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  • 1
    Electronic Resource
    Electronic Resource
    Imaging pattern formation: Bridging the pressure gap (1995)
    Springer
    Applied physics 61 (1995), S. 569-574 
    Details
    ISSN: 1432-0630
    Keywords: PACS: 82.65. Tv; 82.40.Ck
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract.  Two surface-sensitive optical imaging methods, Ellipso-Microscopy for Surface Imaging (EMSI) and Reflection Anisotropy Microscopy (RAM) are introduced. They allow imaging of pattern formation on surfaces, e.g., due to submonolayer coverages of adsorbates, at any arbitrary pressure. In spatio-temporal pattern formation during heterogeneously catalysed reactions this bridges the ‘pressure gap’ between well-defined UHV experiments and ‘real catalysis’. For the CO oxidation on Pt(110), the parameter space for pattern formation was extended up to 100 mbar, i.e., by 5 orders of magnitude compared to earlier investigations by Photo-Emission Electron Microscopy (PEEM) which had to be conducted below 10-3 mbar. With increasing pressure, the synchronisation mechanisms responsible for the observed pattern showed a gradual shift from reaction-diffusion to thermal-kinetic coupling unveiling previously unseen features of pattern formation in catalysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01542865
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  • 2
    Electronic Resource
    Electronic Resource
    UV photostimulated desorption of ammonia from Cu(111) (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 3414-3430 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Upon irradiation with 193 and 308 nm laser light photoinduced desorption of ammonia from Cu(111) was studied at three coverages less than one monolayer (ML). The linear power dependence of the desorption yield and angle-resolved translational energy distributions of desorbed molecules indicate that desorption occurs due to an electronic excitation rather than a thermal process. Polarization measurements indicate an excitation process which is mediated by hot substrate electrons. The isotope effect, i.e., the ratio of the cross sections for photostimulated desorption (at 193 nm) of NH3 and ND3, respectively, decreases from 4.1 ± 1.2 to 1.9 ± 0.5 when the coverage—with respect to the substrate atom density—was raised from ≈0.02 to ≈0.14 ML. The magnitude of this isotope effect suggests that the energy which is required to break the molecule–surface bond is acquired in an intramolecular coordinate during a short-lived electronic excitation. We propose that for high vibrational excitation on the ground-state potential energy surface (PES), efficient coupling of the inversion mode with the molecule–surface coordinate leads to desorption. In order to illustrate the suggested desorption mechanism at a semiquantitative level, we performed trajectory calculations on a two-dimensional model potential energy surface. The results predict that desorption occurs rapidly within a few vibrational periods of the umbrella mode (Tvib∼35 fs)—with comparable energy release into the translational and vibrational degrees of freedom. Ammonia is furthermore expected to desorb in an inverted geometry, i.e., with the hydrogen atoms pointing towards the surface as opposed to the adsorption geometry with the nitrogen end bound to the surface. Angular distributions of flux and mean translational energy are strongly peaked around the surface normal. Their width can be attributed to thermal motions parallel to the surface prior to excitation. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469215
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  • 3
    Electronic Resource
    Electronic Resource
    Stress-induced recrystallization of a protein crystal by electron irradiation (1999)
    [s.l.] : Macmillan Magazines Ltd.
