700 research outputs found
Adlayer core-level shifts of random metal overlayers on transition-metal substrates
We calculate the difference of the ionization energies of a core-electron of
a surface alloy, i.e., a B-atom in a A_(1-x) B_x overlayer on a
fcc-B(001)-substrate, and a core-electron of the clean fcc-B(001) surface using
density-functional-theory. We analyze the initial-state contributions and the
screening effects induced by the core hole, and study the influence of the
alloy composition for a number of noble metal-transition metal systems. Data
are presented for Cu_(1-x)Pd_x/Pd(001), Ag_(1-x) Pd_x/Pd(001), Pd_(1-x)
Cu_x/Cu(001), and Pd_(1-x) Ag_x/Ag(001), changing x from 0 to 100 %. Our
analysis clearly indicates the importance of final-state screening effects for
the interpretation of measured core-level shifts. Calculated deviations from
the initial-state trends are explained in terms of the change of inter- and
intra-atomic screening upon alloying. A possible role of alloying on the
chemical reactivity of metal surfaces is discussed.Comment: 4 pages, 2 figures, Phys. Rev. Letters, to appear in Feb. 199
The structure of epitaxial V2O3 films and their surfaces : a medium energy ion scattering study
Medium energy ion scattering, using 100 keV H+ incident ions, has been used to investigate the growth of epitaxial films, up to thicknesses of ~200 Å, of V2O3 on both Pd(111) and Au(111). Scattered-ion energy spectra provide a measure of the average film thickness and the variations in this thickness, and show that, with suitable annealing, the crystalline quality is good. Plots of the scattering yield as a function of scattering angle, so-called blocking curves, have been measured for two different incidence directions and have been used to determine the surface structure. Specifically, scattering simulations for a range of different model structures show poor agreement with experiment for half-metal (….V’O3V) and vanadyl (….V’O3V=O) terminations, with and without surface interlayer relaxations. However, good agreement with experiment is found for the modified oxygen-termination structure, first proposed by Kresse et al., in which a subsurface V half-metal layer is moved up into the outermost V buckled metal layer to produce a VO2 overlayer on the underlying V2O3, with an associated layer structure of ….O3VV’’V’O3
Metastable precursors during the oxidation of the Ru(0001) surface
Using density-functional theory, we predict that the oxidation of the
Ru(0001) surface proceeds via the accumulation of sub-surface oxygen in
two-dimensional islands between the first and second substrate layer. This
leads locally to a decoupling of an O-Ru-O trilayer from the underlying metal.
Continued oxidation results in the formation and stacking of more of these
trilayers, which unfold into the RuO_2(110) rutile structure once a critical
film thickness is exceeded. Along this oxidation pathway, we identify various
metastable configurations. These are found to be rather close in energy,
indicating a likely lively dynamics between them at elevated temperatures,
which will affect the surface chemical and mechanical properties of the
material.Comment: 11 pages including 9 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Stability of sub-surface oxygen at Rh(111)
Using density-functional theory (DFT) we investigate the incorporation of
oxygen directly below the Rh(111) surface. We show that oxygen incorporation
will only commence after nearly completion of a dense O adlayer (\theta_tot =
1.0 monolayer) with O in the fcc on-surface sites. The experimentally suggested
octahedral sub-surface site occupancy, inducing a site-switch of the on-surface
species from fcc to hcp sites, is indeed found to be a rather low energy
structure. Our results indicate that at even higher coverages oxygen
incorporation is followed by oxygen agglomeration in two-dimensional
sub-surface islands directly below the first metal layer. Inside these islands,
the metastable hcp/octahedral (on-surface/sub-surface) site combination will
undergo a barrierless displacement, introducing a stacking fault of the first
metal layer with respect to the underlying substrate and leading to a stable
fcc/tetrahedral site occupation. We suggest that these elementary steps,
namely, oxygen incorporation, aggregation into sub-surface islands and
destabilization of the metal surface may be more general and precede the
formation of a surface oxide at close-packed late transition metal surfaces.Comment: 9 pages including 9 figure files. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Evaluation of the chimere model at high resolution over Europe : focus on urban area
International audienc
Surface Core Level Shifts of Clean and Oxygen Covered Ru(0001)
We have performed high resolution XPS experiments of the Ru(0001) surface,
both clean and covered with well-defined amounts of oxygen up to 1 ML coverage.
