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By employing diverse surface sensitive synchrotron radiation spectroscopies we demonstrate that the fluorine content of initial C

The occupied and unoccupied electronic structures of three high spin TM(acac)

We have investigated the Pt(110) surface by He atom scattering. The (1 x 2) deconstruction transition at 1087 K is shown to fall in 2D Ising universality class with antiphase domain walls proliferating predominantly along the [001] direction. The surface roughening is observed at 1160 ± 10 K through a power law broadening of the anti-phase diffraction peak along the [001] direction. The two distinct phase transition confine the temperature range in which the surface is disordered but still bound to a common height level.

We investigated the local structure around N and Sb atoms in GaAsSbN/GaAs epilayers as a function of growth conditions and annealing time via soft and hard x-ray absorption spectroscopies in order to find out if short range ordering (SRO) in the group-V sublattice is present. SRO is one of the potential origins of the huge blueshift of the band gap observed upon annealing in these materials. By combining a Sb K- and L- and N K-edge x-ray absorption fine structure spectroscopy analysis, we demonstrate that neither strong Sb clustering nor preferential Sb-N association is possible, and that Sb atoms see a random number of N next nearest neighbors except for growth temperatures smaller than 400 degrees C, for which Sb-N neighbors in the type-V sublattice are in excess with respect to statistical disorder. On the other hand, the evolution of SRO around N anions (breaking of nitrogen pairs and randomization) can play a role in the annealing-induced band gap blueshift. Varying growth conditions and concentration modifies the band gap but, surprisingly, it does not affect the position of the conduction band minimum when Sb is incorporated.

The line shape of the Auger decay of adatoms is studied by a joint theoretical and experimental effort, the former within a DFT framework, and the latter with synchrotron radiation measurements. We investigate the $K{L}_{2,3}V$ Auger deexcitation of Na on Al(111), a system with different adsorption geometries. In particular, we study the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\text{ }\ifmmode^\circ\else\textdegree\fi{}$ phase at $1/3$ ML (monolayer) and the more complex $(2\ifmmode\times\else\texttimes\fi{}2)$ structure at $1/2$ ML coverage. From the comparison between theory and experiment, we unambiguously determine features that allow for the determination of the adsorption environment from the adatom Auger spectrum.

The resonant photoemission of Cd0.82Mn0.18Te single crystals has been studied in the vicinity of the Mn 2p --> 3d absorption threshold. A clear resonant behavior is detected for valence band states at the Mn 2p --> 3d absorption threshold, while a weak normal Auger emission is singled out in the valence band region for photon energies well above the main absorption edge. Resonant Raman-Auger and normal Auger emissions are found to coexist above the absorption edge. This effect is ascribed to the presence of a manifold of localized states arising from the Mn 2p --> 3d Coulomb and exchange interaction, which prevents the delocalization in the intermediate state of the autoionization process and favors the Raman-Auger process. (C) 2004 Elsevier B.V. All rights reserved.

We observe that ultrathin Fe/Cu3Au(001) films in the 6-13 Angstrom thickness range, beyond the thickness of pseudomorphism breakdown at room temperature, exhibit a temperature dependent structural phase transition in the range T(c)approximate to345-380 K. In the high temperature state the Fe film becomes pseudomorphic, while breakdown of pseudomorphism reversibly occurs as the system is cooled below the transition temperature. The difference between substrate and overlayer thermal expansion coefficient is highlighted as the driving force for the observed transition.

We present a practical method for the analysis of spectroscopic binaries, reconstructing the lines of the two components of the system. We show that the problem of the separation of binary spectra can be solved in an easy way, under most common conditions. One pair of observations may be sufficient, if taken at different orbital phases of the system, preferably at opposite quadratures. The separation procedure is discussed analytically, and a technique is described, which allows to restore the secondary lines in few steps. An algorithm is also provided, which derives the radial velocity of the secondary star, by directly analysing a difference line-profile obtained from the two input spectra. The efficiency of the method is tested, by reconstructing artificial line-profiles and simulated binary spectra as well. Then the procedure is applied to the eclipsing binary IZ Per , revealing for the first time its faint secondary spectrum.

