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Auger photoelectron coincidence spectroscopy APECS has been used to examine the electron correlation and itinerance effects in transition metals Cu, Ni and Co. It is shown that the LVV Auger, in coincidence with 2p photoelectrons, spectra can be represented using atomic multiplet positions if the 3d shell is localized atomic like and with a self convoluted valence band for band like itinerant materials as explained using the Cini Sawatzky model. For transition metals, the 3d band changes from band like to localized with increasing atomic number, with the possibility of a mixed behavior. Our result shows that the LVV spectra of Cu can be represented by atomic multiplet calculations, those of Co resemble the self convolution of the valence band and those of Ni are a mixture of both, consistent with the Cini Sawatzky model

The chemical shift of core level binding energies makes electron spectroscopy for chemical analysis (ESCA) a workhorse analytical tool for science and industry. For some elements, close lying and overlapping spectral features within the natural life time broadening restrict applications. We establish how the core level binding energy chemical shift can be picked up experimentally by the additional selectivity through Auger electron photoelectron coincidence spectroscopy (APECS). Coincident measurement of Ni 3p photoemission with different MVV Auger regions from specific decay channels, narrows the 3p core-levels to a width of 1.2 eV, resolves the spin–orbit splitting of 1.6 eV and determines the chemical shift of Ni 3p levels of a Ni(111) single crystal and its oxidized surface layer to 0.6 eV.

In this work, we present the CoESCA station for electron-electron coincidence spectroscopy from surfaces, built in a close collaboration between Uppsala University and Helmholtz-Zentrum Berlin at the BESSY II synchrotron facility in Berlin, Germany. We start with a detailed overview of previous work in the field of electron-electron coincidences, before we describe the CoESCA setup and its design parameters. The system is capable of recording shot-to-shot resolved 6D coincidence datasets, i.e. the kinetic energy and the two take off angles for both coincident electrons. The mathematics behind extracting and analysing these multi-dimensional coincidence datasets is introduced, with a focus on coincidence statistics, resulting in fundamental limits of the signal-to-noise ratio and its implications for acquisition times and the size of the raw data stream. The functionality of the CoESCA station is demonstrated for the example of Auger electron - photoelectron coincidences from silver surfaces for photoelectrons from the Ag 3d core levels and their corresponding MNN Auger electrons. The Auger spectra originating from the different core levels, 3d$_{3/2}$ and 3d$_{5/2}$ could be separated and further, the two-hole state energy distributions were determined for these Auger decay channels., 14 pages, 6 figures

For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in- and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves., AF acknowledges funding by the Helmholtz Virtual Institute VI 419 and the ERC Advanced Investigator Grant No. 669531 EDAX. NM acknowledges funding by the ERC Advanced Investigator Grant No. 321319 ESUX and support from the Carl Tryggers foundation for Scientifc Research CTS. DS-P and MM acknowledge funding from EU-FP7 project THINFACE (grant No. 607232), Spanish MINECO (grant No. MAT2016-78293-C6-4-R), and Basque Dep.de Educación and UPV/EHU (grant No. IT-756-13).

Ni $LVV$ Auger spectra, in coincidence with the corresponding $2{p}_{1/2}$, $2{p}_{3/2}$, and $6\phantom{\rule{0.28em}{0ex}}\text{eV}$ satellite photoelectrons, have been used to examine electron correlation and itinerance effects in Ni. In coincidence with the $2{p}_{3/2}$ core level, the Auger spectral shape is represented by localized $3{d}^{8}$ and itinerant valence final states with an additional $3{d}^{7}$ Auger shake-up contribution. The spectra in coincidence with the $6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ satellite probe the decay of localized $2{p}^{5}3{d}^{9}$ double hole states, leading to $3{d}^{7}$ final states. It is found that a fraction of the double hole states delocalize before the Auger decay. A similar delocalization is observed for the double hole states produced by the ${L}_{2}{L}_{3}{M}_{45}$ Coster-Kronig process, and the delocalization rates have been determined.