For the clean surface we detected two distinct components in the Ru 3d_{5/2}
core level spectra, for which a definite assignment was made using the high
resolution Angle-Scan Photoelectron Diffraction approach. For the p(2x2),
p(2x1), (2x2)-3O and (1x1)-O oxygen structures we found Ru 3d_{5/2} core level
peaks which are shifted up to 1 eV to higher binding energies. Very good
agreement with density functional theory calculations of these Surface Core
Level Shifts (SCLS) is reported. The overriding parameter for the resulting Ru
SCLSs turns out to be the number of directly coordinated O atoms. Since the
calculations permit the separation of initial and final state effects, our
results give valuable information for the understanding of bonding and
screening at the surface, otherwise not accessible in the measurement of the
core level energies alone.Comment: 16 pages including 10 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Tropospheric Ozone In A Mountain Forest Area: Spatial Distribution And Its Relation With Meteorology And Emission Sources
Biological injuries to forest ecosystems in two lateral valleys of Valtellina (Italy) have been
studied. The selected areas are characterized by different forest novel decline symptoms and
ozone concentration levels. Analyses of meteorological and air quality data collected by fixed
and mobile stations located in the two valleys are presented. Ozone concentration has been
measured both by passive samplers and continues analyzers and the ozone vertical gradient in
one valley has been determined. In order to investigate the relation between emission sources,
ambient ozone levels and plant biological injuries, a modeling research project has been
started. As preliminary results the wind field obtained by the application of two diagnostic
meteorological models, MINERVE and CALMET, and the computation of the biogenic
emissions are presented. Some examples of spatial distribution and temporal trend of the most
important pollutants emitted by plants are discussed
Potential, core-level and d band shifts at transition metal surfaces
We have extended the validity of the correlation between the surface
3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d
transition metal series and to the neighboring elements Sr and Ag via accurate
first-principles calculations. We find that the correlation is quasilinear and
robust with respect to the differencies both between initial and final-state
calculations of the SCLS's and two distinct measures of the SDBS's. We show
that despite the complex spatial dependence of the surface potential shift
(SPS) and the location of the 3d and 4d orbitals in different regions of space,
the correlation exists because the sampling of the SPS by the 3d and 4d
orbitals remains similar. We show further that the sign change of the SCLS's
across the transition series does indeed arise from the d band-narrowing
mechanism previously proposed. However, while in the heavier transition metals
the predicted increase of d electrons in the surface layer relative to the bulk
arises primarily from transfers from s and p states to d states within the
surface layer, in the lighter transition metals the predicted decrease of
surface d electrons arises primarily from flow out into the vacuum.Comment: RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.
Palliative Electrochemotherapy in Vulvar Carcinoma: Preliminary Results of the ELECHTRA (Electrochemotherapy Vulvar Cancer) Multicenter Study
Vulvar cancer (VC) is a rare disease of which recurrence poses management problems due to patients’ advanced age and comorbidities, and to the localization of the disease. Palliative treatments, allowing local disease control in patients previously treated with multimodal therapies or with comorbidities, are lacking. In this study we tested electrochemotherapy (ECT) on recurrent VC refractory to standard therapies to assess the tumor response and to define the selection criteria for patient’s candidate to ECT. This is a multicenter observational study carried out in five Italian centers. Data about patients and tumor characteristics, treatment, toxicity, and clinical response were recorded. In all procedures, intravenous bleomycin was administered according to European Standard Operative Procedure ECT (ESOPE) guidelines. Sixty-one patients, with a median age 79 years (range: 39–85) and mainly affected by squamous cellular carcinoma (91.8%), were treated with ECT. No serious adverse events were reported. Patients were discharged after three days (median, range: 0–8 days). Two months after ECT, the clinical response rate was 83.6% and was not related to age, body mass index, International Federation of Gynecology and Obstetrics (FIGO) stage, number of treated nodules, or previous treatments. ECT is a safe procedure with a favorable cost-effectiveness ratio and should be considered as a treatment option for local disease control in patients unsuitable for standard therapies
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