While the demonstrated power conversion eff i ciency of organic photovoltaics (OPVs) now exceeds 10%, new design rules are required to tailor interfaces at the molecular level for optimal exciton dissociation and charge transport in higher effi ciency devices. We show that molecular shape-complementarity between donors and acceptors can drive performance in OPV devices. Using core hole clock (CHC) X-ray spectroscopy and density functional theory (DFT), we compare the electronic coupling, assembly, and charge transfer rates at the interface between C 60 acceptors and fl at- or contorted-hexabenzocorone (HBC) donors. The HBC donors have similar optoelectronic properties but differ in molecular contortion and shape matching to the fullerene acceptors. We show that shape-complementarity drives self-assembly of an intermixed morphology with a donor/acceptor (D/A) ball-and-socket interface, which enables faster electron transfer from HBC to C 60 . The supramolecular assembly and faster electron transfer rates in the shape complementary heterojunction lead to a larger active volume and enhanced exciton dissociation rate. This work provides fundamental mechanistic insights on the improved effi ciency of organic photovoltaic devices that incorporate these concave/convex D/A materials.

The α phase of Sn/Ge(111) has been investigated from 120 K up to 500 K, using synchrotron radiation core-level photoemission. By means of photoelectron diffraction experiments, we verified that the rippled structure of the low-temperature (3×3) phase is preserved in the (√3×√3)R30° phase at room temperature, thus confirming the order-disorder character of the phase transition. We also found that at least two components are present in the Sn 4d core-level spectra up to 500 K, i.e., about 300 K above the onset of the transition from the low-temperature (3×3) phase to the (√3×√3)R30° phase, thus excluding the occurrence of any displacive transition.

The gap state that appears upon reduction of TiO2 plays a key role in many of titania's interesting properties but its origin and spatial localization have remained unclear. In the present work, the TiO2(110) surface is reduced in a chemically controlled way by sodium adsorption. By means of resonant photoelectron diffraction, excess electrons are shown to be distributed mainly on subsurface Ti sites strikingly similar to the defective TiO2(110) surface, while any significant contribution from interstitial Ti ions is discarded. In agreement with first principles calculations, these findings demonstrate that the distribution of the band gap charge is an intrinsic property of TiO2(110), independent of the way excess electrons are produced.

We report the detection of 10 new double–line eclipsing binary systems, with first measurements of masses and radii. These results are the preliminary outcome of an observational campaign for detecting new double–line eclipsing systems, carried out at the Observatoire de Haute Provence (OHP) in France, with the 1.52-m telescope and its CCD high–resolution spectrograph Aurelie.

The photoemission spectra of the Sn 4d electrons from the (3×3)-Sn/Ge(111) surface present two components which are attributed to inequivalent Sn atoms in T4 bonding sites. This structure has been explored by photoelectron diffraction experiments performed at the ALOISA beamline of the Elettra storage ring in Trieste (Italy). The modulation of the intensities of the two Sn components, caused by the backscattering of the underneath Ge atoms, has been measured as a function of the emission angle at fixed kinetic energies and vice versa. The bond angle between Sn and its nearest neighbor atoms in the first Ge layer ( Sn–Ge 1) has been measured by taking polar scans along the main symmetry directions and it was found almost equivalent for the two components. The corresponding bond lengths are also quite similar, as obtained by studying the dependence on the photoelectron kinetic energy with the photon polarization and the collection direction parallel to the Sn–Ge 1 bond orientation (bond emission). A clear difference between the two bonding sites is observed when studying the energy dependence at normal emission, where the sensitivity to the Sn height above the Ge atom in the second layer is enhanced. The (3×3)-Sn/Ge(111) is thus characterized by a structure where the Sn atom and its three nearest neighbor Ge atoms form a rather rigid unit that presents a strong vertical distortion with respect to the underneath atom of the second Ge layer.