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

The thermal stability and the structural evolution of graphene grown on Fe(110) has been studied upon annealing in ultra-high vacuum conditions (UHV, P ≤ 10−9 mbar) and in the presence of gaseous ethylene at a pressure of ∼10−6 mbar by grazing incidence X-ray diffraction. It was observed that upon annealing at temperatures below 630 °C, graphene on Fe is thermally stable. Exposure to ethylene at these temperatures promotes the formation of graphene while inhibiting its deterioration. Annealing graphene/Fe(110) at temperatures above 630 °C results in a fast degradation of graphene followed by carburization of the sample, that is the irreversible formation of various iron carbides, with the most common phases being Fe3C (cementite) and Fe7C3 (Eckstrom-Adcock carbide). Annealing of the carburized sample does not result in the formation of a detectable graphitic structure.

International audience; We demonstrate that the possibility of monitoring relative photoionization cross sections over a large photon energy range allows us to study and disentangle shake processes and intramolecular inelastic scattering effects. In this gas-phase study, relative intensities of the carbon 1s photoelectron lines from chemically inequivalent carbon atoms in the same molecule have been measured as a function of the incident photon energy in the range of 300–6000 eV. We present relative cross sections for the chemically shifted carbon 1s lines in the photoelectron spectra of ethyl trifluoroacetate (the “ESCA” molecule). The results are compared with those of methyl trifluoroacetate and S-ethyl trifluorothioacetate as well as a series of chloro-substituted ethanes and 2-butyne. In the soft X-ray energy range, the cross sections show an extended X-ray absorption fine structure type of wiggles, as was previously observed for a series of chloroethanes. The oscillations are damped in the hard X-ray energy range, but deviations of cross-section ratios from stoichiometry persist, even at high energies. The current findings are supported by theoretical calculations based on a multiple scattering model. The use of soft and tender X-rays provides a more complete picture of the dominant processes accompanying photoionization. Such processes reduce the main photoelectron line intensities by 20–60%. Using both energy ranges enabled us to discern the process of intramolecular inelastic scattering of the outgoing electron, whose significance is otherwise difficult to assess for isolated molecules. This effect relates to the notion of the inelastic mean free path commonly used in photoemission studies of clusters and condensed matter.

For practical electronic device applications of graphene nanoribbons (GNRs), it is essential to have abrupt and well-defined contacts between the ribbon and the adjacent metal lead. By analogy with graphene, these contacts can induce electron or hole doping, which may significantly affect the I/V characteristics of the device. Cu is among the most popular metals of choice for contact materials. In this study, we investigate the effect of in situ intercalation of Cu on the electronic structure of atomically precise, spatially aligned armchair GNRs of width N = 7 (7-AGNRs) fabricated via a bottom-up method on the Au(788) surface. Scanning tunneling microscopy data reveal that the complete intercalation of about one monolayer of Cu under 7-AGNRs can be facilitated by gentle annealing of the sample at 80 °C. Angle-resolved photoemission spectroscopy (ARPES) data clearly reflect the one-dimensional character of the 7-AGNR band dispersion before and after intercalation. Moreover, ARPES and core-level photoemission results show that intercalation of Cu leads to significant electron injection into the nanoribbons, which causes a pronounced downshift of the valence and conduction bands of the GNR with respect to the Fermi energy (ΔE ~ 0.5 eV). As demonstrated by ARPES and X-ray absorption spectroscopy measurements, the effect of Cu intercalation is restricted to n-doping only, without considerable modification of the band structure of the GNRs. Post-annealing of the 7-AGNRs/Cu/Au(788) system at 200 °C activates the diffusion of Cu into Au and the formation of a Cu-rich surface Au layer. Alloying of intercalated Cu leads to the recovery of the initial position of GNR-related bands with respect to the Fermi energy (EF), thus, proving the tunability of the induced n-doping. [Figure not available: see fulltext.] (Less)

Means to measure the temporal evolution following a photo-excitation in conjugated polymers are a key for the understanding and optimization of their function in applications such as organic solar cells. In this paper we study the electronic structure dynamics by direct pump-probe measurements of the excited electrons in such materials. Specifically, we carried out a time-resolved photoelectron spectroscopy (TRPES) study of the polymer PCPDTBT by combining an extreme ultraviolet (XUV) high harmonic generation source with a time-of-flight spectrometer. After excitation to either the 1st excited state or to a higher excited state, we follow how the electronic structure develops and relaxes on the electron binding energy scale. Specifically, we follow a less than 50 fs relaxation of the higher exited state and a 10 times slower relaxation of the 1st excited state. We corroborate the results using DFT calculations. Our study demonstrates the power of TRPES for studying photo-excited electron energetics and dynamics of solar cell materials.

