Showing papers by "Fritz Haber Institute of the Max Planck Society published in 1999"

Phonon- Versus Electron-Mediated Desorption and Oxidation of CO on Ru(0001)

Abstract: Heating of a ruthenium surface on which carbon monoxide and atomic oxygen are coadsorbed leads exclusively to desorption of carbon monoxide. In contrast, excitation with femtosecond infrared laser pulses enables also the formation of carbon dioxide. The desorption is caused by coupling of the adsorbate to the phonon bath of the ruthenium substrate, whereas the oxidation reaction is initiated by hot substrate electrons, as evidenced by the observed subpicosecond reaction dynamics and density functional calculations. The presence of this laser-induced reaction pathway allows elucidation of the microscopic mechanism and the dynamics of the carbon monoxide oxidation reaction.

Palladium Nanocrystals on Al 2 O 3 : Structure and Adhesion Energy

Abstract: We have investigated the structure and morphology of nanosized palladium clusters supported by a thin ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ film on NiAl(110) using scanning tunneling microscopy. Well-ordered clusters with a diameter above $\ensuremath{\approx}40\AA{}$ were imaged with atomic resolution, allowing a crystallographic identification of the cluster facets. A new method to obtain quantitative information on the work of adhesion (adhesion energy) of metal clusters deposited on oxides is introduced. For Pd on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ we obtain a value of ${W}_{\mathrm{adh}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.8\ifmmode\pm\else\textpm\fi{}0.2\mathrm{J}/{\mathrm{m}}^{2}$. This result is at variance with values recently derived on the basis of ab initio density-functional theory.

Dual-path mechanism for catalytic oxidation of hydrogen on platinum surfaces

Abstract: The catalytic formation of water from adsorbed hydrogen and oxygen atoms on Pt(111) was studied with scanning tunneling microscopy and high resolution electron energy loss spectroscopy. The known complexity of this reaction is explained by the strongly temperature dependent lifetime of the product ${\mathrm{H}}_{2}\mathrm{O}$ molecules on the surface. Below the desorption temperature water reacts with unreacted O adatoms to ${\mathrm{OH}}_{\mathrm{ad}}$, leading to an autocatalytic process; at higher temperatures sequential addition of H adatoms to ${\mathrm{O}}_{\mathrm{ad}}$ with normal kinetics takes place.

Electrocatalytic Activity of Ru-Modified Pt(111) Electrodes toward CO Oxidation

Abstract: The electrochemical deposition of Ru on Pt(111) electrodes has been investigated by electron diffraction, Auger spectroscopy, and cyclic voltammetry in a closed UHV transfer system. At small coverages Ru formed a monatomic commensurate layer, at higher coverage mostly small islands with a bilayer height were detected. When the Pt was almost completely covered by Ru, three-dimensional clusters developed. The island structure of Ru changed upon electrooxidation of CO, reflecting an enhanced mobility of Ru. Adsorption and electrooxidation of CO have been studied on such Ru-modified Pt(111) electrodes using cyclic voltammetry and in situ FTIR spectroscopy. Compared to the pure metals, the Ru−CO bond is weakened, the Pt−CO bond strengthened on the modified electrodes. The catalytic activity of the Ru/Pt(111) electrode toward CO adlayer oxidation is higher than that of pure Ru and a PtRu alloy (50:50). It is concluded that the electrooxidation of CO takes place preferentially at the Ru islands, while CO adsorbe...

Criticality and scaling in evolutionary ecology.

Abstract: Fluctuations in ecological systems are known to involve a wide range of spatial and temporal scales, often displaying self-similar (fractal) properties. Recent theoretical approaches are trying to shed light on the nature of these complex dynamics. The results suggest that complexity in ecology and evolution comes from the network-like structure of multispecies communities that are close to instability. If true, these ideas might change our understanding of how complexity emerges in the biosphere and how macroevolutionary events could be decoupled from microevolutionary ones.

