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XANES spectra of transition metal compounds
Abstract
An overview is given of the interactions that determine the XANES spectral shapes of transition metal compounds. The interactions are divided into ground state effects, final state effects and transition effects. The metal L edges, metal K edges and ligand K edges are analysed with respect to these interactions. The importance of XANES is partly due to its wide versatility in measurement conditions. XANES spectra can be measured using a number of sample environments, ranging from vacuum to ambient pressures for soft x-rays and up to extreme conditions with hard X-rays. These in-situ XANES spectra can be measured with a spatial resolution of 10 to 30 nm. XANES spectral shapes can be used as resonant channels in resonant photoemission, resonant x-ray emission or resonant diffraction experiments. This gives rise to a large number of resonant techniques that also allow the detection of site, valence, spin and symmetry selective XANES spectra and/or XANES spectra revealing information with a resolution better than its lifetime broadening.
... A considerable limitation of nearedge spectra is their poor spectroscopic resolution, with substantial broadening due to the short core-hole lifetime smearing out spectroscopic details. Additional sources of broadening for mercury L-edges 26 contribute to broad XAS spectra, showing little resolved sensitivity to chemical speciation. ...
... Image stacks of particles were collected at energies of 700 to 717 eV across the Fe L 3 absorption edge to determine chemical oxidation state of Fe. The near edge peak splitting of XANES spectra indicates the oxidation state of Fe in the sample with the initial peak intensity indicative of Fe(II) content and the preceding peak intensity indicative of Fe(III) content (de Groot, 2009). Based on the difference in the height of these two peaks, regions of particles are separated into four categories: pure Fe(II), Fe(II) rich mixed valence, Fe(III) rich mixed valence, and Fe(III) pure (von der Heyden et al., 2012;Hawkings et al., 2018). ...
It is now clear that, in some parts of the ocean, inputs of hydrothermal iron (Fe) can make a more significant contribution to the Fe inventory than previously thought. While the Fe isotopic signature of seawater has proved useful for distinguishing between inputs of Fe from atmospheric deposition and seafloor sediments, the Fe isotope signature of hydrothermal vent fluids may change during mixing of vent fluids and seawater. To better constrain the processes leading to these changes, the Fe isotopic compositions (δ56Fe) of dissolved and total dissolvable Fe have been determined in high temperature vent fluids and the buoyant hydrothermal plumes at the Beebe and the Von Damm vent fields, which are located along the ultraslow Mid-Cayman spreading center in the Caribbean Sea.
Our results show that the δ56Fe value of dissolved Fe in the earliest stages of buoyant plume formation was lower (as low as −4.08‰) than measured in a high temperature, low-Mg, vent fluid sample (−0.28‰). This indicates that the iron isotopic signature of dissolved Fe is principally controlled by oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides that preferentially incorporate heavy Fe isotopes. In support of this, the δ56Fe value of labile particulate Fe was higher than the δ56Fe value of dissolved Fe. Nevertheless, at Beebe, the δ56Fe value of total dissolvable Fe increased as the proportion of Fe predicted to have been lost from the plume increased, consistent with preferential fall-out of Fe-sulfides that are enriched in light Fe isotopes. The very low δ56Fe values of dissolved Fe in the Beebe buoyant plume are consistent with (i) the high Fe/H2S ratio of the vent fluids, and (ii) high Fe(II) oxidation rates, relative to other vent sites.<br/
... Image stacks of particles were collected at energies of 700 to 717 eV across the Fe L 3 absorption edge to determine chemical oxidation state of Fe. The near edge peak splitting of XANES spectra indicates the oxidation state of Fe in the sample with the initial peak intensity indicative of Fe(II) content and the preceding peak intensity indicative of Fe(III) content (de Groot, 2009). Based on the difference in the height of these two peaks, regions of particles are separated into four categories: pure Fe(II), Fe(II) rich mixed valence, Fe(III) rich mixed valence, and Fe(III) pure (von der Heyden et al., 2012;Hawkings et al., 2018). ...
Iron (Fe) limits primary productivity and nitrogen fixation in large regions of the world's oceans. Hydrothermal supply of Fe to the global deep ocean is extensive; however, most of the previous work has focused on examining high temperature, acidic, focused flow on ridge axes that create "black smoker" plumes. The contribution of other types of venting to the global ocean Fe cycle has received little attention. To thoroughly understand hydrothermal Fe sources to the ocean, different types of vent site must be compared. To examine the role of more diffuse, higher pH sources of venting, a hydrothermal plume above the Von Damm vent field (VDVF) was sampled for Total dissolvable Fe (unfiltered, TDFe), dissolved Fe (<0.2 µm, dFe) and soluble Fe (<0.02 µm, sFe). Plume particles sampled in situ were characterized using scanning electron microscopy and soft X-ray spectromicroscopy. The VDVF vents emit visibly clear fluids with particulate Fe (TDFe-dFe, >0.2 µm) concentrations up to 196 nmol kg −1 comparable to concentrations measured in black smoker plumes on the Mid-Atlantic Ridge. Colloidal Fe (cFe) and sFe increased as a fraction of TDFe with decreasing TDFe concentration. This increase in the percentage of sFe and cFe within the plume cannot be explained by settling of particulates or mixing with background seawater. The creation of new cFe and sFe within the plume from the breakdown of pFe is required to close the Fe budget. We suggest that the proportional increase in cFe and sFe reflects the entrainment, breakdown and recycling of Fe bearing organic particulates near the vents. Fe plume profiles from the VDVF differ significantly from previous studies of "black smoker" vents where formation of new pFe in the plume decreases the amount of cFe. Formation and removal of Fe-rich colloids and particles will control the amount and physico-chemical composition of dFe supplied to the deep ocean from hydrothermal systems. This study highlights the differences in the stabilization of hydrothermal Fe from an off-axis diffuse source compared to black smokers. Off-axis diffuse venting represent a potentially significant and previously overlooked Fe source to the ocean due to the difficulties in detecting and locating such sites.
... X-Ray Absorption Near Edge Structure (XANES) spectroscopy probes the transition from a core level to the bound state and valence orbitals, providing a tool to get insight into chemical information [14][15][16][17][18]. The energy position of the absorption edge changes with the oxidation state. ...