Synopsis We demonstrate that the possibility of monitoring relative photoionization cross sections over a large photon energy range extending to hard x-rays allows studying and disentangling shake processes and intramolecular inelastic scattering effects. An “EXAFS”-type of oscillatory behaviour of the relative C 1s cross sections is observed in a wide photon energy range up to 1 keV due to multiple intramolecular elastic scattering of the photoelectron. These effects are also present in solids and constitute a significant part of all elastic and inelastic scattering effects occurring in solids.

For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in- and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase – not layer stacking variations – causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves.

We investigate the bottom-up growth of N = 7 armchair graphene nanoribbons (7-AGNRs) from the 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules on Ag(111) with the focus on the role of the organometallic (OM) intermediates. It is demonstrated that DBBA molecules on Ag(111) are partially debrominated at room temperature and lose all bromine atoms at elevated temperatures. Similar to DBBA on Cu(111), debrominated molecules form OM chains on Ag(111). Nevertheless, in contrast with the Cu(111) substrate, formation of polyanthracene chains from OM intermediates via an Ullmann-type reaction is feasible on Ag(111). Cleavage of C–Ag bonds occurs before the thermal threshold for the surface-catalyzed activation of C–H bonds on Ag(111) is reached, while on Cu(111) activation of C–H bonds occurs in parallel with the cleavage of the stronger C–Cu bonds. Consequently, while OM intermediates obstruct the Ullmann reaction between DBBA molecules on the Cu(111) substrate, they are required for the formation of polyanthracene chains on Ag(111). If the Ullmann-type reaction on Ag(111) is inhibited, heating of the OM chains produces nanographenes instead. Heating of the polyanthracene chains produces 7-AGNRs, while heating of nanographenes causes the formation of the disordered structures with the possible admixture of short GNRs.

Previously recorded and published photoelectron spectroscopic data for mercury in the gas phase has been reanalyzed. The life-time broadenings have been determined for a large number of core levels. It is then seen that a recent detailed derivation of core-level line-widths based on X-ray emission spectroscopy give life-time widths that are generally too large. The 4d 3/2 4d 5/2 nd Coster–Kronig (CK) transition is also discussed. We find that the additional broadening of the 4d 3/2 level for mercury metal is indeed due to a CK decay, in contrast to recent claims. In atomic mercury, however, the CK process in energetically forbidden. In spite of this we find that the 4d 3/2 level is broadened also in this case. We propose that this is due to a mixing between the 4d 3/2 hole state and discrete 4d 5/2 nd states.

We present a combined core-level spectroscopy and low-energy electron diffraction study of the evolution of thin CuI layers on graphene/Ni(111) during annealing. It has been found that the annealing of the CuI/graphene/Ni(111) system up to 160 degrees C results in the formation of an ordered CuI overlayer with a (root 3 x root 3) R30 degrees structure on top of the graphene surface. At annealing temperatures of about 180 degrees C or higher, the CuI overlayer decomposes with a simultaneous intercalation of Cu and I atoms underneath the graphene monolayer on Ni(111). Nearly complete intercalation of graphene by Cu and I atoms can be achieved by deposition of about 20 angstrom of CuI, followed by annealing at 200 degrees C. The intercalated graphene layer is p-doped due to interfacial iodine atoms.

