2,225 research outputs found
The structure of atomic nitrogen adsorbed on Fe(100)
Nitrogen atoms adsorbed on a Fe(100) surface cause the formation of an ordered c(2 Ă 2) overlayer with coverage 0.5. A structure analysis was performed by comparing experimental LEED IâV spectra with the results of multiple scattering model calculations. The N atoms were found to occupy fourfold hollow sites, with their plane 0.27 Ă
above the plane of the surface Fe atoms. In addition, nitrogen adsorption causes an expansion of the two topmost Fe layers by 10% (= 0.14 Ă
). The minimum r-factor for this structure analysis is about 0.2 for a total of 16 beams. The resulting atomic arrangement is similar to that in the (002) plane of bulk Fe4N, thus supporting the view of a âsurface nitrideâ and providing a consistent picture of the structural and bonding properties of this surface phase
Recognition of alterations induced by erly vaccinia surface anigens and dependence of virus-specific lysis on H-2 antigen concentration on target cells
Existence of a âHotâ Atom Mechanism for the Dissociation of O<sub>2</sub> on Pt(111)
The dissociation of O2 on a Pt(111) surface was studied by variable temperature scanning tunneling microscopy at 150â106 K.âThe two oxygen atoms created by the dissociation appear in pairs, with average distances of two lattice constants. Since thermal random walk sets in only at around 200 K, with a diffusion barrier of 0.43 eV and a preexponential factor of 10â6.3cm2sâ1, the distribution of distances at around 160 K evidences nonthermal processes during the dissociation. It is concluded that transient ballistic motion exists, where the short range traveled is in agreement with recent molecular dynamics studies
The surface state and catalytic properties of Pt black after O<sub>2</sub>-H<sub>2</sub> cycles
XPS and UPS of a Pt black catalyst after customary H2-O2 regeneration shows considerable amounts of residual C as well as surface OH/H2O species. Surface C could not be removed even by O2 at 800 K. Oxygenates are stable even after H2 treatment up to 750 K. Their chemical state has been tentatively identified by comparing XPS and UPS results. Catalytic transformations of n-hexane on Pt black treated analogously is reported and the effect of surface species on catalytic properties discussed. Possible consequences of the presence of stable surface OH/H2O species on H2-O2 titrations are mentioned
{\it Ab initio} NMR chemical shifts and quadrupolar parameters for phases and their precursors
The Gauge-Including Projector Augmented Wave (GIPAW) method, within the
Density Functional Theory (DFT) Generalized Gradient Approximation (GGA)
framework, is applied to compute solid state NMR parameters for in
the , , and aluminium oxide phases and their gibbsite
and boehmite precursors. The results for well-established crystalline phases
compare very well with available experimental data and provide confidence in
the accuracy of the method. For -alumina, four structural models
proposed in the literature are discussed in terms of their ability to reproduce
the experimental spectra also reported in the literature. Among the considered
models, the structure proposed by Paglia {\it et al.} [Phys. Rev.