As many metals are essential for plants, excess or deficiency of them or alteration of their uptake, translocation, sequestration or physiological use can have severe consequences for plant growth and fitness. Therefore, investigating the distribution and speciation of metals in tissues is essential to understand plant physiology. We present a method based on non-destructive Synchrotron X-ray microtomography combined with microspectroscopy for studying metal distribution and speciation in plant tissues. By using the Maia detector system and the high flux of the undulator beam at the P06 beamline of the PETRA III synchrotron (at DESY), it was possible to record micro X-ray Absorption Near Edge Structure (μXANES) for every voxel of a tomogram. The metal coordination in regions of interest within the tissue samples could be determined by comparing the XANES with spectra of relevant reference compounds. Metal distribution and coordination were measured in shock frozen hydrated plant leaves in a cryostream, avoiding sample preparation artefacts like liquid cell content redistribution that occurs with other preparation methods, unequal distribution of stains in staining assays, sample degradation by beam damage and thawing, etc. A spatial resolution of 5 μm was selected, which is sufficient to resolve all leaf tissues (epidermis, palisade mesophyll, spongy mesophyll, veins), larger cells and biomineralization hotspots.
As an application example, we studied the effect of infection with Turnip Yellow Mosaic Virus (TYMV) on the Zn distribution and the Zn speciation in duplicates of Noccaea ochroleucum. This non-accumulator plant grown with 100 μM Zn had enough metal to allow collecting significant spectroscopic data. We found that the TYMV infected samples formed biomineralization crystallites, showing strong spectroscopic similarity to Zn silicate.
... The near-edge peak splitting seen in Fe L-edge absorption spectra is indicative of the oxidation state of Fe (Fitzsimmons et al., 2017a;Hoffman et al., 2018;Toner et al., 2009a;von der Heyden et al., 2012). The intensity of the initial near-edge peak is representative of Fe(II) and the intensity of the preceding peak representative of Fe(III) abundance (de Groot, 2009). Measured ...
Iron (Fe) limits or co-limits primary productivity and nitrogen fixation in large regions of the worlds oceans, and the supply of Fe from hydrothermal vents to the deep ocean is now known to be extensive. However, the mechanisms that control the amount of hydrothermal Fe that is stabilised in the deep ocean, and thus dictate the impact of hydrothermal Fe sources on surface ocean biogeochemistry, are unclear. To learn more, we have examined the dispersion of total dissolvable Fe (TDFe), dissolved Fe (dFe) and soluble Fe (sFe) in the buoyant and non-buoyant hydrothermal plume above the Beebe vent field, Caribbean Sea. We have also characterised plume particles using electron microscopy and synchrotron based spectromicroscopy.
We show that the majority of dFe in the Beebe hydrothermal plume was present as colloidal Fe (dFe- sFe = cFe). During ascent of the buoyant plume, a significant fraction of particulate Fe (pFe=TDFe- dFe) was lost to settling and exchange with colloids. Conversely, the opposite was observed in the non-buoyant plume, where pFe concentrations increased during non-buoyant plume dilution, cFe concentrations decreased apparently due to colloid aggregation. Elemental mapping of carbon, oxygen and iron in plume particles reveals their close association and indicates that exchanges of Fe between colloids and particles must include transformations of organic carbon and Fe oxyhydroxide minerals. Notably, sFe is largely conserved during plume dilution, and this is likely to be due to stabilisation by organic ligands, in contrast to the more dynamic exchanges between pFe and cFe.
This study highlights that the size of the sFe stabilising ligand pool, and the rate of iron-rich colloid aggregation will control the amount and physico-chemical composition of dFe supplied to the ocean interior from hydrothermal systems. Both the ligand pool, and the rate of cFe aggregation in hydrothermal plumes remain uncertain and determining these are important intermediate goals to more accurately assess the impact of hydrothermalism on the ocean’s carbon cycle.
Extended X-ray Absorption Fine Structure or EXAFS spectroscopy is commonly used to characterize inorganic materials and in particular, those used in heterogeneous catalysis. This chapter begins by introducing the EXAFS technique including a brief history of its development, the theory behind it, sample environments, data analysis routines and limitations. What then follows is the main body of the work where select examples are presented from literature which demonstrate the power of the technique to characterize catalytic active sites comprising single site (ions/atoms), clusters, nanoparticles and even bulk structures in attempt to show the scope of what is possible with the EXAFS technique as far as the determination of catalyst structure-activity relationships where thermal and electrocatalytic reactions are concerned. The chapter concludes with a summary of the current state of the art as well as an outlook of what is on the horizon as far as the technique is concerned.
Fluoride-ion batteries are an attractive energy storage concept analogous to lithium-ion batteries but feature an inverted paradigm where anions are the principal charge carriers. Insertion hosts that can reversibly insert fluoride ions at room temperature are exceedingly sparse. Here, we report that topochemical insertion of fluoride ions in FeSb2O4 involves Fe²⁺/Fe³⁺ redox but is mediated by multi-center synergies between iron and antimony centers. Separation of the redox center from the p-block coordination site alleviates structural strain by enabling compensatory contraction and expansion of FeO6 and SbO3 polyhedra, respectively. p-block electron lone pairs play a critical role in weakening anion-lattice interactions, enabling reversible fluoride-ion diffusion across microns. The results illuminate the key principle that interactions traceable to stereoactive lone pairs can be used to mediate anion-lattice interactions and suggest that anion insertion hosts can be designed by pairing redox-active transition metals with p-block cations bearing stereochemically active electron lone pairs.
Maghemite-like materials containing Fe(3+) and Cr(3+) in comparable amounts have been prepared by solution-combustion synthesis. The conditions of synthesis and the magnetic properties are described. These materials are ferrimagnetic and are much more stable than pure iron maghemite since their maghemite-hematite transformation takes place at about ∼700 °C instead of ∼300 °C, as usually reported. These materials were studied by synchrotron radiation X-ray diffraction (XRD) and by X-ray absorption fine structure (XAFS) of the K-absorption edge of two elements. High-resolution XRD patterns were processed by means of the Rietveld method. Thus, maghemites were studied by XAFS in both Fe and Cr K-edges to clarify the short-range structure of the investigated systems. Pre-edge decomposition and theoretical modeling of X-ray absorption near edge structure transitions were performed. The extended X-ray absorption fine structure (EXAFS) spectra were fitted considering the facts that the central atom of Fe is able to occupy octahedral and tetrahedral sites, each with a weight adjustment, while Cr occupies only octahedral sites. Interatomic distances were determined for x = 1, by fitting simultaneously both Fe and Cr K-edges average EXAFS spectra. The results showed that the cation vacancies tend to follow a regular pattern within the structure of the iron-chromium maghemite (FeCrO3).