The temperature-dependent behavior of the Rh(111)-(2X2)-10 phase was investigated by He-atom scattering (HAS) and low-energy electron diffraction. The adsorption system undergoes an order-disorder phase transition at T-c=280+/-5 K, with critical exponents found to be consistent with the four-state Potts model. Beyond the phase transition the HAS specular peak intensity exhibits a strong and reversible increase. This finding points toward a reduction of the surface charge-density corrugation induced by the phase transition itself. Around 160 K, hydrogen adsorbed on the Rh(111)-(2X2)-10 surface reacts with oxygen to form water, and drives the overlayer in an out-of-equilibrium condition which is characterized by a dramatic domain-wall proliferation.

We report photoelectron diffraction (PED) experiments of weakly sub-stoichiometric TiO2(100) rutile surfaces. Apart from standardcore-level PED from the Ti-2p3/2 line, we have studied valence band PED from the defect induced Ti-3d states in the insulating band gap. For maximum yield, the latter were resonantly excited at the Ti-2p absorption edge. The PED patterns have been analyzed within the forward scattering approximation as well as by comparison with simulated PED patterns obtained in multiple scattering calculations. The analysis shows that the defect induced Ti-3d charge is mainly located on the second layer Ti atoms.

We measured NEXAFS spectra on both C and N K-edges of an ordered monolayer of lead phthalocyanine adsorbed onto an InSb(001) 4 x 2/c(8 x 2) substrate. We compared the spectra with the calculated density of unoccupied states of an isolated molecule. We demonstrate that molecules lay flat on the surface, determine the energy position of the lowest excited state, and show that it is site dependent. Finally, we compare the core exciton structure with the valence exciton structure previously measured by two different valence spectroscopic techniques and discuss their differences in terms of electron-nuclear and electron-hole Coulomb interactions.

We have investigated the controversy surrounding the (sqrt[3] x sqrt[3]) R30 degrees structure of self-assembled monolayers of methylthiolate on Au(111) by first principles molecular dynamics simulations, energy and angle resolved photoelectron diffraction, and grazing incidence x-ray diffraction. Our simulations find a dynamic equilibrium between bridge site adsorption and a novel structure where 2 CH3S radicals are bound to an Au adatom that has been lifted from the gold substrate. As a result, the interface is characterized by a large atomic roughness with both adatoms and vacancies. This result is confirmed by extensive photoelectron and grazing incidence x-ray diffraction measurements.

The charge distribution of the defect states at the reduced TiO(2)(110) surface is studied via a new method, the resonant photoelectron diffraction. The diffraction pattern from the defect state, excited at the Ti-2p-3d resonance, is analyzed in the forward scattering approach and on the basis of multiple scattering calculations. The defect charge is found to be shared by several surface and subsurface Ti sites with the dominant contribution on a specific subsurface site in agreement with density functional theory calculations.

PHYS. REV. B

We observe that ultrathin Fe/Cu(3)Au(001) films in the 6-13 A thickness range, beyond the thickness of pseudomorphism breakdown at room temperature, exhibit a temperature dependent structural phase transition in the range T(c) approximately 345-380 K. In the high temperature state the Fe film becomes pseudomorphic, while breakdown of pseudomorphism reversibly occurs as the system is cooled below the transition temperature. The difference between substrate and overlayer thermal expansion coefficient is highlighted as the driving force for the observed transition.

The effects of annealing on the structure of ultra thin Fe films (4-10 ML) deposited at 150 K on Ag(001) were studied by synchrotron radiation photoelectron diffraction (PED) and x-ray diffraction (XRD). The occurrence of a surfactant-like stage, in which a single layer of Ag covers the Fe film is demonstrated for films of 4-6 ML heated at 500-550 K. Evidence of a stage characterized by the formation of two Ag capping layers is also reported. As the annealing temperature was increased beyond 700 K the surface layers closely resembled the structure of bare Ag(001) with the residual presence of subsurface Fe aggregates. The data illustrate a film dissolution path which is in agreement with recent theoretical models [J. Roussel Phys. Rev. B 60, 13890 (1999)]

Short-range ordering in the form of an excess of In-N bonds with respect to the random case has been recently predicted for the dilute nitride alloy InxGa1-xAs1-yNy. This ordering induces a blue shift of the optical band gap and could represent a fundamental materials limitation. In this report we provide a quantitative determination of the effect of annealing on the short-range ordering in InxGa1-xAs1-yNy, using x-ray- absorption spectroscopy and state-of-the-art analysis methods. We find that in annealed samples short-range ordering is weak, one order of magnitude smaller than predicted.