Comment on "Bottom-Up Graphene-Nanoribbon Fabrication Reveals Chiral Edges and Enantioselectivity"

The development of photoelectron spectroscopy since the early days of the technique is discussed. The focus is on the interaction between instrumental development and scientific achievements. In particular the opportunities provided by the increasingly brilliant synchrotron radiation sources are discussed. The contribution is focused on core level studies. The recent development is demonstrated by using selected examples obtained at today's most advanced synchrotron radiation facilities. The spectral resolution and intensity that can be reached at these facilities reveal new effects and provide detailed information on the investigated systems. The examples are mainly taken from studies of atoms and molecules where different effects can be most accurately identified and separated. UBjL

International audience; Various 4–50 nm in size diamond nanoparticles prepared by different synthesis methods and their fluorinated derivatives were studied by NEXAFS, solid state NMR and FTIR spectroscopy. C 1s and F 1s NEXAFS spectra of as-prepared and fluorinated nanodiamonds (NDs and F-NDs) were analyzed based on a comparison with the known ones of reference compounds (graphitized carbon nanodiscs, phenol and amino acid molecules, graphite oxide and monofluoride). It has been found that all the studied diamond nanoparticles have crystalline diamond cores and their surfaces are covered with graphite-like carbon clusters. These clusters are partially amorphized and oxidized with the formation of functional groups C–OH, C═O, or O═C–OH and the properties of these surface shells depend on the synthesis method of nanodiamonds. The fluorination of diamond nanoparticles has a purely superficial character; it almost completely cleans the NDs particles from carbon clusters and saturates dangling bonds on the surface of the diamond nanoparticles with F atoms forming covalent σ(C–F) bonds. NEXAFS data are further supported by NMR and FTIR spectroscopy, leading to similar conclusions concerning the properties of various NDs and the chemical bonding between C and F atoms in F-NDs. A combination of NEXAFS, solid state NMR and FTIR spectroscopy is demonstrated to be very efficient in investigating various NDs and their functionalized derivatives

Atomically precise graphene nanoribbons (GNRs) can be fabricated via thermally induced polymerization of halogen containing molecular precursors on metal surfaces. In this paper the effect of substrate reactivity on the growth and structure of armchair GNRs (AGNRs) grown on inert Au(111) and active Cu(111) surfaces has been systematically studied by a combination of core-level X-ray spectroscopies and scanning tunneling microscopy. It is demonstrated that the activation threshold for the dehalogenation process decreases with increasing catalytic activity of the substrate. At room temperature the 10,10′-dibromo-9,9′-bianthracene (DBBA) precursor molecules on Au(111) remain intact, while on Cu(111) a complete surface-assisted dehalogenation takes place. Dehalogenation of precursor molecules on Au(111) only starts at around 80 °C and completes at 200 °C, leading to the formation of linear polymer chains. On Cu(111) tilted polymer chains appear readily at room temperature or slightly elevated temperatures. An...

Biographical item: "Prof. Carl Nordling 1931-2016 Obituary" in Journal Of Electron Spectroscopy And Related Phenomena, vol 224, Special Issue: SI, pp 107-108

The C is photoelectron spectrum of ethyl trifluoroacetate (CF3-CO-O-CH2-CH3), also known as the 'ESCA molecule', is the most illustrative showcase of chemical shifts in photoelectron spectroscopy. ...

The new instrument for ambient-pressure X-ray photoelectron spectroscopy at the Swedish synchrotron radiation facility MAX IV Laboratory is presented. The instrument is based on the use of a retractable and exchangeable high-pressure cell, which implies that ultrahigh-vacuum conditions are retained in the analysis chamber and that dual ambient pressure and ultrahigh-vacuum use is possible., The new instrument for near-ambient-pressure X-ray photoelectron spectroscopy which has been installed at the MAX II ring of the Swedish synchrotron radiation facility MAX IV Laboratory in Lund is presented. The new instrument, which is based on a SPECS PHOIBOS 150 NAP analyser, is the first to feature the use of retractable and exchangeable high-pressure cells. This implies that clean vacuum conditions are retained in the instrument’s analysis chamber and that it is possible to swiftly change between near-ambient and ultrahigh-vacuum conditions. In this way the instrument implements a direct link between ultrahigh-vacuum and in situ studies, and the entire pressure range from ultrahigh-vacuum to near-ambient conditions is available to the user. Measurements at pressures up to 10−5 mbar are carried out in the ultrahigh-vacuum analysis chamber, while measurements at higher pressures are performed in the high-pressure cell. The installation of a mass spectrometer on the exhaust line of the reaction cell offers the users the additional dimension of simultaneous reaction data monitoring. Moreover, the chosen design approach allows the use of dedicated cells for different sample environments, rendering the Swedish ambient-pressure X-ray photoelectron spectroscopy instrument a highly versatile and flexible tool.