Feature summary:
This Feature describes several methods for the characterization of magnetic nanoparticles in biological matrices such as cells and tissues. The article focuses on sample preparation and includes several case studies where multiple techniques were used in conjunction.
We use resonant photoelectron spectroscopy at the Fe2p and the O1s absorption edges to report on spectroscopic investigation of Fe-oxides nanoparticles. We discuss the spectroscopic details like multiple Auger decays, satellite emission in the X-ray absorption process and the core level data. We explain these data by a novel mechanism in which the resonant excitation between Fe3d and Fe4s states are involved. This mechanism is observed when the photo-excited electron becomes polarized by the electron-hole pairs existing in the conduction and valence bands.
In this contribution, we describe some features of atomic layer deposition (ALD) investigated by means of synchrotron radiation photoelemission spectroscopy (SR-PES). In particular, we show how the surface sensitivity of SR-PES combined with the in situ nature of our investigations can point out interactions between the substrate and ALD precursors. We observed changes on all substrates investigated, included Si, GaAs, Ru and their surface oxides. These interactions are extremely important during the first ALD cycles and induce modifications in the substrate, which might lead to its functionality enhancement.
In this work, we calculate band structures, the density of state and chemical bonding of the metallic niobium (Nb) and its mono-oxide (NbO) crystals in their solid states using the Density Functional Theory and X-ray Absorption Near Edge Spectroscopy. The electronic properties of Nb and NbO are investigated using the Finite Difference Method. These theoretical results are found in good agreement with the most recent experimental data. Our calculations reveal that the NbO crystal behaves like a superconductor.
We report a connection between the local structure of low-level Fe impurities and vacancies as the cause of ferromagnetic behavior observed in strontium titanate single crystals (STO), which were implanted with Fe and Si ions at different doses then annealed in oxygen. The effects of Fe doping and post-implantation annealing of STO were studied by X-ray Absorption Near Edge Structure (XANES) spectroscopy and Superconducting Quantum Interference Device magnetometry. XANES spectra for Fe and Ti K- and L-edge reveal the changes in the local environment of Fe and Ti following the implantation and annealing steps. The annealing in oxygen atmosphere partially healed implantation damages and changed the oxidation state of the implanted iron from metallic Fe0 to Fe2+/Fe3+ oxide. The STO single crystals were weak ferromagnets prior to implantation. The maximum saturation moment was obtained after our highest implantation dose of 2 × 1016 Fe atom/cm2, which could be correlated with the metallic Fe0 phases in addition to the presence of O/Ti vacancies. After recrystallization annealing, the ferromagnetic response disappears. Iron oxide phases with Fe2+ and Fe3+ corresponding to this regime were identified and confirmed by calculations using Real Space Multiple Scattering program (FEFF9).
The strange shape of experimental TEY XANES spectra from GaxIn1−xP measured at the phosphorus L2,3 edges as a function of photon glancing angle of incidence is explained by taking into account, for the first time, the energy dependence of the non-resonant photoelectron background arising from modulation of the X-ray radiation field by mirror reflection from the sample surface. The adjustment of the relative contribution of the resonant and nonresonant absorption to the TEY signal as well as the optical parameters of the sample allows us to fit the experimental data reasonably well. We perform the detailed analysis of the resonant white line shape in TEY XANES spectrum at different photon glancing angles depending on the optical parameters of the sample and the relative amount of the nonresonant contribution.
Ab-initio study using the density-functional theory (DFT) method to investigate the role of MgO, a basic material to oxidative dehydrogenation of C3H8 and n-C4H10 using V2O5 catalyst. The detailed calculations suggest that V2O5/MgO instead of compound Mg3(VO4)2 can be used for ODH of both alkanes but V2O5 supported on MgO may not be the most suitable catalyst for the ODH of n-C4H10. And when preparing the catalyst for ODH of n-C4H10 it should be avoided creating Mg3(VO4)2, a low catalytic activity.
In this contribution, we describe some features of atomic layer deposition (ALD) investigated by means of synchrotron radiation photoelemission spectroscopy (SR-PES). In particular, we show how the surface sensitivity of SR-PES combined with the in situ nature of our investigations can point out interactions between the substrate and ALD precursors. We observed changes on all substrates investigated, included Si, GaAs, Ru and their surface oxides. These interactions are extremely important during the first ALD cycles and induce modifications in the substrate, which might lead to its functionality enhancement.
Swimming in Iron Pools
Because iron is essential for marine phytoplankton growth, its availability limits the primary productivity of the oceans. Iron is typically bioavailable only when present in a dissolved state; however, a large fraction of the total iron in the oceans exists as tiny solid-phase particles ranging in size from a few nanometers to a few micrometers. von der Heyden et al. (p. 1199 ) used high-resolution x-ray microscopy and spectroscopy to characterize the distribution of iron particles along two transects of the Southern Ocean. Analysis of a number of individual particles reveals strong variation in iron oxidation state, particle mineralogy, and substitution of aluminum for iron—all of which control the solubility, and hence bioavailability, of iron.
Theoretical near edge X-ray absorption fine structure (NEXAFS) spectra describing oxygen 1s core excitation have been evaluated for the differently coordinated oxygen species appearing near the V2O3(0001) surface with half metal layer V′OV termination. Adsorption of oxygen above vanadium centers of the V′OV terminated surface (Ot
V′O termination) results in very strongly bound vanadyl oxygen, which has also been considered for core excitation in this study. The angle-resolved spectra are based on electronic structure calculations using ab initio density functional theory (DFT) together with model clusters. Experimental NEXAFS spectra for V2O3(0001) show a rather strong dependence of peak positions and relative intensities on the photon polarization direction. This dependence is well described by the present theoretical spectra and allows us to assign spectral details in the experiment to specific O 1s core excitations where final state orbitals are determined by the local binding environments of the differently coordinated oxygen centers. As a result, a combination of the present theoretical spectra with experimental NEXAFS data enables an identification of differently coordinated surface oxygen species at the V2O3(0001) surface.
Dramatic advances in the understanding of x-ray absorption fine structure (XAFS) have been made over the past few decades, which have led ultimately to a highly quantitative theory. This review covers these developments from a unified multiple-scattering viewpoint. The authors focus on extended x-ray absorption fine structure (EXAFS) well above an x-ray edge, and, to a lesser extent, on x-ray absorption near-edge structure (XANES) closer to an edge. The discussion includes both formal considerations, derived from a many-electron formulation, and practical computational methods based on independent-electron models, with many-body effects lumped into various inelastic losses and energy shifts. The main conceptual issues in XAFS theory are identified and their relative importance is assessed; these include the convergence of the multiple-scattering expansion, curved-wave effects, the scattering potential, inelastic losses, self-energy shifts, and vibrations and structural disorder. The advantages and limitations of current computational approaches are addressed, with particular regard to quantitative experimental comparisons.