B {\bf 71}, 224115 (2005)] shows the best agreement. We attempt to link the
theoretical NMR parameters to the local geometry. Chemical shifts depend on
coordination number but no further correlation is found with geometrical
parameters. Instead our calculations reveal that, within a given coordination
number, a linear correlation exists between chemical shifts and Born effective
charges
Electrochemical machining of stainless steel microelements with ultrashort voltage pulses
An electrochemical pulse technique enables the fabrication of three-dimensional microelements from stainless steel. The method is based on the application of ultrashort (nanosecond) voltage pulses, whereupon electrochemical reactions are locally confined with submicrometer precision. Employing properly shaped tool electrodes enables the machining of freestanding cantilevers or microstructures directly to a metal sheet. Due to gentle removal of the material, the grain structure of the material is revealed without any chemical or mechanical modifications. This is demonstrated by measuring the vibration frequency of a cantilever, and agrees well with the value derived from the bulk material properties
Surface Structure and Catalytic Oxidation Oscillations
A cellular automaton model is used to describe the dynamics of the catalytic
oxidation of on a surface. The cellular automaton rules account
for the structural phase transformations of the substrate, the reaction
kinetics of the adsorbed phase and diffusion of adsorbed species. The model is
used to explore the spatial structure that underlies the global oscillations
observed in some parameter regimes. The spatiotemporal dynamics varies
significantly within the oscillatory regime and depends on the harmonic or
relaxational character of the global oscillations. Diffusion of adsorbed
plays an important role in the synchronization of the patterns on the substrate
and this effect is also studied.Comment: Latex file with six postscript figures. To appear in Physica
Interactions between alkali metals and oxygen on a reconstructed surface: An STM study of oxygen adsorption on the alkali-metal-covered Cu(110) surface
Room-temperature adsorption of oxygen on potassium- and cesium-precovered Cu(110) surfaces was studied by scanning tunneling microscopy. Depending on the alkali-metal precoverage, two different scenarios exist for the structural evolution of the surfaces. For alkali-metal coverages Ξalkâ€0.13 ML [Ξalk=0.13 corresponds to the (1Ă3) missing-row reconstructed Cu(110) surface], oxygen adsorption leads first to a transient contraction of the missing rows into islands of a (1Ă2) structure. After longer exposures it causes the local removal of the alkali-metal-induced reconstruction, and the (2Ă1) Cu-O ââadded-rowââ structure with ΞO=0.5 is formed. In this structure the alkali-metal atoms are incorporated in the Cu-O chains. For higher alkali-metal precoverages, in the range of the (1Ă2) reconstruction (Ξalkâ0.2), more than one-half a monolayer of oxygen can be incorporated into the (1Ă2) phase with only a minor structural effect before, at higher oxygen coverages, complex oxygenâalkali-metalâCu structures with oxygen coverages well above 0.5 ML are formed. The saturation oxygen coverage is drastically enhanced beyond ΞO=0.5, the quasisaturation value of the clean surface. Based on mass-transport arguments the substrate is reconstructed for all ratios of oxygen and alkali metal investigated here. Hence, adsorbate-substrate interactions are essential for these structures; they are not dominated by interactions between alkali metals and oxygen, i.e., by adsorbate-adsorbate interactions
Kinetic oscillations in the NO + CO reaction on Pt(100): Experiments and mathematical modeling
The reaction of NO and CO on Pt(100) exhibits two branches of steady state production of N2 and CO2 and the occurrence of kinetic oscillations. This system was studied under steady flow conditions in the 10â6mbar total pressure range using lowâenergy electron diffractionâ(LEED), work function measurement, and mass spectrometry for determination of the reaction rate. These studies revealed that kinetic oscillations can only be initiated from one of the two stable reaction branches. Two separate existence regions were detected in which the oscillations are always damped. Oscillations can be very reproducibly excited by slight decreases in temperature. The 1Ă1 hex phase transition of the surface structure was observed to take place only in one of the two regions of reaction rate oscillations. Its influence seems to be of minor relevance to the mechanism of oscillations as oscillations in one region occur on the surface that maintains a 1Ă1 structure. The experiments were modeled by a set of coupled differential equations based on knowledge about the elementary reaction steps. The model calculations reproduced the steady states of the reaction as well as the occurrence of kinetic oscillations in different ranges in excellent agreement with experimental observation. In the model, the phase transition also has no relevance for the oscillation mechanism. The occurrence of oscillations can be rationalized in terms of a periodic sequence of autocatalytic ââsurface explosionsââ and the restoration of an adsorbateâcovered surface. The damping, experimentally observed, is attributed to insufficient spatial coupling between different regions of the surface
Exoelectron emission during oxidation of Cs films
During oxidation of thin Cs films, a nonadiabatic surface reaction manifests itself in the emission of electrons. This effect was investigated in detail by combining measurements of the current and of energy distributions of these exoelectrons with studies on the electronic properties of the surface by means of ultraviolet photoelectron spectroscopy and metastable deexcitation spectroscopy. Exoelectron emission occurs via Auger deexcitation of the empty state derived from the O2 affinity level. This process is confined to the stage Cs2O2âCsO2 in which resonance ionization of the affinity level of the impinging O2 molecule upon crossing the Fermi level EF is efficiently suppressed due to the absence of metallic states near EF. A kinetic model based on the successive steps involved in the oxidation of Cs is developed which describes qualitatively well all the experimental findings
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