he structure of Fe films grown on the (001) surface of a Cu3Au single crystal at room temperature has been investigated by means of grazing incidence x-ray diffraction (GIXRD) and photo/Auger-electron diffraction (ED) as a function of thickness in the (3-36)-Angstrom range. The combination of GIXRD and ED allows one to obtain quantitative information on the in-plane spacing a from the former technique, and the ratio between the vertical spacing c and a, from the latter one. At low coverage the film grows pseudomorphic to the face-centered-cubic substrate. The experimental results obtained on a film of 8 Angstrom thickness clearly indicate the overcoming of the limit for pseudomorphic growth. Above this limit the film is characterized by the coexistence of the pseudomorphic phase with another tetragonally strained phase gamma, which falls on the epitaxial line of ferromagnetic face-centered cubic Fe. Finally, the development of a body-centered phase alpha, whose unit cell is rotated by 45degrees with respect to the substrate one, has been clearly observed at similar to17 Angstrom. alpha is the dominating phase for film thickness above similar to25 Angstrom and its lattice constant evolves towards the orthomorphic phase in strict quantitative agreement with epitaxial curves calculated for body-centered tetragonal iron phases.

We report on the structural and magnetic properties of thin Fe films grown on the Cu(1 1 0) surface. In-plane grazing-incidence X-ray diffraction has been used to measure the lateral lattice spacing of the Fe films. Complementary information about the structure of the topmost layers has been obtained by means of Auger and photoelectron diffraction. The Fe film grows pseudomorphic with the substrate up to a thickness of about 0.8 nm. The diffraction feature of a new phase is observed at 1.6 nm, with a corresponding interplanar distance close to the bulk body centered cubic (bcc) Fe one, which is eventually recovered at higher thickness (6.4 nm). From comparison between X-ray diffraction and photoelectron diffraction, it is suggested that the bcc-like Fe grows on the (1 0 0) surface with its [1 1 0] axis oriented along the [0 0 1] substrate direction. The photoelectron diffraction data also indicate a strong faceting in the [1 10] substrate direction. This morphology is believed to contribute to the in-plane uniaxial magnetic anisotropy observed by Kerr effect and Brillouin light scattering from spin waves.

The Ge(001) surface is known to undergo phase transitions from c(4 x 2) to (2 x 1) at low temperature and from (2 x 1) to (I x 1) at high temperature. In the former case the phase transition is shown to occur at 240 K and to fall into the 2D Ising universality class. The high temperature phase transition takes place above 900 K and is characterized by (2 x 1) domain wall proliferation, thus indicating an order-disorder character. We have also observed an increase in the density of steps, but these are shown to be only partially involved in the disordering of the (2 x 1) phase.

We present new measurements of He atom inelastic scattering from the (001) surface of Copper. Our results confirmed the recent data obtained in Gottingen and show that at large momentum transfer in the [Formula: see text] direction the intensity of the longitudinal resonance is larger than that of the Rayleigh wave. We interpret these results by evaluating the reflection coefficient in the Distorted Wave Born Approximation. By taking the He surface interaction potential as a sum of nonspherical pairwise potential we are able to explain quantitatively the behavior of the experimental spectra.

The experimental information accumulated in recent years on elastic and inelastic scattering of He and Ne atoms from metal surfaces is analysed by describing the atom-surface interaction as a superposition of pseudo-pairwise potentials. These are chosen according to the quite accurate potentials presently available for van der Waals pairs and take a phenomenological account for many body effects. The emerging picture is quite satisfactory and allows the physical interaction of a given atom-surface system to be predicted with excellent accuracy. In particular the coupling of He atoms to the surface vibrational modes is considered with attention focused on He inelastic scattering from Cu(001).

not Available.