High-resolution Fe K-edge XANES spectra of a series of crystalline Fe2+- and Fe3+-bearing model compounds were measured in an effort to correlate characteristics of the pre-edge feature with oxidation state and local coordination environment of Fe atoms. The model compounds comprise 30 natural minerals and synthetic compounds, with Fe coordination environments ranging from 4 to 12 O atoms for Fe2+, including 5-coordinated trigonal bipyramidal Fe2+, and from 4 to 6 O atoms For Fe3+. Most pre-edge spectra show two components (due to crystal-field splitting) that are located just above the Fermi level. The most useful characteristics of the Fe-K pre-edge for determining Fe oxidation state and coordination number are the position of its centroid and its integrated intensity. The separation between the average pre-edge centroid positions for Fe2+ and Fe3+ is 1.4 +/- 0.1 eV. Thus, the position of the pre-edge feature can be used as a measure of the average Fe-redox state, with the average pre edge position for mixed Fe2+-Fe3+ compounds occurring between positions for Fe2+ and Fe3+. The lowest pre-edge normalized heights and integrated intensities are observed for the most centrosymmetric sites of Fe, in agreement with previous studies (see Waychunas et al. 1983). Examination of the pre-edge features of mechanical mixtures of phases containing different proportions of Fe2+ and Fe3+ suggests that the pre-edge position and intensity for these mixtures can vary quite non-linearly with the average redox state of Fe. However, distinctly different trends of pre-edge position vs, pre-edge intensity can be observed, depending on the coordination environment of Fe2+ and Fe3+, with an accuracy in redox determination of +/- 10 mol% provided that the site geometry for each redox state is known. These methods have been used to estimate the Fe3+/Fe2+ ratio in 12 minerals (magnetite, vesuvianite, franklinite, rhodonite, etc.) containing variable/unknown amounts of Fe2+/Fe3+.
By resonant inelastic x-ray scattering in the soft x-ray regime we probe the dynamical multiple-spin correlations in the antiferromagnetic cuprates La(2)CuO(4) and CaCuO(2). High resolution measurements at the copper L(3) edge allow the clear observation of dispersing bimagnon excitations. Theory based on the ultrashort core-hole lifetime expansion fits the data on these coherent spin excitations without free parameters.
In studies of the electronic structure of solids, the augmented plane wave (APW) method is the basis for the solution of the Kohn–Sham equations of density functional theory (DFT). The different versions and developing steps are discussed in terms of linearization, full potential, local orbitals, mixed basis sets, relativistic effects and computational aspects, as employed in the WIEN2k code.
We have investigated the behavior of the 2p3s3p, 2p3p3p, and 2p3s3s Auger lines of NiO, a model compound in the class of strongly correlated 3d systems, while varying the photon energy across the Ni L-3 and L-2 absorption edges. The experimental data are discussed in comparison with a theoretical model based on a charge-transfer multiplet approach. When the excitation energy is below the L-3 resonance, we observe the 2p3p3p and 2p3s3p peaks at a constant binding energy. This behavior is typical of nonradiative resonant Raman scattering. If the photon energy is increased further, the 2p3p3p and 2p3s3p lines rapidly transform into constant kinetic energy features, showing a normal Auger behavior. The transition from Raman- to Auger-like behavior takes place for photon energies lower than the ones corresponding to excitations of the photoelectron into ligand-hole states. This might indicate the participation of inelastic processes in the recombination of the core hole involving energies much smaller than the NiO gap, or the possible presence of nonlocal effects. On the high photon energy side of the L-3 edge, the constant kinetic energy of the 2p3p3p and 2p3s3p peaks is systematically larger than the one observed for an excitation well above the L-2,L-3 edges. We attribute this behavior to the intervention of an intermediate state of 2p(5)3d(10) character, which has very little weight but is strongly enhanced at resonance. [S0163-1829(99)06215-3].
We report on a Ni L-2,L-3 edge x-ray absorption spectroscopy study in RNiO3 perovskites. These compounds exhibit a metal-to-insulator (MI) transition as temperature decreases. The L-3 edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the MI transition in RNiO3 perovskites in terms of modifications in the Ni3+ crystal field splitting that induces a spin transition from an essentially low-spin to a mixed-spin state.
The purpose of this work is to compare the two different procedures to calculate the L(2,3) x-ray absorption spectra of transition-metal compounds: (1) the semi-empirical charge transfer multiplet (CTM) approach and (2) the ab initio configuration-interaction (CI) method based on molecular orbitals. We mainly focused on the difference in the treatment of ligand field effects and the charge transfer effects in the two methods. The reduction of multiplet interactions due to the solid state effects has been found by the ab initio CI approach. We have also found that the mixing between the original and the charge transferred configurations obtained by the ab initio CI approach is smaller than that obtained by the CTM approach, since charge transfer through the covalent bonding between metal and ligand atoms has been included by taking the molecular orbitals as the basis functions.
We develop a general procedure to analyse the pre-edges in 1s x-ray absorption near edge structure (XANES) of transition metal oxides and coordination complexes. Transition metal coordination complexes can be described from a local model with one metal ion. The 1s 3d quadrupole transitions are calculated with the charge-transfer multiplet program. Tetrahedral coordination complexes have more intense pre-edge structures due to the local mixing of 3d and 4p states, implying a combination of 1s 3d quadrupole and 1s 4p dipole transitions. Divalent transition metal oxides can be described similar to coordination complexes, but for trivalent and tetravalent oxides, additional structures are visible in the pre-edge region due to non-local dipole transitions. The 1s 4p dipole transitions have large cross section at the 3d-band region due to the strong metal-metal interactions, which are oxygen mediated. This yields large intensity in the 3d-band region but at a different energy than the local 1s 3d quadrupole transitions because of smaller core-hole effects due to the delocalization of the excited electron.
