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The adsorption and reactions of NO2 on the Ag(111) surface. Part II: Adsorption at 25K and annealing to 300K
No full textThe adsorption and reactions of NO2 on the Ag(111) surface. Part I: XPS/UPS and annealing studies between 90K and 300K
No full textMECHANISMS RESPONSIBLE FOR THE SHIFTS OF CORE-LEVEL BINDING-ENERGIES BETWEEN SURFACE AND BULK ATOMS OF METALS
No full textInterpretations are given for the core-level binding energies, BE's, of the surface and bulk atoms of metals; in particular, the origin of possible differences, or shifts, between these BE's is considered. The mechanisms which are responsible for the BE shifts are identified and characterized through theoretical analyses of the surface electronic structure. The dominant mechanisms for the BE shifts are those which lead to the initial-state changes in the orbital energies and, hence, the Koopmans' Theorem BE's. This is particularly important because it means that the shifts can be directly related to the chemistry and physics of the systems. The surface electronic structure is described with molecular-orbital wavefunctions for cluster models of the surface; this theoretical approach is particularly well suited for the study of local chemical interactions. The different mechanisms act in a cancelling fashion which leads to rather small net shifts between the BE's of bulk and surface atoms. The mechanisms which explain the surface and bulk core-level BE shifts are rather general and may provide a basis for understanding other cases of core-level BE shifts
Evidence for oxygen-island formation on Al(111): Cluster-model theory and x-ray photoelectron spectroscopy
No full textThe O 1s x-ray photoelectron spectroscopy spectrum for Al(111)/O at 300 K shows two components whose behavior as a function of time and variation of detection angle are consistent with either (a) a surface species represented by the higher binding-energy (BE) component and a subsurface species represented by the lower BE component, or (b) small close-packed oxygen islands with the interior atoms represented by the lower BE component and the perimeter atoms by the higher BE component. We have modeled both situations using ab initio Hartree-Fock wave functions for clusters of Al and O atoms. For an O atom in a threefold site, it was found that a below-surface position gave a higher O 1s BE than an above-surface position, incompatible with interpretation (a). This change in the O 1s BE could arise because the bond for O to Al may have a more covalent character when the O is below the surface than when it is above the surface. We present evidence consistent with this view. An O adatom island with all the O atoms in threefold sites gives calculated O 1s BE's which are significantly higher for the perimeter O atoms. Further, the results for an isolated O island without the Al substrate present also give higher BE¿s for the perimeter atoms. Both these results are consistent with interpretation (b). Published scanning-tunneling-microscopy data supports the suggestion that the chemisorbed state consists of small, close-packed islands, whereas the presence of two vibrational modes in high-resolution electron-energy-loss spectroscopy data has been interpreted as representing surface and subsurface oxygen atoms. In light of the present results, we suggest that a vibrational interpretation in terms of interior and perimeter adatoms should be considered
X-ray photoelectron spectroscopy of oxygen adsorbates on Al(111): Theory experiment
Full text linkWe present the result of polar angle resolved x¿ray photoemission spectroscopy on Al(111)/O and cluster calculations of the O(1s) binding energy (BE) for various model situations. In the experimental data two O(1s) peaks are observed, separated by 1.3 eV. The angular behavior (depth¿resolution) could indicate that the lower BE peak is associated with an O atom under the surface, and the higher BE peak with an O atom above the surface. Equally, it could indicate oxygen islands on the surface where the perimeter atoms have a higher O(1s) BE than the interior atoms. The cluster calculations show that the former interpretation cannot be correct, since an O ads below the surface has a higher calculated O(1s) BE than one above. Cluster calculations simulating oxygen islands are, however, consistent with the experimental data
ORIGIN OF THE BA CORE-LEVEL BINDING-ENERGY DIFFERENCE BETWEEN TETRAGONAL AND ORTHORHOMBIC YBA2CU3O7-DELTA
No full textSintered bulk samples of the orthorhombic, superconducting, phase of YBa2Cu3O7-delta exhibit a congruent-to 1.5 eV Ba core-level decrease compared to the tetragonal, nonsuperconducting phase. This is also observed for thin-film material in the two phases. Using ab initio cluster calculations, we show that shifts of this magnitude are explainable by isotropic contraction of the Ba-O bond length in the 0.01- to 0.02-angstrom range, values compatible with the known lattice-parameter changes between the phases. Finally, even in situ cleaved good-quality single crystals of the orthorhombic phase show a chemically shifted Ba component for some regions of the surface, which may be due to Ba-O bond-length contractions at the surface-vacuum interface
SIMS studies of adsorption and reaction at the metal surfaces: correlation with other surface sensitive techniques
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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