By employing diverse surface sensitive synchrotron radiation spectroscopies we demonstrate that the fluorine content of initial C60F48 deposited at room temperature on Ag(111) varies with molecular coverage. At the very early stages of deposition, C60F48 fully de-fluorinates and transforms into C60. Strong indications of silver fluoride formation are provided. The chemical footprint of fluorinated fullerenes emerges at relatively low molecular coverage indicating that the degree of fullerene de-fluorination decreases (from total to partial de-fluorination) as molecules are deposited. De-fluorination stops well before the substrate surface is completely covered by fullerenes. At the molecular level, the fluorine loss observed by spectroscopic techniques are supported by scanning tunneling microscopy imaging. Both molecules and metal surface are importantly involved in the process., This work has been supported by the Spanish Government under the projects PID2019-110907GB-I00, MAT2017-85089- C2-1-R (AEI/FEDER, UE) and the ‘‘Severo Ochoa’’ Program for Centres of Excellence in R&D (CEX2019-000917-S). This project has received funding from the EU-H2020 research and innovation programme under grant agreement No 654360 having benefitted from the access provided by CNR-IOM in Trieste and Grenoble within the framework of the NFFA-Europe Transnational Access Activity (ID: 929).

Note: Univ ljubljana,jozef stefan inst,ljubljana,slovenia. univ trieste,dipartmento fis,i-34127 trieste,italy. Crottini, A, IST NAZL FIS MAT,LAB TASC,PADRICIANO 99,I-34012 TRIESTE,ITALY. ISI Document Delivery No.: XT124 Cited References: BATRA IP, 1986, PHYS REV B, V34, P8246 CIRACI S, 1986, PHYS REV B, V33, P4294 CVETKO D, 1992, MEAS SCI TECHNOL, V3, P997 EDWARDS KA, 1996, PHYSICA B, V221, P201 FEIBELMAN PJ, 1983, PHYS REV B, V27, P1991 FEIBELMAN PJ, 1984, PHYS REV B, V29, P6463 FERRER S, 1995, PHYS REV LETT, V75, P1771 HINCH BJ, 1989, EUROPHYS LETT, V10, P341 HINCH BJ, 1991, VACUUM, V42, P309 HRICOVINI K, 1991, SPRINGER SERIES SURF, V24, P589 JAKLEVIC RC, 1971, PHYS REV LETT, V26, P89 JALOCHOWSKI M, 1992, PHYS REV B, V45, P13607 JALOCHOWSKI M, 1992, PHYS REV B, V46, P4693 LAPUJOULADE J, 1992, SPRINGER SERIES SURF, V27 LENT CS, 1984, SURF SCI, V139, P121 MEYERHEIM HL, 1995, PHYS REV B, V52, P16830 SCHULTE FK, 1976, SURF SCI, V55, P427 TORRELLES X, 1996, SURF SCI, V364, P242 YANG WS, 1995, PHYS REV B, V51, P7571 ZENG H, 1995, PHYS REV LETT, V74, P582 ZHANG Y, 1993, SURF SCI, V293, L821 ZHAO RG, 1993, PHYS REV B, V48, P8462 NR 22 TC 13 PU AMERICAN PHYSICAL SOC PI COLLEGE PK PA ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA SN 0031-9007 Reference LOEQ-ARTICLE-1997-016doi:10.1103/PhysRevLett.79.1527 Record created on 2007-08-31, modified on 2017-05-12

The Fe62Ni20Cr18 valence band was studied by scanning the photon energy across the 2p(3/2) core-level threshold of each element of the alloy. A resonant enhancement of the weak 3d-like features was observed. In pure transition metals, similar valence band resonances are explained by a radiation-less Raman de-excitation emission, which is active at threshold and degenerate with the two-hole satellite of direct photoemission. Present structures are associated to satellite features occurring in Fe62Ni20Cr18, and their intensities and binding energies are compared to those of the pure metal components. The alloy satellite resonant behaviour reveals some peculiar modi. cations of: a) the crossover between the radiationless Raman scattering and the Auger emission regimes; b) the ratio of the relative intensities of the main and satellite peaks. We mainly assign these differences to the hetero-nuclear bonds in the alloy.