The structural environment of substitutional Cr3+ ion in MgAl2O4 spinel has been investigated by Cr K-edge Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) spectroscopies. First-principles computations of the structural relaxation and of the XANES spectrum have been performed, with a good agreement to the experiment. The Cr-O distance is close to that in MgCr2O4, indicating a full relaxation of the first neighbors, and the second shell of Al atoms relaxes partially. These observations demonstrate that Vegard's law is not obeyed in the MgAl2O4-MgCr2O4 solid solution. Despite some angular site distortion, the local D3d symmetry of the B-site of the spinel structure is retained during the substitution of Cr for Al. Here, we show that the relaxation is accomodated by strain-induced bond buckling, with angular tilts of the Mg-centred tetrahedra around the Cr-centred octahedron. By contrast, there is no significant alteration of the angles between the edge-sharing octahedra, which build chains aligned along the three four-fold axes of the cubic structure.
Magnetic atoms at surfaces are a rich model system for solid-state magnetic bits exhibiting either classical or quantum behaviour. Individual atoms, however, are difficult to arrange in regular patterns. Moreover, their magnetic properties are dominated by interaction with the substrate, which, as in the case of Kondo systems, often leads to a decrease or quench of their local magnetic moment. Here, we show that the supramolecular assembly of Fe and 1,4-benzenedicarboxylic acid molecules on a Cu surface results in ordered arrays of high-spin mononuclear Fe centres on a 1.5 nm square grid. Lateral coordination with the molecular ligands yields unsaturated yet stable coordination bonds, which enable chemical modification of the electronic and magnetic properties of the Fe atoms independently from the substrate. The easy magnetization direction of the Fe centres can be switched by oxygen adsorption, thus opening a way to control the magnetic anisotropy in supramolecular layers akin to that used in metallic thin films.
The modern chemical industry uses heterogeneous catalysts in almost every production process. They commonly consist of nanometre-size active components (typically metals or metal oxides) dispersed on a high-surface-area solid support, with performance depending on the catalysts' nanometre-size features and on interactions involving the active components, the support and the reactant and product molecules. To gain insight into the mechanisms of heterogeneous catalysts, which could guide the design of improved or novel catalysts, it is thus necessary to have a detailed characterization of the physicochemical composition of heterogeneous catalysts in their working state at the nanometre scale. Scanning probe microscopy methods have been used to study inorganic catalyst phases at subnanometre resolution, but detailed chemical information of the materials in their working state is often difficult to obtain. By contrast, optical microspectroscopic approaches offer much flexibility for in situ chemical characterization; however, this comes at the expense of limited spatial resolution. A recent development promising high spatial resolution and chemical characterization capabilities is scanning transmission X-ray microscopy, which has been used in a proof-of-principle study to characterize a solid catalyst. Here we show that when adapting a nanoreactor specially designed for high-resolution electron microscopy, scanning transmission X-ray microscopy can be used at atmospheric pressure and up to 350 degrees C to monitor in situ phase changes in a complex iron-based Fisher-Tropsch catalyst and the nature and location of carbon species produced. We expect that our system, which is capable of operating up to 500 degrees C, will open new opportunities for nanometre-resolution imaging of a range of important chemical processes taking place on solids in gaseous or liquid environments.
The oxygen 1s x-ray-absorption edges of a series of 3d-transition-metal oxides have been measured. The structures at the edge arise from covalent mixing of the metal and oxygen states, which introduces oxygen p character in unoccupied states of mainly metal character. The spectra can be divided into two regions: The first is a double-peaked sharp structure near threshold, which can be related to the metal 3d states; the second is a broader structure 5–10 eV above the edge and is related to the metal 4s and 4p bands. We attribute the oxygen p character up to 15 eV above threshold to mainly oxygen 2p character. The data are analyzed in terms of ligand-field and exchange splittings. It is shown that the splitting between the two sharp peaks near threshold is related closely to the ligand-field splitting, but the relative intensities of the peaks are not fully explained at the present time.
The L2,3 x-ray-absorption edges of 3d0 compounds are calculated with use of an atomic description of the 2p63d0 to 2p53d1 excitation, with the inclusion of the crystal field. For reasons of clarity, we confine ourselves to d0 compounds in octahedral symmetry, but the same approach is applicable to all other dN compounds in any point-group symmetry. The experimental spectra of FeTiO3, Sc2O3, ScF3, CaF2, and the potassium halides are well reproduced by the present calculations, including the previously misinterpreted small leading peaks. The splitting between the two main peaks in both the L3 and L2 edge are related, though not equal, to the crystal-field splitting. Comparison to experiment showed that the broadening of the main multiplet lines is different. This can be related to Coster-Kronig Auger processes for the L2 edge and to a solid-state broadening which is a combination of vibrational (phononic) and dispersional broadenings. With the full treatment of the atomic multiplets, the atomic effects can be separated from solid-state effects, which offers a better description of the latter. This includes vibrational broadenings, the covalent screening of the intra-atomic Coulomb and exchange interactions, via the position of small leading peaks, and surface effects. The same general framework can be used to discuss crystal-field effects in both lower symmetries, with the possibility of polarization-dependent spectra (e.g., TiO2), and partly filled d bands.
The metal 2p x-ray-absorption spectra (or L2,3 edges) of 3d
transition-metal compounds are calculated, using atomic multiplet theory
with inclusion of the cubic crystal field. A general overview of the
effect of the cubic crystal field on the shape of the 3dN to
2p53dN+1 excitation spectrum is given for 14
common valencies of 3d transition-metal ions. Comparison to some
high-resolution 2p x-ray-absorption spectra shows excellent agreement,
which confirms the validity of the approach. Possible refinements of the
theory, including lower-symmetry calculations and the inclusion of
configuration interaction, are discussed.
We report on strong dipole transitions to 3d orbitals of neighboring Co atoms in the Co 1s x-ray absorption pre-edge. They are revealed by applying high-resolution resonant x-ray emission spectroscopy (RXES) to compounds containing CoO6 clusters. When contrasted to quadrupole local 1s3d excitations, these non-local 1s3d transitions are identified by their energy dispersion and angular dependence, their sensitivity to second-shell effects (i.e. the connection mode of the CoO6 octahedra and the bond lengths), and an upwards energy shift of 2.5 eV due to the poorer screening of the core hole. The experiment reveals that the intensity of the non-local transitions gauges the oxygen-mediated 4p-O-3d intersite hybridization. We propose a revised interpretation of the pre-edges of transition metal compounds. Detailed analysis of these new features in the pre-edge offers a unique insight in the oxygen mediated metal-metal interactions in transition metal-based systems, which is a crucial aspect in orbital ordering and related electronic and magnetic properties. In addition, the exceptional resolving power of the present 1s2p RXES experiment allows us to demonstrate the coherent second-order nature of the underlying scattering process.