Spectral properties of carbon black

Abstract: The internal structure of carbon black particles considerably influences the optical behavior of the material, apart from the shape and agglomeration state of the primary particles. In this paper the correlation between internal structure and spectral behavior of carbon black is investigated experimentally. The carbon blacks were produced by resistive heating of graphite electrodes and condensation in a cooling gas atmosphere. The internal structure of the primary carbon black particles was investigated by high-resolution transmission electron microscopy, electron energy loss spectroscopy, 13 C NMR spectroscopy, and Raman spectroscopy. The primary particles were found to consist of bent or plane structural subunits. The UV π−π* absorption feature of the produced carbon blacks varies in position between 196 and 265 nm depending on the state of bending of the graphene layers in the subunits of particles and/or the dimensions of the plane graphitic microcrystallites and the incorporation of hydrogen. The different curvature radii of the graphene layers or the sizes of microcrystallites can be summarized by an integral dimension like the ratio of sp2/sp3 hybridized carbon atoms. In the mid-infrared spectral region, the absolute value of the absorption coefficient κ is dominated by a continuous absorption due to free charge carriers which are also influenced by the ratio of sp2/sp3 hybridized carbon in the primary particles. The appearance of prominent bands is related to the existence of functional groups, like C–Hn, CO and/or C–O–C.

Metal oxide heteroepitaxy: Stranski-Krastanov growth for iron oxides on Pt(111)

Abstract: The epitaxial growth of iron oxide films on Pt(111) substrates was investigated by scanning tunneling microscopy and low-energy electron diffraction. The film growth was accomplished by repeated cycles of iron deposition and subsequent oxidation at $p({\mathrm{O}}_{2}{)=10}^{\ensuremath{-}6}\mathrm{mbar}.$ For oxidation temperatures of 870 K second and third FeO(111) layers grow layer by layer, whereas for oxidation temperatures of 1000 K only one FeO(111) monolayer is formed. On top of the FeO(111) films a homogeneous nucleation of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ islands takes place, resulting in a Stranski-Krastanov growth for iron oxides on Pt(111). The islands grow in the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ bulk structure laterally much faster than vertically, forming flat platelets with heights up to 100 \AA{} and hexagonal and triangular basal planes 1000--5000 \AA{} in diameter. The islands only expose low index ${1\ifmmode\bar\else\textasciimacron\fi{}11}$ and ${21\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}}$ facet planes, and their growth can be described by an Ostwald ripening mechanism that takes place during each oxidation cycle. Eventually the islands coalesce and form smooth ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ films at least 150 \AA{} thick. The atomic and mesoscopic surface roughness of these films depends on the growth temperature, where the latter ranges between 40 and 100 \AA{} on a length scale of 1 \ensuremath{\mu}m. By a high-pressure oxidation at $p({\mathrm{O}}_{2}{)=10}^{\ensuremath{-}1}\mathrm{mbar}$ the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ films were transformed into well-ordered $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}(0001)$ films with similar surface morphologies. In all oxide phases formed the hexagonal oxygen (111) planes are aligned to the Pt(111) substrate surface lattice. The film growth is discussed in terms of surface and interfacial energies, oxidation and growth kinetics, as well as thermodynamic stability ranges of the different oxide phases.

Effect of Mn 2+ concentration in ZnS nanoparticles on photoluminescence and electron-spin- resonance spectra

Abstract: Organically capped zinc sulfide nanoparticles doped with different manganese concentrations were prepared under similar conditions. Only the doping concentration was varied. Photoluminescence and electron-spin-resonance (ESR) investigations show some new results. At an optimum concentration of Mn doping a maximum in the photoluminescence is reached, whereas photoluminescence quenching occurs at higher concentrations. ESR investigations show that the spectra arise due to four different contributions of Mn ions, viz., (1) $\mathrm{Mn}{(S}_{I})$ in tetrahedral cationic substitution site with ${T}_{d}$ symmetry, (2) isolated Mn ions at the surface or interstitial locations ${(S}_{\mathrm{II}})$ with octahedral symmetry $({\mathrm{O}}_{h}),$ (3) Mn-Mn dipolar interactions ${(S}_{\mathrm{III}}),$ and (4) exchange-coupled Mn clusters ${(S}_{\mathrm{IV}})$ in various proportions. Linewidths for all these ${(S}_{I}$--${S}_{\mathrm{IV}})$ differ from each other. Identification of these components suggests that ${S}_{I}$ may be responsible for the photoluminescence increase, whereas ${S}_{\mathrm{II}}$--${S}_{\mathrm{IV}}$ may be responsible for the luminescence quenching in nanoparticles.

