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D. W. Jepsen

Bio: D. W. Jepsen is an academic researcher from IBM. The author has contributed to research in topics: Low-energy electron diffraction & Electron diffraction. The author has an hindex of 25, co-authored 42 publications receiving 2188 citations.

Papers
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Journal ArticleDOI
J. E. Demuth1, P. M. Marcus1, D. W. Jepsen1
TL;DR: In this paper, a low-energy-electron-diffraction intensity-energy spectra was calculated for Ni (001), (110), and (111) surfaces between 10 and 220 eV by the layer-Korringa-Kohn-Rostoker method and compared with recent room-temperature experimental results.
Abstract: Elastic low-energy-electron-diffraction intensity-energy spectra are calculated for Ni (001), (110), and (111) surfaces between 10 and 220 eV by the layer-Korringa-Kohn-Rostoker method and compared with recent room-temperature experimental results The calculation uses the Wakoh self-consistent muffin-tin potential, retains eight phase shifts, and includes finite temperature effects (assuming a Debye spectrum) An effective Debye temperature of 335\ifmmode^\circ\else\textdegree\fi{}K is found from the temperature dependence of spectral intensities, an energy-dependent imaginary potential roughly of the form $\ensuremath{\beta}=085{E}^{\frac{1}{3}}$ for electron energy $E$ (in eV) is determined by matching features of the calculated spectra to experiment, and the best values of the first interlayer spacing are found to be 176 \AA{} (the bulk spacing) +002 \ifmmode\pm\else\textpm\fi{} 002 \AA{} on the (001) surface, 124 - (006\ifmmode\pm\else\textpm\fi{}002) \AA{} on the (110) surface, and 203 - (0025\ifmmode\pm\else\textpm\fi{}0025) \AA{} on the (111) surface With these parameters, excellent agreement with observed spectra is obtained in positions and shapes of peaks for several beams and a large number of incident angles For all faces a small systematic deviation in peak positions is found with a constant 11-eV inner potential, suggesting an inner potential varying from the expected static value of 135 at low energies to about 9 eV near 220 eV Comparison of relative intensities between calculation with the above $\ensuremath{\beta}(E)$ and experiment suggests that excitation of $3p$ electrons from Ni significantly enhances electron absorption above 65 eV

208 citations

Journal ArticleDOI
P. M. Marcus1, J. E. Demuth1, D. W. Jepsen1
TL;DR: In this paper, the use of LEED intensity spectra for surface structure determination is discussed, and guidelines are suggested for successful application to surface structure, and the special suitability of the muffin-tin potential for the LEED calculation is brought out; the model of the complex crystal potential used for the calculation, and systematic determination of the four parameters of the model from the data for clean Ni are described.

204 citations

Journal ArticleDOI
J. E. Demuth1, D. W. Jepsen1, P. M. Marcus1
TL;DR: In this paper, the degree to which surface atomic arrangement influences the bonding geometry of sulfur atoms on Ni surfaces was investigated by an analysis of experimental low-energy electron diffraction data, showing that the sulfur atoms reside in high-coordination sites, i.e., the atomic hollows of the surface.
Abstract: The degree to which the surface atomic arrangement influences the bonding geometry of sulfur atoms on (001), (110), and (111) Ni surfaces is investigated by an analysis of experimental low-energy electron diffraction data. For all surfaces, the sulfur atoms reside in high-coordination sites---the atomic hollows of the surface; all nearest-neighbor Ni-S bond lengths are less than those of stable bulk compounds.

180 citations

Journal ArticleDOI
TL;DR: In this article, the inconsistency between low-energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) analyses concerning the orientation of molecular CO when chemisorbed in a Ni(001) surface was resolved.
Abstract: We have resolved the inconsistency between low-energy electron diffraction (LEED) and ultraviolet photoemission spectroscopy (UPS) analyses concerning the orientation of molecular CO when chemisorbed in a $c(2\ifmmode\times\else\texttimes\fi{}2)$ structure on the Ni(001) surface. Dynamical analysis of new LEED data indicates that CO stands perpendicular to the Ni surface with the C atom on top of a Ni atom; preliminary results for the Ni-C and C-O separations are 1.72 and 1.15 \AA{}, respectively.

111 citations


Cited by
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Journal ArticleDOI
TL;DR: Based on density functional theory calculations, kinetic measurements, microkinetic and Monte Carlo simulations, thermogravimetric analysis (TGA) experiments, extended X-ray absorption spectroscopy (EXAFS) measurements, and experimental results from the literature, this paper presented a detailed and comprehensive mechanistic picture of the steam reforming process on a Ni catalyst.

1,002 citations

Journal ArticleDOI
TL;DR: In this paper, the double-R reliability factor (RR) was introduced, where R*RR is the variance of R. This enables quantitative statements to be made about the significance of minima in R.
Abstract: A new R-factor is defined which is insensitive to intensity discrepancies between theory and experiment but retains a simple functional form. An error analysis of R introduces another new quantity, the 'double-R' reliability factor, RR, where R*RR is the variance of R. This enables quantitative statements to be made about the significance of minima in R.

661 citations

Book ChapterDOI
01 Jan 1984
TL;DR: The steam reforming process converts hydrocarbons into mixtures of hydrogen, carbon monoxide, carbon dioxide, and methane as discussed by the authors, which is used also for the well-knowm process for improvement of the octane number of gasoline.
Abstract: The steam reforming process converts hydrocarbons into mixtures of hydrogen, carbon monoxide, carbon dioxide, and methane $$C_n H_m + nH_2 O \to nCO + \left( {n + \frac{m}{2}} \right)H_2 \left( { - \Delta H_{298}^0 < 0} \right) $$ (1) $$ CO + H_2 O \rightleftarrows CO_2 + H_2 \left( { - \Delta H_{298}^0 = 41.2kJ\,mol^{ - 1} } \right) $$ (2) $$ CO + 3H_2 \rightleftarrows CH_4 + H_2 O\left( { - \Delta H_{298}^0 = 206.2kJ\,mol^{ - 1} } \right) $$ (3) The expression reforming is misleading since it is used also for the well-knowm process for improvement of the octane number of gasoline [1]. In the gas industry, reforming has generally been used for “the changing by heat treatment of a hydrocarbon with high heating value into a gaseous mixture of lower heating value” [2].

655 citations

Journal ArticleDOI
TL;DR: In this article, a compilation of measured attenuation lengths of low-energy electrons in solids in the energy range (40 to 2000 eV) normally employed in X-ray photoelectron and Auger-electron spectroscopy is presented.

610 citations