We determine the structural relaxations around paramagnetic impurities (Ti,
Cr, Fe) in corundum (alpha-Al2O3), by combining x-ray absorption near edge
structure (XANES) experiments and ab initio calculations. The structural
relaxations are found to be very local. We then show that XANES is sensitive to
small variations in interatomic distances within the coordination shell of the
absorbing atom. The experiments were carried out on single crystals of ruby and
sapphires. Linear dichroic signals are essential to characterize the geometry
of the impurity site. The calculations were performed within a self-consistent
``non muffin-tin'' framework, that uses pseudopotentials, plane-wave basis set,
and the continued fraction for the absorption cross section.
Resonant inelastic soft x-ray inelastic-scattering (RIXS) spectra of single-crystal iron ilmenite FeTiO3 were measured at the Ti 2p resonance, and additional peaks at −2.5 and −4.5 eV in energy loss were observed. In order to investigate the origin of these peaks, we performed a double-cluster model calculation based on full-multiplet calculation in which, in addition to the interaction with the O 2p ligands of the Ti ion, which is already included in the more conventional single-cluster model, the charge-transfer contribution to Ti 3d from Fe 3d is taken into account. The calculation shows that the peaks may be attributed to the Fe 3d→Ti 3d intermetallic charge-transfer scheme that previously had not been accounted for in RIXS.
O K-edge x-ray absorption near-edge structure (XANES) spectra of α-quartz-type and rutile-type GeOâ polymorphs and of KâGeâOââ have been analyzed using first-principles plane-wave pseudopotential calculations. XANES spectra have been calculated using supercell including core-hole effects and good agreement with experiment has been obtained. In the the case of GeOâ polymorphs, local density of empty states has been performed and peaks in the experimental spectra can be assigned to transitions involving hybridization of the O p orbitals with the Ge s, Ge p, Ge sp, and Ge d orbitals. Furthermore, peak positions in the theoretical spectra appear to be correlated with changes in the Ge-O-Ge angle as well as indirectly with the Ge coordination geometry. Analysis of O K-edge XANES spectra for individual O sites in KâGeâOââ shows that oxygens shared between two fivefold Ge atoms or one fourfold and one fivefold Ge atom exhibit subtle shifts to lower energy of the peaks, which have been previously observed in alkali germanate glasses at and above the germanate anomaly.
A spin-selective and polarization dependent study at the K absorption pre-edge in hematite (alpha-Fe2O3) was performed by means of 1s3p resonant x-ray emission spectroscopy on a single crystal. The experimental results can be modeled using a band structure approach [local density approximation (LDA + U )]. The spin-selective spectra in connection with the calculations firmly establish metal-oxygen and metal-metal orbital hybridizations. A phase shift in the polarization dependent absorption cross section is observed between the first two absorption features showing directionality in the orbital hybridization.
We have investigated the pre-edge features in x-ray absorption spectra of transition metal (TM) monoxides at the metal K edge. By comparing the calculated dipolar and quadrupolar spectra with experiments we assign the first peak at the lowest energy to a direct quadrupolar transition (due to the more effective attraction of the core hole). The following peaks in this region are mainly dipolar in character and reflect the density of states due to the medium range order as long as the radius of the cluster equals the cation-cation shortest distance plus the nearest neighbor cation-anion bond, representing a band-like effect arising from the hybridization of the 4p orbitals of central TM atoms with the 3d-based octahedral molecular orbitals of the higher-neighboring cations.
The strength of the core-hole effect in simulations of electron energy-loss near edge structures or x-ray absorption near edge structures is investigated using ab initio calculations based on the density functional theory. Calculations were performed using the WIEN2K and FEFF codes at different edges for the following model compounds: rutile TiO 2 , MgO, and for some transition metals Ti, Fe, Co, Cu, and Zn. We demonstrate that although a core hole is always present in any core-level spectroscopy experiment, its effect is not always observable. To observe or not a core-hole effect in any experimental situation can, however, be predicted from the examination of the material's ground-state electronic structure, especially of the states just above the Fermi level. Indeed, it is shown that the two important criteria governing the core-hole strength at a given edge depend first on the localization and on the character of the first empty states of the material under investigation, and second on the nature of the compensating charge necessary to maintain the crystal neutrality. These criteria are expected to give a good description of the core-hole effect in the one-particle approximation. However, nothing can be inferred as for the good/bad agreement between calculations and experiments for more complex cases such as excitations of delocalized semicore states or of atomiclike multiplet structures.
The creation of a 1s core hole in a 3d transition metal ion gives rise to an emission spectrum that can be recorded using a crystal analyzer. K shell X-ray spectroscopy using an analyser energy bandwidth of ∼1 eV is sensitive to electron–electron interactions and orbital splittings and preserves the advantages of the hard X-ray probe. We review recent developments in Kα and Kβ spectroscopy and show how the chemical sensitivity of the fluorescence lines can be exploited for selective X-ray absorption studies. When the photo excitation energy is tuned close to the K edge threshold, the phenomenon known as X-ray resonant Raman or resonant inelastic X-ray scattering (RIXS) occurs. RIXS spectroscopy on 3d transition metals at the 1s resonances with lowest incident energies (K pre-edge) is a very recent technique. We discuss basic aspects and demonstrate with several examples its potential as a future routine spectroscopic tool.
Recent developments in density functional theory (DFT) methods applicable to studies of large periodic systems are outlined. During the past three decades, DFT has become an essential part of computational materials science, addressing problems in materials design and processing. The theory allows us to interpret experimental data and to generate property data (such as binding energies of molecules on surfaces) for known materials, and also serves as an aid in the search for and design of novel materials and processes. A number of algorithmic implementations are currently being used, including ultrasoft pseudopotentials, efficient iterative schemes for solving the one-electron DFT equations, and computationally efficient codes for massively parallel computers. The first part of this article provides an overview of plane-wave pseudopotential DFT methods. Their capabilities are subsequently illustrated by examples including the prediction of crystal structures, the study of the compressibility of minerals, and applications to pressure-induced phase transitions. Future theoretical and computational developments are expected to lead to improved accuracy and to treatment of larger systems with a higher computational efficiency. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 895–910, 2000
We report on the performance of a newly constructed synchrotron radiation soft x‐ray beamline. This beamline, dubbed Dragon, is a spherical version of the cylindrical element monochromator design proposed previously. By measuring the K‐edge absorption spectra of condensed nitrogen, it is determined that this monochromator has achieved resolving power 104 at 400‐eV photon energy, using its full 15 by 1 mrad angular acceptance. The ideas and advantages contained in the CEM design have also been experimentally confirmed.