The effect of atomic oxygen adsorption on the structure and electronic properties of monolayer hexagonal boron nitride (h-BN) grown on Ir(111) has been studied using near edge X-ray absorption fine ...

The effect of atomic oxygen adsorption on the structure and electronic properties of monolayer graphite (MG or graphene) grown on Pt(111) and Ir(111) has been studied using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and scanning tunneling microscopy. For comparison, the adsorption of atomic oxygen on highly oriented pyrolytic graphite has been studied under the same conditions. Graphene oxidation predominantly occurs through the formation of epoxy groups and causes atomic-scale buckling of the graphene lattice, as evidenced by an sp2-to-sp3 bonding transformation. The different parts of the graphene/metal moire superstructure show different oxidation dynamics, with the initial formation of epoxy groups in the more bonding “pores”. Upon O adsorption, the nearest C neighbors of epoxy groups get engaged in a stronger bonding with the substrate. As a result, the pores of the graphene mesh become attracted and effectively pinned to the substrate by the O atoms. A ...

Scanning tunneling microscopy (STM) studies of monolayer of metal-free phthalocyanine (H2Pc) adsorbed on Au(111) have shown ordered arrangement of the molecules on the surface. Evaporation of H2Pc onto the Au(111) surface and post annealing of the sample to 670 K results in a densely packed structure of the molecules. The monolayer is characterized by molecules adsorbed with the molecular plane parallel to the substrate surface in a square adsorption unit cell. Furthermore, the high resolution images revealed the orientation of individual molecules. The H2Pc/Au(111) system has also been doped by rubidum and compared to the undoped layers. The Rb affects the molecular adsorption geometry, and a hexagonal unit cell is found for the coadsorption of H2Pc and Rb. Upon doping, highly ordered Rb-induced protrusions are observed at the benzene site of adsorbed molecules.

Resonant inelastic soft X-ray scattering at the 3p resonances in crystalline Ge is presented. Both stationary and dispersive features are observed in a wide energy range above as well as below the ionization limits. These observations are in agreement with theoretical predictions based on a two-step model where the initially excited electron has no influence on the emission step. Excess population of states in the conduction band is found, and discussed in terms of attosecond electron dynamics. (c) 2009 Elsevier B.V. All rights reserved.

We review some aspects of the investigation of surface systems with soft X-rays. Examples are presented, showing how core level spectroscopies provide detailed information on the structure, bonding and chemistry of adsorbates and thin layers on surfaces. Core level spectroscopies employed at high brilliance synchrotron light sources allow furthermore access to electron dynamics in the attosecond domain, as we demonstrate for the core hole clock method.

Core-level binding energy shifts between the free atom and corresponding large clusters, the latter closely approximating the infinite solid, have been experimentally derived for several elemental ...

The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(111) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low ...

The growth of iron phthalocyanine (FePc) on InSb(001) c(8 x 2) at submonolayer coverage has been investigated with scanning tunneling microscopy (STM). FePc adsorbs flat centered on the In rows both at 70 K and at room temperature (RT). However, the shapes of the two-dimensional molecular islands are fundamentally different; while the RT growth results in chainlike structures along the [I 10] direction, as already observed for other Pc's adsorbed on the same surface, the islands are prolonged along [110], i.e., perpendicular to the substrate rows, at 70 K. These observations are explained on the basis of a recently observed new surface phase at low temperature, resulting in structural domains on the surface. The molecular growth front follows the propagating domain boundary that freezes at low temperature.