The electronic properties of a C70 single layer chemisorbed on Cu(1 1 1) surface, in which the charge state has been modified by Na doping, were studied via synchrotron radiation photoemission and absorption spectroscopy. A two-dimensional metallic phase is observed in the whole range of charge states investigated (from 1 up to 3 electrons/C70). Moreover, all the results suggest that the orientation of the molecules, with the C5v axis perpendicular to the surface, is not affected by Na doping.

The C 60 molecules are known to bind covalently to the Pt(1 1 1) surface. We performed a structural study of one C 60 layer on Pt(1 1 1) by using X-ray photoelectron diffraction (XPD). Our results show that the molecules are bonded to the metal surface with their six-carbon rings facing the Pt(1 1 1) surface and with the CC bonds perpendicularly oriented with respect to the direction of the close-packed atom rows. Unlike to what previously found for Cu(1 1 1) and Al(1 1 1), the measured diffraction pattern shows a three-fold symmetry indicating that only one orientation is taken by the chemisorbed molecules. This can be explained by a stronger interaction of the C 60 molecules with the second layer Pt atoms.

The Fe LMM Auger and Auger vacancy satellite spectra of ultrathin Fe films grown on Cu(100) have been measured using photon excitation energies above and below the L 2 ionisation edge, and in coincidence with the Fe 2p 3/2 and Fe 2p 1/2 photoelectrons. A comparison of the relative intensities of the satellites and main Auger transitions indicates that the M 4,5 holes of L 3 M 4,5 double hole states are localised for longer than the L 3 lifetime for the 0.3 and 10 ML coverages but have a lifetime comparable to that of L 3 holes for the 1 ML coverage. The results for the 1 ML coverage are supported by the results of the coincidence experiments. The reasons for the variation in the lifetime of M 4,5 holes are discussed in terms of changes in the number of Fe–Fe neighbours and the density of states at the Fermi energy in these systems.

The atomic-like behavior of the photoionization cross-section of core levels in solids is remarkably displayed in the observation of linear magnetic dichroism in the angular distribution of photoelectrons. Structure-related effects are clearly visible as modulations of the dichroism signal, induced by photoelectron diffraction, a fact that can be exploited to obtain information from magnetic surfaces. A systematic investigation of such effects has been carried out for Fe 2p and 3p core levels on the ALOISA beamline at ELETTRA. We present results for Fe(0 0 1) with a reference calibration for amorphous RCP-Fe grown on Fe30Ni50B20, which provides the atomic-like standard, free from diffraction effects.

The temperature-dependent behavior of the Rh(111)-(2\ifmmode\times\else\texttimes\fi{}2)-1O phase was investigated by He-atom scattering (HAS) and low-energy electron diffraction. The adsorption system undergoes an order-disorder phase transition at ${\mathrm{T}}_{\mathrm{c}}$=280\ifmmode\pm\else\textpm\fi{}5 K, with critical exponents found to be consistent with the four-state Potts model. Beyond the phase transition the HAS specular peak intensity exhibits a strong and reversible increase. This finding points toward a reduction of the surface charge-density corrugation induced by the phase transition itself. Around 160 K, hydrogen adsorbed on the Rh(111)-(2x2)-1O surface reacts with oxygen to form water, and drives the overlayer in an out-of-equilibrium condition which is characterized by a dramatic domain-wall proliferation.

Using He atom scattering, layer-by-layer erosion of InSb(110) by low energy ion bombardment has been observed to proceed via nucleation of vacancy islands, island growth and coalescence. The mechanism is in full agreement with theoretical models developed for crystal growth, with diffusing adatoms substituted by diffusing vacancies. In particular it has been observed that coalescence of the vacancy islands sets in after the removal of small fractions of a monolayer and that the average terrace width increases with increasing number of sputtered monolayers.

Resonant photoemission from the valence band of a (√3 × √3)R30° reconstructed Mn:Ge(111) metallic interface has been carefully analyzed with the aim to track the transition from resonant Raman to normal Auger emission. The transition energy has been compared with the Mn 2p binding energy, as well as with the Mn L(3) absorption edge energy. Close similarities emerge with respect to the case of elemental Mn thin films, suggesting that the excitation dynamics is dominated by the electronic properties of Mn 3d states, in spite of the bonding with Ge atoms. The switching from the resonant Raman Auger (RRAS) to the normal Auger regime is found about 2 eV below the Mn L(3) absorption edge. A change of the lineshape due to the transition from an overall N - 1 electron final state (RRAS channel) to an N - 2 electron final state (normal Auger channel) is evidenced by the analysis of the experimental data, which also allowed the ratio to be tracked between charge delocalization and core-hole time scales as the photon energy is tuned across the Mn L(3) edge.