The analysis of x-ray absorption spectra to determine the electronic and magnetic structure of transition metal compounds is discussed. The models to describe the ground state of transition metal compounds (single-particle, impurity, crystal field) are introduced. Some basic aspects of the interaction of x-rays with matter are recapitulated and the description of x-ray
absorption is separated into single-particle models for the
1s
edges and multiplet models for the
2p
edges. Magnetic circular dichroism is introduced and the six Thole sum rules are discussed. The complications and experimental problems of the sum rules are outlined. The last section briefly mentions some aspects of resonance studies, for which a detailed knowledge of x-ray
absorption is crucial.
We have applied a number of novel X-ray spectroscopic tools to Fe/ZSM-5 systems. Fe/ZSM-5 can be considered as an ideal test-system for the characterization techniques in heterogeneous catalysis. The existence of a large range of sites and structures creates a good testing ground to determine which experimental tools are able to resolve such complex system. In situ soft X-ray absorption provides important information on the valence and electronic structure of iron during treatments, with a time scale down to 30 s. Kβ-detected XANES yields unprecedented resolution for pre-edge structures and using hard X-rays can be used under any condition and treatment, including high-pressures. It can be expected that both in situ soft X-ray absorption and Kβ-detected XANES become ‘standard’ tools for catalysis research, similar to traditional XANES and EXAFS today. The X-MCD is also used in this paper but it will probably remain a rather specialized technique in the field of heterogeneous catalysis. Further new developments for catalysis characterization are for all to be expected from X-ray spectro-microscopy, where one will have the possibility to perform the in situ soft X-ray absorption and Kβ-detected XANES experiments with nanometer size spatial resolution.
The valence and local symmetry of iron in framework-substituted FeZSM-5 with a high Fe dilution (Si/Fe = 360) was studied by means of Kbeta-detected X-ray absorption spectroscopy. This technique combines high-resolution (DeltaE similar to1 eV) fluorescence detection of the 3p to 1s (Kbeta) transition with the X-ray absorption near-edge structure (XANES) at the Fe K-edge. An absorption-like spectrum is recorded by detecting the Kbeta fluorescence intensity as a function of the incident energy that is scanned through the K absorption edge. Kbeta-detected XANES spectra allow for a more precise separation of the weak K pre-edge structure from the main edge as compared to conventional absorption spectroscopy. Subsequent analysis and interpretation of the pre-edge spectral features therefore is more accurate. The pre-edge is sensitive to changes in the local coordination and oxidation state of Fe. Using this technique we were able to quantitatively determine the degree of iron extraction out of a zeolite framework upon steaming. With the use of appropriate reference compounds, the pre-edge analysis was used to monitor the activation of low-loaded, framework-substituted FeZSM-5 (0.3 wt % Fe). Template removal and calcination distort the zeolite framework and induce a deviation from T-d symmetry for incorporated iron. The (deliberate) presence of water at high temperature (T > 500 degreesC) facilitates the hydrolysis of the Si-O-Fe bonds and increases the formation of extraframework iron species. The amount of Fe-III occupying tetrahedral sites in the MFI-type zeolite decreases to 32% and 19%, respectively, for mild- and hard-steamed samples.
This review gives an overview of the presence of multiplet effects in X-ray spectroscopy, with an emphasis on X-ray absorption studies on 3d transition metal ions in inorganic oxides and coordination compounds. The first part of the review discusses the basics of multiplet theory and respectively, atomic multiplets, crystal field effects and charge transfer effects are explained. The consequences of 3d-spin-orbit coupling and of 3d systems in symmetries lower than cubic are discussed. The second part of the paper gives a short overview of all X-ray spectroscopes, where the focus is on the multiplet aspects of those spectroscopies and on the various configurations that play a role in combined spectroscopies such as resonant photoemission, resonant X-ray emission and coincidence spectroscopy. The review is concluded with a section that gives an overview of the use of multiplet theory for 3d coordination compounds. Some new developments are sketched, such as the determination of differential orbital covalence and the inclusion of pi-(back)bonding, (C) 2004 Elsevier B.V. All rights reserved.
In-situ soft X-ray absorption spectroscopy (XAS) has been applied to study the iron redox behavior in overexchanged Fe/ZSM5. The Fe L2,3 XAS and O K spectral shapes of the Fe/ZSM5 surface have been measured during heat treatments and reduction/oxidation cycles. Charge-transfer multiplet calculations provide a detailed
understanding of the L2,3 spectra of iron in Fe/ZSM5. The oxidized form of Fe/ZSM5 contains FeIII ions in an octahedral surrounding, with a total crystal field splitting of ~1.0 eV. This value is significantly smaller than that for Fe2O3, which is indicative of a much weaker Fe-O bonding. The reduced form of Fe/ZSM5 has
FeII ions in a tetrahedral oxygen surrounding. The Fe L2,3 spectra show that iron in calcined Fe/ZSM5 is reduced in 15 min to an average valence state of 2.65, under 10 mbar of pure helium at room temperature. This value has a relative uncertainty on the order of 0.01. Heating in helium up to 350 °C under the same pressure further reduces the iron valence to 2.15. The oxygen spectra show that the autoreduction is accompanied by a loss of molecular oxygen and water. Reoxidation with 5% O2 in helium yields a valence of >2.90 after 10 min.
X-ray absorption spectra of gas-phase VOCl(3) and CrO(2)Cl(2) have been measured in the metal L(2,3)-edge and O K-edge regions. The assignment of the spectral features is based on the relativistic two-component ZORA TDDFT approach. The calculations provide results in excellent agreement with the experimental spectra and prove the importance of including both configuration mixing and spin-orbit coupling in the theoretical description to obtain a reliable simulation of the transition metal L(2,3)-edge. The calculations are extended also to the MnO(3)Cl molecule to discuss the spectral variations along the series of the oxychlorides both in the metal L(2,3) and ligand O K spectra.