Sub-monolayers of gold have been grown at room temperature on the hexagonal boron nitride (h-BN) nanomesh formed on Rh(111), and studied systematically with core level and angle-resolved valence band photoelectron spectroscopy. The results are compared with those for the Au/Rh(111) and Au/h-BN/Pt(111) interfaces. It has been found that on clean Rh(111) substrate gold starts to grow two-dimensionally (2D), while in the presence of a h-BN interlayer it forms islands from the very beginning. In the case of flat h-BN monolayer (on Pt) these islands are essentially three-dimensional (3D) and irregular in size. In contrast, on the h-BN nanomesh (on Rh) gold grows initially as regular islands (predominantly 2D), probably filling the pores of the nanomesh. In addition, the evolution of the Au islands in h-BN/Rh(111) upon annealing has been investigated with core level photoemission and X-ray absorption. The annealing at temperatures between RT and 300 C does not affect the 2D character of Au islands, while further increase in temperature results in formation of 3D islands. At higher temperatures (above 500 C), the gold atoms diffuse through h-BN into the Rh substrate and/or desorb from the surface. However, they do not influence the chemical bond between rhodium and h-BN at any stage of annealing: the nanomesh remains intact. (Less)

Zinc phthalocyanine (ZnPc) adsorbed on the InSb(0 0 1)-c(8 × 2) surface has been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Coverages from sub-monolayer to monolayer (ML) have been investigated. The molecules form ordered structures on the reconstructed rows of the surface with the molecular plane parallel to the surface. A change in the electronic structure between the sub-ML and higher coverages has been observed. Moreover, in order to study the influence of annealing on the electronic and geometric structures, the samples have been heated to elevated temperatures (about 640 K). In addition, multi-layer ZnPc films have been characterized by XPS measurements.

Electronic and atomic structure of hexagonal boron nitride (h-BN) on lattice-mismatched transition metal surfaces has been studied systematically by X-ray absorption and photoelectron spectroscopy and diffraction. The strength of interfacial chemical interaction between h-BN and the substrates is rising in the row Pt(111)-Ir(111)-Rh(111)-Ru(0001). This rise directly correlates with a growing corrugation of the h-BN monolayer. In particular, the h-BN nanomesh has larger pores on Ru(0001) than on the less reactive Rh(111) substrate. Our experiments provide strong evidence for the single-layer model of the nanomesh structure. (C) 2007 Elsevier B.V. All rights reserved.

Synchrotron-based X-ray photoelectron spectroscopy on copper and silver cluster beams created by a magnetron-based gas-aggregation source has allowed mapping the electron density of states (DOS) of free metallic nanoparticles. The cluster DOS profiles obtained in the experiments strongly resemble the infinite solid DOS shapes, but the extracted cluster work-functions are lower than those for the bulk metal. The latter observation is explained by the initial negative charge on most of the clusters, created by the source.

The pioneering work on electron spectroscopy in Uppsala in the 1950s is discussed, focussing primarily on some of the experimental issues. The first experiments were performed using equipment essentially developed for detection of beta radiation. The X-ray sources used to excite the spectra utilized CrKα and CuKα radiation, i.e. today the experiments would be classified as Hard X-ray Photo Electron Spectroscopy (HAXPES).