    Nature 399 (1999), S. 51-54 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Ordering of a system of particles into its thermodynamically stable state usually proceeds by thermally activated mass transport of its constituents. Particularly at low temperature, the activation barrier often hinders equilibration—this is what prevents a glass from crystallizing and a ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/19947
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  • 4
    Electronic Resource
    Electronic Resource
    Oscillatory instabilities during formic acid oxidation on Pt(100), Pt(110) and Pt(111) under potentiostatic control. I. Experimental (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 979-990 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The experimental characterization of the current/outer potential (I/U) behavior during the electrochemical CO oxidation on Pt(100), Pt(110) and Pt(111) is used as the first step towards a thorough investigation of the processes occurring during the electrochemical formic acid oxidation. The CO study is followed by new cyclovoltammetric results during the electrochemical formic acid oxidation on the corresponding Pt single crystals. At high concentrations of formic acid, the cyclovoltammograms revealed a splitting of the large current peak observed on the cathodic sweep into two peaks whose dependence on scan rate and reverse potential was investigated. It turned out that the presence of a sufficiently large ohmic resistance R was crucial for oscillatory instabilities. Given an appropriate resistance, all three Pt surfaces were found to exhibit current oscillations at both low and high formic acid concentrations. On Pt(100) stable mixed-mode oscillations were observed. In addition, the sensitivity of the oscillations to stirring was investigated. Whereas the period-1 oscillations were found to be independent of stirring, the mixed-mode oscillations transformed into simple oscillations with stirring. The mechanism giving rise to instability and oscillations is described. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474450
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  • 5
    Electronic Resource
    Electronic Resource
    Vibrations, coverage, and lateral order of atomic nitrogen and formation of NH3 on Ru(101¯0) (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9313-9319 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociative chemisorption of nitrogen on the Ru(101¯0) surface has been studied using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). To prepare a surface covered by atomic nitrogen we have used ionization-gauge assisted adsorption. A saturation coverage of θN=0.6 is achieved of which about 30% is in the subsurface region. At saturation coverage a ( 2 1−1 1) LEED pattern is observed. The ν(parallel)(Ru–N) mode at 41 meV and the ν⊥(Ru–N) mode at 60 meV are identified. Upon exposing the nitrogen covered surface to hydrogen at 300 K we have observed the formation of NH3 which is characterized by its symmetric bending mode δs(NH3) at 149 meV. At 400 K, NH3 could not be detected. The reaction intermediate NH is stable up to 450 K and has been identified by its vibrational losses ν(Ru–NH) at 86 meV, and ν(N–H) at 408 meV. The TD spectra of mass 14 show three desorption states of nitrogen, Nα at 740 K (from subsurface N), Nβ shifting from 690 to 640 K with increasing coverage, and Nγ at 550 K. The activation energy for desorption via the Nβ state is 120±10 kJ/mol. The TD spectra of mass two showed three desorption states at 450, 550, and 650 K due to the decomposition of NHx. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474042
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  • 6
    Electronic Resource
    Electronic Resource
    Coverage, lateral order, and vibrations of atomic nitrogen on Ru(0001) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 8944-8950 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The N/Ru(0001) system was studied by thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), and high-resolution electron energy-loss spectroscopy (HREELS). Atomic nitrogen was prepared by NH3 decomposition at sample temperatures decreasing from 500 to 350 K during NH3 exposure. A maximum N coverage of θN=0.38 could thus be achieved. (square root of 3), split 2×2 and 2×2 LEED patterns were observed for decreasing θN. After NH3 decomposition and before annealing the sample to a temperature above 400 K, the surface is composed of adsorbed N, H, and NH species. This composite layer exhibits a split (square root of 3) LEED pattern due to domains of size 4 with heavy walls. This phase decays through dissociation of NH leading to sharp first-order type desorption peaks of H2 and N2. From the weak intensity of the ν(Ru–NH) stretch mode it is concluded that NH is adsorbed at threefold-hollow sites. The energy of the ν(Ru–N) mode shifts from 70.5 to 75.5 meV when θN is increased from 0.25 to 0.38. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472624
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  • 7
    Electronic Resource
    Electronic Resource
