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  • 1
    Electronic Resource
    Electronic Resource
    Isothermal measurements of NH3 and ND3 desorption from Cu(001) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7494-7498 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The kinetics of NH3 and ND3 desorption from Cu(001) surface have been measured by time-resolved electron energy loss spectroscopy. The desorption precess is observed to be characterized by pseudo-first-order kinetics. The desorption energy and the prefactor have been determined for both NH3 and ND3. The prefactors are unusually low as compared to those observed for simple diatomic molecules. Moreover, we observed an unusual behavior consistent with an isotope kinetic compensation effect. Both of these observations can be explained within the context of transition state theory by the interplay between isotopic masses and the shifts of frustrated vibrational modes of adsorbed molecules. Our results imply that the conventional Arrhenius form would not be precisely applicable to molecular desorption, if a large enough temperature range could be measured, particularly for hydrogen-containing molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460180
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  • 2
    Electronic Resource
    Electronic Resource
    Desorption and molecular interactions on surfaces: CO/Cu(001) and Cu(011) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 2281-2293 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report time-resolved electron energy loss spectroscopy results for the desorption of CO from Cu(001) and Cu(011). The measurements were performed isothermally and effectively at constant coverage. We present an analysis based on a quasiequilibrium model that expresses the pseudo-first-order desorption rate constant in terms of a virial expansion. The results of this analysis demonstrate the existence of small, long-range attractive interactions as well as the expected short-range repulsive interactions between adsorbed molecules. The interplay between these two types of interactions leads to a kinetic compensation effect that can be easily understood in terms of vanishing virial coefficients. Thus, in these simple systems, the origin of the kinetic compensation effect lies in the statistical thermodynamics of the adsorbed phase, while the dynamical events associated with the desorption event apparently play a lesser role. The relationship of this analysis technique to others is investigated, and its generality is emphasized.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459900
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  • 3
    Electronic Resource
    Electronic Resource
    Time-resolved electron energy loss spectroscopy study of water desorption from Ag(011) (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 7964-7971 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present time-resolved electron energy loss spectroscopy (EELS) results for the desorption of submonolayer coverages of H2O and D2O from Ag(011). The measurements were performed isothermally and effectively at constant coverage. At coverages above a few percent of a monolayer, we observe that the desorption is characterized by zero-order kinetics. That is, the desorption rate is independent of coverage. We hypothesize that this results from the formation of two-dimensional water islands on the surface at all coverages and temperatures measured. The desorption process is governed in part by the dissociation of water from the edges of these islands. Rapid diffusion on the surface maintains a quasiequilibrium between the molecules bound to islands and isolated adsorbed water molecules, thereby leading to pseudo-zero-order kinetics. This result is interpreted upon general thermodynamic grounds, and also using a simple kinetic model which leads to pseudo-zero-order desorption kinetics. An unusual hydrogen kinetic isotope effect is reported which lends support to our interpretation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457214
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  • 4
    Electronic Resource
    Electronic Resource
    Coverage-dependent desorption measurements for CO/Ag(011) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 8592-8598 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report time-resolved electron energy loss spectroscopy results for the thermal desorption of CO physisorbed onto Ag(011). The measurements were performed isothermally and effectively at constant coverage. We observe that the logarithm of the measured pseudo-first-order desorption rate constant increases approximately linearly with coverage over the temperature and coverage ranges accessible to our measurements. The results are analyzed in terms of a quasiequilibrium model in which the desorption rate constant is expressed analytically as a virial expansion. Using this analysis, we determine that at low coverage the molecules adsorb with their axes parallel to the surface plane and are oriented with respect to the crystalline axes. We find that long-range interactions extending out to 7–8 neighbor shells must be included to fit our results. The magnitude of these long-range interactions is reasonably well described by the gas phase intermolecular potential if screening by substrate image charges is included.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461238
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  • 5
    Electronic Resource
    Electronic Resource
    Lateral interactions and corrugation in physisorption systems: CH4/Cu(100) (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 7808-7814 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present and analyze isothermal adsorption and desorption measurements for methane physisorbed on a Cu(100) surface to characterize the interplay between molecule–surface and molecule–molecule interactions. A 2D gas–liquid phase boundary for the methane layer is observed and partially mapped in coverage and temperature. Comparing our measured critical temperature of 66 K ±2 K to previous work on related systems implies an attractive well between physisorbed molecules of ∼130 K. This reduction compared to the gas phase well can be adequately understood in terms of dielectric screening of the dispersion interaction. By contrast, analyzing the low-coverage adsorption and desorption isotherms using a quasiequilibrium model and imperfect gas statistics, we deduce a lateral attractive intermolecular potential well of only 40–50 K. This drastic reduction of the attractive well from the gas phase value is not consistent with image charge screening, and is difficult to reconcile with the observed critical temperature. This discrepancy is discussed in terms of surface corrugation which dictates different positions for molecule–surface and molecule–molecule potential energy minima. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472562