Bottom-up strategies can be effectively implemented for the fabrication of atomically precise graphene nanoribbons. Recently, using 10,10'-dibromo-9,9'-bianthracene (DBBA) as a molecular precursor to grow armchair nanoribbons on Au(111) and Cu(111), we have shown that substrate activity considerably affects the dynamics of ribbon formation, nonetheless without significant modifications in the growth mechanism. In this paper we compare the on-surface reaction pathways for DBBA molecules on Cu(111) and Cu(110). Evolution of both systems has been studied via a combination of core-level X-ray spectroscopies, scanning tunneling microscopy, and theoretical calculations. Experimental and theoretical results reveal a significant increase in reactivity for the open and anisotropic Cu(110) surface in comparison with the close-packed Cu(111). This increased reactivity results in a predominance of the molecular-substrate interaction over the intermolecular one, which has a critical impact on the transformations of DBBA on Cu(110). Unlike DBBA on Cu(111), the Ullmann coupling cannot be realized for DBBA/Cu(110) and the growth of nanoribbons via this mechanism is blocked. Instead, annealing of DBBA on Cu(110) at 250 °C results in the formation of a new structure: quasi-zero-dimensional flat nanographenes. Each nanographene unit has dehydrogenated zigzag edges bonded to the underlying Cu rows and oriented with the hydrogen-terminated armchair edge parallel to the [1-10] direction. Strong bonding of nanographene to the substrate manifests itself in a high adsorption energy of -12.7 eV and significant charge transfer of 3.46e from the copper surface. Nanographene units coordinated with bromine adatoms are able to arrange in highly regular arrays potentially suitable for nanotemplating.

In the present work we review a number of research directions addressed at the HIKE end-station at the BESSY II storage ring at the Helmholtz-Zentrum Berlin, HZB, using hard X-ray photoelectron spectroscopy (HAXPES). The emphasis of this review is on the specific properties of the technique, which are required in order to address different scientific questions at the HIKE beamline. UBjL

In this study, metal-free phthalocyanine has been deposited on a conducting glass surface by two methods: by spreading the molecular powder directly on the substrate in air and by vapor sublimation under ultra-high vacuum conditions (evaporation). The films have been characterized by means of core level X-ray Photoemission Spectroscopy, X-ray Absorption Spectroscopy (XAS) and Ultra Violet and Visible absorption spectroscopy (UV-Vis). Our results show that the two deposition methods produce molecular overlayers in different polymorphic phases; the UV-Vis measurements indicate that the film obtained by powder deposition is of x-phase type whereas sublimation leads to an α-polymorph structure. The XAS results show that in the powder deposited film the molecules are mainly oriented parallel to the surface. This is opposite to the case of the vapor deposited film, where the molecules mainly are oriented orthogonal to the surface.

Non-radiative decay of core excitations at the B 1s and N 1s absorption edges of bulk hexagonal boron nitride (h-BN) has been studied with resonant Auger spectroscopy. In order to reproduce the bulk properties of the sample while keeping it electrically conducting, we have grown reasonably thick (more than 20nm) polycrystalline films of h-BN on a Ni(111) surface by thermal cracking of borazine vapor. The probability of the participator Auger process in the non-radiative decay of the B 1s(-1) pi* and N 1s(-1) pi* excitations has been found to be very high (31%) and rather low (below 0.5%), respectively. The drastic difference between the participator Auger decay probabilities of these two excitations has been explained in terms of different localization of the electrons promoted into the lowest unoccupied pi* state on the cationic (B) and anionic (N) sites. (Less)

The role of the transition metal 3d states in the adsorption of h-BN on Cu(1 1 1) and Ni(1 1 1) has been studied by NEXAFS and PE spectroscopies and by work function measurements. Based on the analysis of drastic differences in the spectra from h-BN/Cu(1 1 1) and h-BN/Ni(1 1 1), it has been concluded that the strength of chemical bonding at the interfaces under study is determined mainly by the strength of the TM 3d–h-BN π orbital hybridization. On Ni(1 1 1) h-BN is found to be strongly chemisorbed, while on Cu(1 1 1) only weak chemisorption is observed. On Ni(1 1 1) the h-BN monolayer is probably metallic. On Cu(1 1 1) the monolayer should possibly also be considered as metallic, however its metallic properties are much less pronounced, and the situation is somewhat uncertain.

We present new results for the inner valence levels of clusters of the three inert gases Ar, Kr and Xe based on photoelectron spectroscopy studies. The inner valence levels are compared to the localised core levels and to the delocalised outer valence levels. This comparison shows a gradual change from a relatively localised behaviour for Ar inner valence 3s, over the intermediate case of Kr inner valence 4s, to a more delocalised behaviour for Xe inner valence 5s. This change correlates well with the ratio between the orbital sizes and the interatomic distances. The Kr4s intermediate case is found to exhibit characteristics of both localised and delocalised behaviour.