    Oscillatory instabilities during formic acid oxidation on Pt(100), Pt(110) and Pt(111) under potentiostatic control. II. Model calculations (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 991-1003 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A kinetic model is developed for the electrocatalytic oxidation of formic acid on Pt under potentiostatic control. The model development proceeds stepwise via a simple model of the electrocatalytic CO oxidation. The full model consists of four coupled, nonlinear ordinary differential equations. The scanned and stationary current/outer potential (I/U) behavior, stationary current oscillations, two-parameter bifurcation diagrams and stirring effects are simulated using realistic model parameters. The numerical findings are found to be consistent with the experimental results given by Strasser et al. The model reproduces period-1 as well as mixed-mode oscillations. Furthermore, a mechanistic analysis of the model was performed: two suboscillators are identified whose characteristics allow a plausible interpretation of the observed dynamics. After a classification of the suboscillators into previously described categories, an attempt is made to identify the minimal mechanistic requirements for electrochemical current oscillations. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474451
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  • 8
    Electronic Resource
    Electronic Resource
    Electronic states of the C6H6/Cu{111} system: Energetics, femtosecond dynamics, and adsorption morphology (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 9155-9165 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two-photon-photoemission (2PPE) spectroscopy is employed to characterize electronic states of a bilayer C6H6/Cu{111} system at 85 K. The unoccupied benzene π* e2u state is observed with a binding energy of 4.6 eV above the Fermi level. This result agrees with inverse-photoemission (IPE) data and provides a case where the determination of the binding energy is identical for 2PPE and IPE. The π* e2u state is assigned in the 2PPE scheme as a final state which is the first observed final state in 2PPE of adsorbate-surface systems. The dependence of the electron dynamics on the morphology of an incomplete adsorption layer is also investigated. Two (n=1)-like image potential states A and B are observed which presumably originate from two different C6H6 adsorption geometries in the bilayer regime. The two image states A and B are characterized by electron effective masses of 1.1 and 1.9 me, binding energies of 3.30 and 3.45 eV above the Fermi level, and lifetimes of 40 and 20 fs, respectively. The dielectric continuum model and the Kronig–Penney model are employed to simulate the origin of (n=1)-like image states. The work function decreases from 4.9 eV at clean Cu{111} to 4.0 eV at bilayer coverage. The change of the work function and the observation of two image states suggest the redefining of the ratio of the numbers of benzene molecules in the first and the second layers of the bilayer regime to approximately 1:1 instead of 1:2, as previously reported. 2PPE is shown to be sensitive to the changes of morphologies, local work functions, and adsorbate-surface potentials during the layer formation. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477468
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  • 9
    Electronic Resource
    Electronic Resource
    Interaction of oxygen with Al(111) at elevated temperatures (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 1740-1747 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction of oxygen with Al(111) was investigated by STM at temperatures between 350 and 530 K, by annealing an oxygen precovered surface and by adsorption of oxygen on the hot surface. For exposures up to 10 L and temperatures up to 470 K a considerable part of the oxygen exists still in the chemisorbed state, another part transforms into Al oxide. In contrast to 300 K chemisorbed Oad atoms are mobile at elevated temperatures, and compact, hexagonal (1×1)Oad islands develop by an ordinary nucleation and growth scheme. This evidences attractive interactions between the oxygen atoms on (1×1) sites. From the lateral distribution of Oad islands a diffusion barrier of 1.0–1.1 eV is derived. The imaging of the islands of the (1×1) phase by STM depends on their size, which is understood by a different imaging of the Oad/Al adsorbate complexes at the island borders. Defects in the islands and bright features at the edges are interpreted as nuclei of aluminum oxide. Additional features which appear as topographic holes may be attributed to nonconducting Al oxide grains. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475546
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  • 10
    Electronic Resource
    Electronic Resource
    Light emission during the agglomeration of silver clusters in noble gas matrices (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 5137-5142 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The agglomeration of small silver clusters in noble gas matrices to form larger ones may be accompanied by the emission of light. Spectral analysis reveals that part of radiation intensity can be attributed to fluorescence from excited metal atoms, dimers and trimers the formation of which results from cluster/cluster agglomeration as a consequence of the gain in binding energy. The remaining spectral features must be assigned to excited clusters Agn, with n≥4. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475919
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