Showing papers by "Fritz Haber Institute of the Max Planck Society published in 1995"

Six-dimensional quantum dynamics of adsorption and desorption of H2 at Pd(100): Steering and steric effects.

Abstract: We report the first six-dimensional quantum dynamical calculations of dissociative adsorption and associative desorption. Using a potential energy surface obtained by density functional theory calculations, we show that the increase of the sticking probability with decreasing kinetic energy at low kinetic energies in the system ${\mathrm{H}}_{2}/\mathrm{Pd}(100)$, which is usually attributed to the existence of a molecular adsorption state, is due to dynamical steering. In addition, we examine the influence of rotational motion and orientation of the hydrogen molecule on adsorption and desorption.

Electronic structure of ultrathin ordered iron oxide films grown onto Pt(111).

Abstract: It has recently been shown that well-ordered iron oxide films can be prepared epitaxially onto Pt(111) surfaces. We have investigated a one-monolayer-thick film with FeO stoichiometry and a ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$(111) multilayer. Moreover, we prepared well-ordered multilayers with \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ stoichiometry. Core and valence level photoemission data clearly reveal that the ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ and \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ multilayers on Pt(111) are chemically identical to thick single crystals and polycrystalline powder samples. The x-ray-absorption near-edge structures (XANES) at the O K edge arise from the hybridization between O 2p and Fe 3d states as well as Fe 4sp states, showing that the latter also are important for the bonding interaction of the oxides. Polarization-dependent XANES measurements at the O K edge reveal a FeO bond length in the monolayer of 2.23\ifmmode\pm\else\textpm\fi{}0.22 \AA{}, which almost corresponds to the bond length in the bulk FeO (2.15 \AA{}). The electronic structure of the iron oxides derived from the set of spectra presented will be described in terms of the ligand-field theory taking into consideration the exchange interaction between the iron 3d electrons.

Electron cryomicroscopy and angular reconstitution used to visualize the skeletal muscle calcium release channel.

Abstract: We exploit the random orientations of ice-embedded molecules imaged in an electron cryomicroscope to determine the three-dimensional structure of the Ca(2+)-release channel from the sarcoplasmic reticulum (SR) in its closed state, without tilting the specimen holder. Our new reconstruction approach includes an exhaustive search of all different characteristic projection images in the micrographs and the assignment of Euler angle orientations to these views. The 30 A map implied reveals a structure in which the transmembrane region exhibits no apparent opening on the SR lumen side. The extended cytoplasmic region has a hollow appearance and consists, in each monomer, of a clamp-shaped and a handle-shaped domain.

Local Chemical Reactivity of a Metal Alloy Surface.

Abstract: The chemical reactivity of a metal alloy surface is studied by density functional theory investigating the interaction of ${\mathrm{H}}_{2}$ with NiAl(110). The energy barrier for ${\mathrm{H}}_{2}$ dissociation is largely different over the Al and Ni sites without, however, reflecting the barriers over the single component metal surfaces. This local chemical behavior is due to the covalent nature of the ( ${\mathrm{H}}_{2}$ ${\ensuremath{\sigma}}_{g}$)-(Ni ${3d}_{{z}^{2}}$) and ( ${\mathrm{H}}_{2}$ ${\ensuremath{\sigma}}_{u}^{*}$)-(Ni ${3d}_{\mathrm{xz}}$) interactions. Thus, it cannot be described in terms of the Harris-Andersson model (i.e., Pauli repulsion and its weakening by empty $d$ states).

Two-Dimensional Imaging of Potential Waves in Electrochemical Systems by Surface Plasmon Microscopy

Abstract: The potential dependence of resonance conditions for the excitation of surface plasmons was exploited to obtain two-dimensional images of the potential distribution of an electrode with high temporal resolution. This method allows the study of spatiotemporal patterns in electrochemical systems. Potential waves traveling across the electrode with a speed on the order of meters per second were observed in the bistable regime of an oscillatory electrochemical reaction. This velocity is close to that of excitation waves in nerve fibers and is far greater than the velocity of reaction-diffusion waves observed in other chemical systems.

Statistics of Topological Defects and Spatiotemporal Chaos in a Reaction-Diffusion System.

Abstract: The transition between rotating spirals and spatiotemporal chaos (``spiral turbulence'') in an excitable reaction-diffusion system is investigated by means of statistics of topological defects of the concentration field. The ratio of the variance and the mean value of the number of defects show a significant deviation from unity at this point. A change of the defect dynamics within the turbulent regime reveals a strong similarity to a liquid-to-gas transition, while a liquid-solid-like transition occurs upon a jumplike return to the region of spiral stability.

Angle-resolved photoelectron spectroscopy of C60.

Abstract: Angle-resolved photoelectron spectra of gaseous ${\mathrm{C}}_{60}$ were recorded in the photon energy regions from 21 to 108 eV and from 295 to 320 eV. Partial cross sections \ensuremath{\sigma} and the angular distribution anisotropy parameter \ensuremath{\beta} vary significantly with photon energy, particularly in the near-threshold region of the valence and the core ionization regimes. Some of these effects may be attributed to scattering of the outgoing photoelectron by the atoms of the ionized ${\mathrm{C}}_{60}$ molecule. Our results indicate that the observed satellites of the C(1s) main line are most likely of shake-up character. Low-energy electrons emitted below the shake-off threshold indicate the occurrence of K-shell vacancy filling double Auger decay.