Resonant core spectroscopies are applied to a study of the excited electron transfer dynamics on a low-femtosecond time scale in systems of aromatic molecules (isonicotinic acid and bi-isonicotinic acid) adsorbed on a rutile TiO 2 (110) semiconductor surface. Depending on which adsorbate state is excited, the electron is either localized on the adsorbate in an excitonic effect, or delocalizes rapidly into the substrate in less than 5 fs (3 fs) for isonicotinic acid (bi-isonicotinic acid). The results are obtained by the application of a variant of resonant photoemission spectroscopy.

The adsorption of monolayers of the pyridine-carboxylic acid monomers (isonicotinic acid, nicotinic acid, and picolinic acid) on rutile TiO2(1 1 0) has been studied by means of X-ray photoemission spectroscopy. An investigation of the O 1s spectra shows that the molecular carboxylic groups are deprotonated and, hence, that the molecules bind to the surface in a bidentate mode. Moreover, the binding energy of those core levels that are related to the pyridine ring atoms shift as a function of molecule relative to the substrate O 1s and Ti 3p levels, while the position of the core levels related to emission from the carboxylic group are constant relative to the substrate levels. The molecule-dependent shifts are attributed to local intermolecular interactions that determine the proximity of adjacent molecular rings and thus the core-hole screening response of the neighbouring molecules. We propose a simple molecular arrangement for each case which satisfies the known constraints. (C) 2003 Elsevier B.V. All rights reserved. (Less)

In this paper the first resonant photoemission and partial electron yield studies of free clusters are presented. The resonant photoemission spectra of free argon clusters have been recorded with excitation energies determined from the cluster X-ray absorption spectrum near the Ar 2p ionization threshold. The decay of 2p(-1)4s and 2p(-1)3d cluster surface and "bulk" excited states has been investigated in detail. The combination of resonant photoemission and partial electron yield methods has allowed a selective probing of the electronic and geometric structure, yielding new results concerning the interplay between the energy of the excited states, their orbital character and the local atomic coordination. These new results demonstrate the power of resonant core level spectroscopy when applied to free clusters, as well as the advantages of free clusters for the studies of the bulk and surface energetics of nanoscale solids. (C) 2002 Elsevier Science B.V. All rights reserved. (Less)

The authors review the use of core-level resonant photoemission and resonant Auger spectroscopy to study femtosecond charge-transfer dynamics. Starting from simple models of the relevant processes, they examine the rationale for this approach and illustrate the approximations and known subtleties for the inexperienced experimentalist. Detailed analysis of case studies of increasing complexity are taken up, as well as the connection to related approaches using both valence excitation and the core-level fluorescent channel.

The pioneering years of photoelectron spectroscopy in Uppsala are discussed, especially the work leading to the discovery of the core level chemical shifts. At a very early stage of the project, th ...

Synchrotron radiation facilities routinely operate in a multi-bunch regime, but applications relying on time-of-flight schemes require single bunch operation. Here we show that pulse picking by resonant excitation in a storage ring creates in addition to the multi-bunch operation a distinct and separable single bunch soft X-ray source. It has variable polarization, a photon flux of up to 107–109 ph s−1/0.1%BW at purity values of 104–102 and a repetition rate of 1.25 MHz. The quasi-resonant excitation of incoherent betatron oscillations of electrons allows horizontal pulse separation at variable (also circular) polarization accessible for both, regular 30 ps pulses and ultrashort pulses of 2–3 ps duration. Combined with a new generation of angularly resolving electron spectrometers this creates unique opportunities for time-resolved photoemission studies as confirmed by time-of-flight spectra. Our pulse picking scheme is particularly suited for surface physics at diffraction-limited light sources promising ultimate spectral resolution. Although synchrotron facilities routinely operate in a multi-bunch regime for maximum average brilliance, studies relying on time-of-flight schemes require single-bunch operation. Here, Holldack et al.isolate and apply single bunch X-ray pulses from multibunch radiation using pulse picking by resonant excitation.