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  • 6
    Electronic Resource
    Electronic Resource
    Effect of structural incoherence on the low-angle diffraction pattern of synthetic multilayer materials (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 905-912 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The sequential, layering techniques used to prepare multilayer materials result in significant structural incoherence due to deviations from the intended thicknesses within an elemental layer and local deviations from the average due to islanding of the depositing elements during deposition. We demonstrate that if the domain size of the structural incoherence is large compared with the wavelength of the scattering radiation, the structural incoherence manifests itself in the low-angle diffraction pattern by attenuating the intensity of the subsidiary maxima relative to the Bragg maxima. We also show that the subsidiary maxima in the low-angle diffraction pattern of a multilayer result from incomplete destructive interference from all of the interfaces, not just from the top and bottom surface of the film. A technique for incorporating structural incoherence when modeling the diffraction pattern of a multilayer structure is presented. The ability of this model to simulate the experimental diffraction pattern of an iron-silicon multilayer is demonstrated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354857
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  • 7
    Electronic Resource
    Electronic Resource
    Nonlinear optical spectroscopy of the Ag(111) surface in an electrolyte and in vacuum (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 5535-5546 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper, we investigate the electronic structure of a metal surface in the presence of aqueous electrolyte and an applied potential by optical second harmonic generation (SHG). We have obtained the detailed wavelength dependence (λSH=300–350 nm) of the SH response from Ag(111) in both an aqueous electrolyte and in ultrahigh vacuum (UHV) and find that, when the Ag(111) electrode is biased at the potential of zero charge (PZC), the SH response can be strongly correlated with the SH response in UHV. For the surface in both environments, there is a sharp peak near 3.82 eV. Possible contributing factors to this peak are discussed. In the electrochemical environment, the effect of applied potential on the SH response at longer wavelengths (nonresonant regime) is consistent both with previous observations at fixed frequencies and predictions of the surface charge density (SCD) model. At resonant wavelengths, there is a dramatic deviation from behavior predicted by the SCD model, a result consistent with previous experiments at discrete wavelengths.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465946
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  • 8
    Electronic Resource
    Electronic Resource
    Isothermal coverage dependent measurements of NH3 and ND3 desorption from Cu(001) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 5355-5363 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present time-resolved electron energy loss spectroscopy measurement of isothermal desorption of NH3 and ND3 from Cu(001) as a function of coverage. The desorption process is characterized by pseudo-first-order kinetics. The pseudo-first-order rate constant increases approximately exponentially as a function of adsorbed ammonia coverage. The results of an analysis based on a quasiequilibrium approximation which expresses the desorption rate in terms of a virial expansion in adsorbate coverage are reported. This demonstrates that the intermolecular interactions are dominated by short range repulsive lateral interactions, in qualitative accord with expectations of dipolar interactions. The results also suggest the existence of longer-range attractive interactions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461650
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  • 9
    Electronic Resource
    Electronic Resource
    Substrate-mediated dispersion interaction effects in the properties of a physisorbed gas (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 4152-4159 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory of substrate-mediated dispersion interactions is applied to analyze a set of quasiequilibrium measurements of adsorption isobars and desorption isotherms of the CH4/Ag(011) physisorption system. The theory provides for the construction of an adsorbate intermolecular potential from the gas phase potential, with the spacing between the metal surface and the adsorbed layer as the sole adjustable parameter. The best fits to the kinetic and thermodynamic data yield a consistent intermolecular potential, with a well depth of approximately two-thirds of the free space value. The limited dynamic range of the measurements does not permit a determination of the form of the intermolecular potential. However, the good agreement of the theory with both data sets lends strong support for the quasiequilibrium model, and supports the proposition that the properties of physisorbed layers can be predicted using gas phase potentials while accounting for substrate-mediated dispersion interaction effects.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466111
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  • 10
    Electronic Resource
    Electronic Resource
    Dispersion compensation high-resolution electron energy-loss spectrometer for time-resolved surface studies (1991)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 62 (1991), S. 1256-1262 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new high-resolution electron energy-loss spectrometer based on the dispersion compensation (DC) concept has been designed, built, and tested. The parallel processing inherent in DC has allowed us to attain signal levels two orders of magnitude in excess of conventional designs while operating at comparable resolution. This is consistent with expectations based on simple model calculations. The resolving power of the spectrometer is not as good as theoretically predicted probably due to uncontrolled beam angle effects. Initial tests on a variety of systems are presented. We analyze the strengths and weaknesses of the design and suggest future improvements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1142008
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