International audience; The gap state that appears upon reduction of TiO2 plays a key role in many of titania's interesting properties but its origin and spatial localization have remained unclear. In the present work, the TiO2(110) surface is reduced in a chemically controlled way by sodium adsorption. By means of resonant photoelectron diffraction, excess electrons are shown to be distributed mainly on subsurface Ti sites strikingly similar to the defective TiO2(110) surface, while any significant contribution from interstitial Ti ions is discarded. In agreement with first principles calculations, these findings demonstrate that the distribution of the band gap charge is an intrinsic property of TiO2(110), independent of the way excess electrons are produced.

We combined measurements of valence-band photoemission and valence-band photoelectron diffraction on Fe3O4 at the Fe 2p-3d resonance. The different structures in the valence band of magnetite due to the different sites of iron were identified experimentally. Specifically the structure near the Fermi level is unambiguously attributed only to octahedral Fe2+ sites ?B-Fe2+?. We showed that tuning the photon energy to the resonance of B-Fe2+, the whole valence band is dominated by signal coming from B sites of iron. Moreover this work shows how resonant photoelectron diffraction is a powerful tool for the study of mixed valence oxides.

We have investigated the structure of nitrogen-hydrogen complexes in ${\text{GaAs}}_{1\ensuremath{-}y}{\text{N}}_{y}$ and ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}{\text{As}}_{1\ensuremath{-}y}{\text{N}}_{y}$ dilute nitride alloys by performing x-ray absorption near-edge structure spectroscopy (XANES). We simulated the spectra based on first-principles calculations of the most recent defective structures proposed in the literature for hydrogenated materials. The comparison between the experimental data and simulations allows us to clarify that the core of the defect is a complex with ${\text{C}}_{2v}$ structure in the neutral charge state, in agreement with the expansion of the lattice parameter measured by x-ray diffraction. Our results are compatible with the presence of H satellites bound to neighboring Ga atoms but not with complexes involving more than two H atoms bound to the same N. Nevertheless, we were not able to determine uniquely the number of H satellites, which may depend on growth conditions. Strain related to the epitaxial growth has a very little effect on the XANES spectra.

Charge transfer processes between donor-acceptor complexes and metallic electrodes are at the heart of novel organic optoelectronic devices such as solar cells. Here, a combined approach of surface-sensitive microscopy, synchrotron radiation spectroscopy, and state-of-the-art ab initio calculations is used to demonstrate the delicate balance that exists between intermolecular and molecule-substrate interactions, hybridization, and charge transfer in model donor-acceptor assemblies at metal-organic interfaces. It is shown that charge transfer and chemical properties of interfaces based on single component layers cannot be naively extrapolated to binary donor-acceptor assemblies. In particular, studying the self-assembly of supramolecular nanostructures on Cu(111), composed of iuorinated copperphthalocyanines (F16CuPc) and diindenoperylene (DIP), it is found that, in reference to the associated single component layers, the donor (DIP) decouples electronically from the metal surface, while the acceptor (F16CuPc) suffers strong hybridization with the substrate © 2009 WILEY-VCH Verlag GmbH & Co. KGaA., This work was supported by the European Community through the Integrated Infrastructure Initiative ‘‘European Light Sources Activities – Synchrotrons and Free Electron Lasers’’. The work was further supported through the Spanish Ministerio de Educacio´n y Ciencia (MAT2007-63083, FIS2007-65702-C02-01), ‘‘Grupos Consolidados UPV/EHU del Gobierno Vasco’’ (IT-319-07, IT-257-07), CSIC, and the European Community throughe-I3 ETSF project (Contract Number 211956), and NANO-ERA CHEMISTRY. We acknowledge support by the ‘‘Red Espanola de Supercomputacion’’ and SGIker ARINA(UPV/EHU). J. M. G.-L. acknowledges funding Spanish MEC through Juan de la Cierva program.

