupied V 3d egΣ states. For the -V=O termination, both features of the spectrum exhibit a shift towards higher energy relative to the spectrum for the -V termination. This shift can be explained by the changes in the electronic structure due to the metal to insulator transition. The XP spectra exhibit enhanced satellite features in the case of the -V=O termination, which can be attributed to poorly screened final states. We also observed a shift of the O 2p band towards lower binding energies for the -V=O terminated surface relative to the -V terminated surface. We tried to explain this phenomenon with a band bending model. Finally, we proposed two models for the surface geometry of the -V=O terminated surface. In the first one, the oxygen atoms sit on top of the vanadium atoms. In the second one, the oxygen atoms sit on quasi regular bulk positions. We performed high resolution electron energy loss spectroscopy (HREELS) measurements and presented a phonon spectrum for each termination. Differences in phonon intensities observed between both surface terminations can be interpreted as a screening effect of electronic carriers. We compared our spectra with a spectrum of the isomorphic Cr2O3(0001) and found out that the metal-oxygen bond is not so strong in V2O3 as in Cr2O3. We studied the water adsorption properties of both surface terminations. The experiment consists of the adsorption of water at 90 K, yielding the formation of ice on the sample surface. The sample then is heated up to 190 K. The species present on the surface at this temperature are analyzed with UPS, XPS and HREELS. The adsorption path seems to depend on both the termination and the exposure. We observed molecularly adsorbed water on both surface terminations for low exposures. The adsorbed water shows only weak interaction with the substrate. For large exposures, water dissociates and OH- groups were detected. When the OH- desorb of the primary -V=O terminated surface, the surface left is -V terminated. In the case of the -V=O terminated surface, the water molecule is assumed to adsorb on the surface vanadium atom through its oxygen atom. The oxygen double bonded to the vanadium can interact with the hydrogen of the water molecule to form a OH radical, breaking its double bond to the vanadium. This dissociation mechanism may imply charge redistribution, explaining why the V 3d emission in UPS increases upon water adsorption. This model explains why the vanadyl oxygen atoms desorb with the OH groups. For the -V terminated surface, we observed a charge transfer from the V 3d substrate to the adsorbate, producing OH- groups. Therefore, we proposed a model in which the vanadium a1g or egΠ orbital forms a Σ bond with oxygen lone-pair orbitals of OH-. We performed CO2 adsorption experiments with UPS, XPS, HREELS and IRAS. The UP results for the -V=O surface exhibit small features which we assigned to physisorbed CO2. The CO2 adsorption on the -V terminated surface is more complex. The analyze of the IRAS results leads us to the conclusion that CO2 adsorbs in a bent configuration. With UPS and XPS, we could evidence the formation of carbonates upon heating up to 200 K. The CO adsorption properties follow a similar trend as for CO2 : only small quantities adsorb on the -V=O surface while the -V surface seems to be much more reactive. On the -V=O surface, CO adsorbs molecularly and we concluded from the angle resolved UPS data that the CO molecule is strongly tilted on the surface. With NEXAFS and IRAS, we showed the formation of CO2 on the -V surface. To our knowledge, we are the first to report a surface effect resulting in a metal to insulator transition. This very complex phenomenon is very exciting for the surface scientist. Further work on V2O3 (0001) should therefore involve theorists in order to explain properly why the formation of vanadyl groups on the surface induces a metal to insulator transition. A simulation of the angle resolved UPS data could determine which model for the surface geometry is correct. Further experimental work could be thermal desorption spectroscopy (TDS) and IRAS with isotopes in order to identify the formation path of CO2 by CO adsorption on the -V terminated surface.
TL;DR: In this article, the structural, electronic, and magnetic properties of this class of systems (II-Ge-V2, II=Zn, Cd, and V=As, P) were investigated by means of first-principles density-functional theory-based codes.