Stochastic multiplicative processes with reset events.

Abstract: We study a stochastic multiplicative process with reset events. It is shown that the model develops a stationary power-law probability distribution for the relevant variable, whose exponent depends on the model parameters. Two qualitatively different regimes are observed, corresponding to intermittent and regular behavior. In the boundary between them, the mean value of the relevant variable is time independent, and the exponent of the stationary distribution equals $\ensuremath{-}2$. The addition of diffusion to the system modifies in a nontrivial way the profile of the stationary distribution. Numerical and analytical results are presented.

Noise-induced breakdown of coherent collective motion in swarms.

Abstract: We consider swarms formed by populations of self-propelled particles with attractive long-range interactions. These swarms represent multistable dynamical systems and can be found either in coherent traveling states or in an incoherent oscillatory state where translational motion of the entire swarm is absent. Under increasing the noise intensity, the coherent traveling state of the swarms is destroyed and an abrupt transition to the oscillatory state takes place.

First-principles calculation of the thermal properties of silver

Abstract: The thermal properties of silver are calculated within the quasiharmonic approximation, by using phonon dispersions from density-functional perturbation theory, and the pseudopotential plane-wave method. The resulting free energy provides predictions for the temperature dependence of various quantities such as the equilibrium lattice parameter, the bulk modulus, and the heat capacity. Our results for the thermal properties are in good agreement with available experimental data in a wide range of temperatures. As a by-product, we calculate phonon frequency and Gr\"uneisen parameter dispersion curves which are also in good agreement with experiment.

Direct and indirect excitation mechanisms in two-photon photoemission spectroscopy of Cu(111) and CO/Cu(111)

Abstract: It is demonstrated that the dependence of the two-photon photoemission ~2PPE! yield on the polarization of the exciting laser light provides detailed information about the excitation mechanism and the orientation of transition dipole moments in the 2PPE process. In particular, it is possible to distinguish between a dir ct two-photon excitation process, where both electronic transitions are induced by the electric fields at the surface, and anindirect mechanism, where the first excitation step occurs in the substrate. In the latter process the intermediate state in 2PPE is populated by scattering of photoexcited hot electrons from the substrate, which are subsequently photoemitted by the second laser pulse. The analysis is applied to 2PPE from clean and CO covered Cu~111!. Furthermore, we have derived analytical expressions for the 2PPE signal based on the optical Bloch equations for a three-level system excited with continuous light beams. They allow us to calculate 2PPE spectra of surface states for a variety of cases. @S0163-1829 ~99!02008-1#

Micellization Dynamics and Impurity Solubilization of the Block-Copolymer L64 in an Aqueous Solution

Abstract: The dynamics of the micellization of Pluronic® L64 (EO13PO30EO13, PEO/PPO = 2/3) was studied by employing the iodine laser temperature-jump technique. These studies show a more complex behavior than previously described. Three relaxation processes were observed, depending on the temperature at which experiments were performed. The first two of these relaxation processes are related to the unimer incorporation into micelles and the redistribution of micelles after incorporation of the unimers. The third relaxation process, which had not been previously observed, occurs at high temperatures and is related to the clustering of micelles into larger aggregates. In addition, the presence of impurities and hydrophobic components was shown for the first time to significantly affect the dynamics of L64 micellization.

Thermodesorption of CO and NO from Vacuum-Cleaved NiO(100) and MgO(100)

Abstract: This paper presents experimental data on the bonding of CO and NO to vacuum-cleaved MgO(100) and NiO(100) and compares them with theoretical results. In the case of CO and NO on NiO(100) we find that the bonding energies obtained for the vacuum-cleaved single crystals agree well with results of recent studies on thin NiO(100) films grown by oxidation of Ni(100) whereas they are at variance with theoretical results. On the other hand, for CO on MgO(100) the experimental data fit well to recent theoretical studies while they contradict studies of adsorption on MgO(100) films grown on Mo(100). The experimentally determined values for the adsorption energies are 0.30 and 0.57 eV for adsorption of CO and NO on NiO(100). respectively, and 0.14 and 0.22 eV for adsorption on MgO(100). We suggest that the stronger bonding to NiO(100) as compared to MgO(100) is due to the influence of the 3d electrons of NiO.