The study of the early stage of Mn growth on GaAs(001)-c(4 × 4) surface has been performed by in situ Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy. Starting from GaAs(001) grown by molecular beam epitaxy, the surface was investigated after Mn deposition and after low temperature annealing at about 250 ∘C. The aim of this work is to understand the mechanism of Mn–As interaction and the behavior of Mn on the GaAs(001) substrate. The results demonstrate the high reactivity and mobility of Mn with the formation of compounds such as Mn subarsenide (MnAsx), MnAs (confirmed by STM results) and, probably, GaMnAs.

The peculiar electrochemical and photophysical properties of porphyrin and fullerene molecules make them promising candidates for the construction of two- and three-dimensional organic-based materials. An important question is how pristine fullerene and porphyrin will organize when deposited on surfaces via in vacuum molecular beam evaporation. Here we show that codeposition of C(70) and Zn-tetraphenyl-porphyrin (ZnTPP) induces the self-assembly of electron-rich flat aromatic molecules at the curved surface of C(70), thus enhancing the chromophore interaction and forming a supramolecular multilayer donor-acceptor structure. While the ground-state electronic spectra almost reflect a simple summation of ZnTPP and C(70) components, the excited-state electrons at the porphyrin macrocycle can rapidly delocalize to the fullerene. The excited charge transfer time scale is faster than 1-2 fs, as shown by resonant photoemission for the core-excited charges.

We have investigated the controversy surrounding the (root 3x root 3)R30 degrees structure of self-assembled monolayers of methylthiolate on Au(111) by first principles molecular dynamics simulations, energy and angle resolved photoelectron diffraction, and grazing incidence x-ray diffraction. Our simulations find a dynamic equilibrium between bridge site adsorption and a novel structure where 2 CH3S radicals are bound to an Au adatom that has been lifted from the gold substrate. As a result, the interface is characterized by a large atomic roughness with both adatoms and vacancies. This result is confirmed by extensive photoelectron and grazing incidence x-ray diffraction measurements.

Here we demonstrate that a molecular dye film composed by a single layer Of C-70 and a single layer of Zn-tetraphenyl-porphyrin (ZnTPP) adsorbed atop can be grown on large substrates using molecular beam epitaxy. The electrostatic interactions between the constituent molecules drive the formation of this ordered donor-acceptor layer. The overlapping between the pi* orbitals of ZnTPP and C-70 allows the delocalisation of the excited electrons from the excited molecule on a timescale of similar to 6 fs. Generally, the ZnTPP/C-70 film, acting as a p-n junction, delocalise more efficiently the excited electrons as compared to pure ZnTPP films on a timescale that competes effectively against loss processes, suggesting its application for a more efficient solar energy conversion in organic-based devices.

The electronic properties and the molecular orientation of Zn-tetraphenyl-porphyrin films deposited on Si(111) have been investigated using synchrotron radiation. For the first time we have revealed and assigned the fine structures in the electronic spectra related to the HOMOs and LUMOs states. This is particularly important in order to understand the orbital interactions, the bond formation and the evolution of the electronic properties with oxidation or reduction of the porphyrins in supramolecular donor-acceptor complexes used in photovoltaic devices., text 11 pages, 4 figures submitted for publication

Photoelectron spectroscopy, low energy electron diffraction, and reflection high energy electron diffraction have been employed to probe the electronic and geometric structure of the TiO2(1 0 0)/K interface. Data have been obtained from two (1 0 0) oriented single crystal samples, which differ in their extent of bulk reduction. It is demonstrated that this parameter is important for both the electronic and geometric structure of ordered TiO2(1 0 0)/K surfaces, formed by annealing to ∼1000 K. Photoelectron spectra, recorded following this preparation, indicate that reduced Ti states are present for the less stoichiometric sample, which exhibits a 1 −1 2 2 surface unit cell. In contrast, the other sample displays a c(2×2) overlayer, with no Ti reduction evidenced.