Abstract: Stimulated by recent experimental observations of room-temperature ferromagnetism of MnxCd1−xGeP2 and MnxZn1−xGeP2, we investigate the structural, electronic, and magnetic properties of this class of systems (II–Ge–V2, II=Zn, Cd, and V=As, P) as a function of Mn concentration and chemical constituents by means of first-principles density-functional- theory-based codes. Our calculations indicate that, for Mn substituting the II element, the antiferromagnetic alignment is the most stable ordering for all the systems studied. For Zn- and Cd-rich systems, the total magnetic moments per Mn atom of the ferromagnetic phase is very close to the ideal value of 5 μB, since the Mn 3d states in the minority spin channel are nearly empty; on the other hand, for Mn rich compounds, the stronger p–d hybridization lowers the total magnetic moment to about 4.4 μB.
TL;DR: In this paper, the Omega Scan method was used to determine the orientation of the blank's surface relative to certain lattice planes or crystal axes. But the accuracy of the Omega scan was only a few arc seconds.
Abstract: Blanks - the thin quartz plates industrially used for oscillators, nowadays produced at rates of millions per week - need thorough controls of their properties. The most important of them, the correct orientation of the plates' surface relative to certain lattice planes or crystal axes, can be determined in a few seconds measuring time with an accuracy of a few arcseconds using the so-called Omega Scan. This method and its further development in the last time will be described in some detail.
TL;DR: In this article, a low-temperature scanning tunnelling microscope was used to detect standing wave patterns on a monolayer-high Pd layer whose wavelength is longer than that of the substrates.
Abstract: Modification of electron density in the surface states of Au(111) and Cu(111) surfaces by Pd deposition is demonstrated. Using a low-temperature scanning tunnelling microscope, electronic standing wave patterns are observed on a monolayer-high Pd layer whose wavelength is longer than that of the substrates. The observation indicates an upward shift of the energy level of the surface states and reduced electron density in the states on the overlayer. A standing wave pattern depending on magnetic field was observed around an impurity atom on the Au(111) surface.
12 May 2002
TL;DR: In this paper, the authors measured the two-pulse correlation (2PC) of the respective reaction yield and detected the yield of the desorbed product species with a quadrupole mass spectrometer as a function of the pulsepulse delay.
Abstract: The dynamics of the energy transfer processes at surfaces is of fundamental importance for a microscopic understanding of chemical reaction at solid surfaces, e.g. in heterogeneous catalysis. In conventional chemistry where phonons and electrons are always in thermal equilibrium, no detailed information can be obtained on the pathway of the energy flow between the substrate and the adsorbate. However, when a metal substrate is excited by an intense femtosecond laser pulse, a transient non-equilibrium of the electronic and phononic temperatures is created, which lasts on the order of the electron-phonon coupling time (for ruthenium ~1 ps) and allows to distinguish electron-mediated from phonon-mediated reactions [1]. The appropriate experimental approach is to measure the two-pulse correlation (2PC) of the respective reaction yield. To this end, the fundamental output of an amplified Ti:sapphire femtosecond laser system is split in two equally intense portions and then both beams are sent time-delayed onto the sample mounted in ultrahigh vacuum chamber. The yield of the desorbed product species is detected with a quadrupole mass spectrometer as a function of the pulse-pulse delay.
TL;DR: In this article, a model of interacting motile chaotic elements is proposed, where chaotic elements are distributed in space and interact with each other through interactions depending on their positions and their internal states.
Abstract: A model of interacting motile chaotic elements is proposed. The chaotic elements are distributed in space and interact with each other through interactions depending on their positions and their internal states. As the value of a governing parameter is changed, the model exhibits successive phase changes with novel pattern dynamics, including spatial clustering, fusion and fission of clusters and intermittent diffusion of elements. We explain the manner in which the interplay between internal dynamics and interaction leads to this behavior by employing certain quantities characterizing diffusion, correlation, and the information cascade of synchronization. Keywords: collective motion, coupled map system, interacting motile elements