The effect of water on the formation of strongly bound oxygen on silver surfaces

Abstract: Interaction of water with an oxygenated Ag(111) surface leads to an enhancement of the surface restructuring and an activated formation of hydroxyl groups (OH) located stably on the surface and incorporated in the subsurface region, as evidenced by means of reflection electron microscopy (REM) and in situ Raman spectroscopy. Dehydroxylation of OHads at elevated temperatures releases the strongly bound oxygen species labelled Oγ at the surface, and offers an alternative to the energetically less favorable pathway for the direct formation of the Oγ species from molecular oxygen.

On the relation between catalytic performance and microstructure of polycrystalline silver in the partial oxidation of methanol

Abstract: Electrolytic silver was investigated as partial oxidation catalyst for the conversion of methanol to formaldehyde. Using the mass spectrometric technique as on-line detector the relation between feed composition and temperature was determined allowing us to conclude that two simultaneous reaction pathways operate under steady state conditions. The microstructure was analysed by XRD and STM. A pronounced restructuring of the surface on the mesoscopic scale was detected. The atomic structure of the γ oxygen phase was determined on the (111) face of facets grown during reaction. The two reaction pathways find their counterparts in two distinctly different surface microstructures providing different geometries for the respective active sites. After prolonged time on stream the high surface mobility of the silver atoms removes all mesoscopic restructuring without changing the conversion characteristics. The observed restructuring is thus considered as a frozen large scale image of the continuously changing surface under reaction conditions.

Delay-induced chaos in catalytic surface reactions : no reduction on pt(100)

Abstract: Deterministic chaos has been observed in the $\mathrm{NO}+\mathrm{CO}$ and $\mathrm{NO}+{\mathrm{H}}_{2}$ reactions on Pt(100). A mathematical model is proposed that explains the origin as being due to delays in the response of a population of reacting adsorbate islands globally coupled via the gas phase. The dynamical equations of this model yield a sequence of period-doubling bifurcations resulting in chaos.

Accelerating fronts in an electrochemical system due to global coupling.

Abstract: We examine spatiotemporal patterns during the reduction of peroxodisulfate to sulfate at silver ring electrodes in an electrochemical cell. The reaction displays bistable behavior in the current-voltage characteristics under potentiostatic conditions. The two stable states are characterized by different voltage drops across the double layer. We investigate transitions in the bistable region between the different states. Spatiotemporally resolved measurements of the voltage drop across the electrode-electrolyte interface show that the transition occurs via fronts. We are able to trigger these fronts by local perturbations of the electric field and thus to examine their dependence on operating parameters. It is shown that the movement of the fronts results from migration currents (movement of ions due to the electric field), and hence the system is described by reaction-migration-diffusion equations. The velocity of the fronts increases with time; hence they are accelerated. This contrasts with usual front solutions of reaction-diffusion systems, which possess a constant velocity in one-dimensional geometries. This acceleration is shown to be a consequence of a global, or more precisely nonlocal, coupling mediated through the electric field.

Catalysis on Microstructured Surfaces - Pattern-Formation during Co Oxidation in Complex Pt Domains

Abstract: The exploration of pattern formation by reaction-diffusion systems in complex bounded domains has begun only recently. While theoretical and numerical information points to a strong interaction between patterns and boundaries, experiments are rare and for heterogeneous catalytic reactions practically nonexistent. By constructing (using microlithography) catalytic surfaces of arbitrary shape and size, we are able to study this interaction for the catalytic oxidation of CO on Pt(110). Experiments along these lines shed light on issues such as anisotropic diffusion and the behavior of individual defects. In addition, certain geometries give rise to patterns that have not been observed on the untreated catalyst and bring to light surface mechanisms that have no analog in homogeneous reaction-diffusion systems. Simple domains of controlled size constitute paradigms that make the comparisons between theory and experiment more fruitful, as we demonstrate through modeling and simulation of such examples. This approach opens the way for systematically probing certain aspects of pattern formation unique to heterogeneous catalysis.

Observation of front bifurcations in controlled geometries: From one to two dimensions.

Abstract: The introduction of microstructures in the form of quasi-one-dimensional channels on a Pt(100) surface allows the detection of front bifurcations for concentration waves from adsorbed O and CO species accompanying their catalytic reaction. At the exits of these channels into larger circular domains, the fronts can be reflected depending on the width of the channels and the external parameters. These findings are reproduced within a reaction-diffusion model.

Transition between fronts and spiral waves in a bistable surface reaction.

Abstract: The transition between wave fronts and spiral waves in the CO oxidation on a Pt(110) surface has been investigated by means of photoemission electron microscopy and treated theoretically by simulations with a reaction-diffusion model under bistable conditions. Periodic external forcing of spirals near the transition to front behavior led to a complex pattern best characterized as a pulsating spiral-front hybrid, thus confirming the bistable nature of the underlying reaction kinetics.

Structure determination of an alkali metal-co coadsorption phase : ni(111)-k/co

Abstract: The structure of a Ni(111)- $(2\ifmmode\times\else\texttimes\fi{}2)$--K $/n$CO $(n\ensuremath{\approx}2)$ coadsorption phase has been determined using K $2p$, C $1s$, and O $1s$ scanned energy mode photoelectron diffraction, and compared with results from pure CO and K adsorption phases having similar coverages. Coadsorbed K has little influence on the local hollow site geometry of the adsorbed CO molecules; the K remains in the atop site of the pure K layer, but the K-Ni bond length increases substantially (by $0.15\ifmmode\pm\else\textpm\fi{}0.05$ \AA{}), as does the Ni-Ni outer layer spacing. The results conflict with aspects of current interpretations of spectroscopic data from such systems.