Mutual synchronization and clustering in randomly coupled chaotic dynamical networks

Abstract: We introduce and study systems of randomly coupled maps where the relevant parameter is the degree of connectivity in the system. Global (almost-) synchronized states are found (equivalent to the synchronization observed in globally coupled maps) until a certain critical threshold for the connectivity is reached. We further show that not only the average connectivity, but also the architecture of the couplings is responsible for the cluster structure observed. We analyze the different phases of the system and use various correlation measures in order to detect ordered nonsynchronized states. Finally, it is shown that the system displays a dynamical hierarchical clustering which allows the definition of emerging graphs.

Crystal structures and growth mechanism for ultrathin films of ionic compound materials: FeO(111) on Pt(111)

Abstract: The growth and atomic structures of epitaxial iron-oxide films on Pt(111) were studied with scanning tunneling microscopy and high-resolution low-energy electron diffraction. During the initial layer-by-layer growth of FeO(111) four different structures are formed as the coverage increases to 2.5 monolayers, then a three-dimensional growth of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)$ islands begins. The structural transformations demonstrate that the relaxations within the FeO(111) films and the Stranski-Krastanov growth mode are induced by electrostatic surface energies, which dominate the energetics of thin film systems made up of ionic compound materials.

Negative activation energies in CO oxidation over an icosahedral Au/Mg(OH)2 catalyst

Abstract: In the course of our studies on CO oxidation over Au/Mg(OH)2 we have discovered a catalyst which exhibits an apparent negative activation energy when studied under ultra‐dry conditions (80 ppb moisture content). A review of current literature suggests that the oxidation of CO may occur by a reaction between CO and OH radicals and not by oxygen as previously thought. Substantial differences in catalytic behaviour between low and high temperature suggest that the reaction is complex and that more than one reaction pathway is present.

Structure of the 13-fold symmetric portal protein of bacteriophage SPP1

Abstract: We have determined the three-dimensional structure of bacteriophage SPP1 portal protein (gp6) using electron microscopy at liquid-helium temperatures and angular reconstitution The 13-fold symmetric gp6 oligomer is a turbine-shaped structure with three distinct regions: a conical stem with a central channel; the turbine wings region; and a fringe of small 'tentacles' at the end of the channel exposed to the viral head interior The tentacle region appears flexible and may be associated with a particular function — sensing when the correct amount of DNA has been packaged The three-dimensional structure of the gp6 SizA mutant, which packages a smaller chromosome, reveals significant differences in that region

Femtosecond electron dynamics at adsorbate-metal interfaces and the dielectric continuum model

Abstract: overlayers adsorbed on Cu(111). With increasing number of adsorbate layers the binding energies of the image potential states are found to decrease while their lifetimes increase (except for the second image potential state on 2 to 3 ML Xe/Cu(111)). These trends are most pronounced for nitrogen, where the binding energy of the first image potential state decreases by a factor of 3.5 from 0 to 2 ML N2/Cu(111); at the same time the lifetime increases from 22 to 700 fs. The results are discussed in the framework of the dielectric continuum model, which approximates the adsorbate layers by a dielectric slab in front of the metal surface. For Xe, the agreement between measured and calculated lifetimes improves significantly if the full dispersion curve of the Xe 6s conduction band is taken into account.

Remote triggering of waves in an electrochemical system

Abstract: In the potentiostatic electrochemical oxidation of formic acid on a platinum ring electrode under bistable conditions, an appropriate perturbation at one location of the ring can cause the emergence of a wave on the opposite side (remote triggering). These findings can be rationalized in terms of the nonlocal coupling function of the system and are theoretically reproduced by solution of the corresponding reaction-migration equation.