Katalysator unter der Lupe : Die Untersuchung der Reduktion an einem Fischer‐Tropsch‐Katalysatorpartikel mithilfe von In‐situ‐Röntgentransmissionsmikroskopie im Rastermodus zeigte bei einer räumlichen Auflösung von 35 nm eine ungleichmäßige Verteilung von Fe ⁰ ‐, Fe ²⁺ ‐ und Fe ³⁺ ‐Spezies. Regionen mit unterschiedlichen Reduktionseigenschaften können abgegrenzt werden; ihr Zustandekommen lässt sich auf der Grundlage lokaler chemischer Wechselwirkungen und der Katalysatormorphologie erklären. magnified image
Iron K-edge X-ray absorption pre-edge features have been calculated using a time-dependent density functional approach. The influence of functional, solvation, and relativistic effects on the calculated energies and intensities has been examined by correlation of the calculated parameters to experimental data on a series of 10 iron model complexes, which span a range of high-spin and low-spin ferrous and ferric complexes in O(h) to T(d) geometries. Both quadrupole and dipole contributions to the spectra have been calculated. We find that good agreement between theory and experiment is obtained by using the BP86 functional with the CP(PPP) basis set on the Fe and TZVP one of the remaining atoms. Inclusion of solvation yields a small improvement in the calculated energies. However, the inclusion of scalar relativistic effects did not yield any improved correlation with experiment. The use of these methods to uniquely assign individual spectral transitions and to examine experimental contributions to backbonding is discussed.
A study of high-resolution X-ray emission and X-ray absorption spectroscopy was carried out. X-ray absorption is a synchrotron-based characterization technique that can be divided into near-edge spectroscopy (XANES) and extended X-ray absorption fine structure. The results revealed that X-ray absorption spectra are analyzed either with density functional theory for the 1s core levels or multiplet theory for all other edges.
The measurement and calculation of X-ray absorption near-edge structure at very low energy can provide important information about a metallic center in biological compounds. A rapid overview of the biological applications of this technique is given, then a new method of calculating the spectra is presented. This technique, based on the use of the finite-difference method to solve the Schrödinger equation, is especially precise and potentially applicable to metalloproteins. Examples of its use on an oxide and an organic compound illustrate the kind of spectroscopic information that can be obtained.
The first quantitative analyses are reported of the Fe
K
-edge polarized X-ray absorption near-edge structure (XANES) of a single crystal of the iron protein carbonmonoxy-myoglobin (MbCO) and of its cryogenic photoproduct Mb*CO. The CO—Fe–heme local structure has been determined using a novel fitting procedure, named
MXAN
, which is able to fit the XANES part (from the edge to about 200 eV) of experimental X-ray absorption data. This method is based on the comparison between the experimental spectrum and several theoretical spectra that are generated by changing the relevant geometrical parameters of the site around the absorbing atom. The theoretical spectra are derived in the framework of the full multiple-scattering approach. The
MXAN
procedure is able to recover information about the symmetry and atomic distances, and the solution is found to be independent of the starting conditions. The extracted local structure of Mb*CO includes an Fe—CO distance of 3.08 (7) Å, with a tilting angle between the heme normal and the Fe—C vector of 37 (7)° and a bending angle between the Fe—C vector and the C—O bond of 31 (5)°
X-ray absorption spectroscopy has been utilized to obtain the L-edge multiplet spectra for a series of non-heme ferric and ferrous complexes. Using these data, a methodology for determining the total covalency and the differential orbital covalency (DOC), that is, differences in covalency in the different symmetry sets of the d orbitals, has been developed. The integrated L-edge intensity is proportional to the number of one-electron transition pathways to the unoccupied molecular orbitals as well as to the covalency of the iron site, which reduces the total L-edge intensity and redistributes intensity, producing shake-up satellites. Furthermore, differential orbital covalency leads to differences in intensity for the different symmetry sets of orbitals and, thus, further modifies the experimental spectra. The ligand field multiplet model commonly used to simulate L-edge spectra does not adequately reproduce the spectral features, especially the charge transfer satellites. The inclusion of charge transfer states with differences in covalency gives excellent fits to the data and experimental estimates of the different contributions of charge transfer shake-up pathways to the t(2g) and e(g) symmetry orbitals. The resulting experimentally determined DOC is compared to values calculated from density functional theory and used to understand chemical trends in high- and low-spin ferrous and ferric complexes with different covalent environments. The utility of this method toward problems in bioinorganic chemistry is discussed.
L(2,3) X-ray absorption spectra of aqueous [Ru(II)(bpy)3]2+ have been recorded in its ground and excited states, 50 ps after short pulse laser excitation. Significant changes in both the XANES (X-ray Near-Edge Absorption Structure) and the EXAFS (Extended X-ray Absorption Fine Structure) regions of the excited state complex are detected. The XANES line shapes have been quantitatively simulated using a crystal field multiplet code in trigonal symmetry. In addition, spectral changes in the EXAFS region of both ground and excited states are analyzed in order to extract structural parameters of their corresponding molecular structures. We obtain a Ru-N bond contraction by approximately 0.03 angstroms in the excited-state complex, as compared to the ground-state compound. This contraction results from electrostatic and polarization contributions, limited by steric constraints on the bpy ligands.
Distinct spectral features at the Fe L-edge of the two compounds K3[Fe(CN)6] and K4[Fe(CN)6] have been identified and characterized as arising from contributions of the ligand pi orbitals due to metal-to-ligand back-bonding. In addition, the L-edge energy shifts and total intensities allow changes in the ligand field and effective nuclear charge to be determined. It is found that the ligand field term dominates the edge energy shift. The results of the experimental analysis were compared to BP86 DFT calculations. The overall agreement between the calculations and experiment is good; however, a larger difference in the amount of pi back-donation between Fe(II) and Fe(III) is found experimentally. The analysis of L-edge spectral shape, energy shift, and total intensity demonstrates that Fe L-edge X-ray absorption spectroscopy provides a direct probe of metal-to-ligand back-bonding.
The Borrmann effect-a dramatic increase in transparency to X-ray beams-is observed when X-rays satisfying Bragg's law diffract through a perfect crystal. The minimization of absorption seen in the Borrmann effect has been explained by noting that the electric field of the X-ray beam approaches zero amplitude at the crystal planes, thus avoiding the atoms. Here we show experimentally that under conditions of absorption suppression, the weaker electric quadrupole absorption transitions are effectively enhanced to such a degree that they can dominate the absorption spectrum. This effect can be exploited as an atomic spectroscopy technique; we show that quadrupole transitions give rise to additional structure at the L(1), L(2) and L(3) absorption edges of gadolinium in gadolinium gallium garnet, which mark the onset of excitations from 2s, 2p(1/2) and 2p(3/2) atomic core levels, respectively. Although the Borrmann effect served to underpin the development of the theory of X-ray diffraction, this is potentially the most important experimental application of the phenomenon since its first observation seven decades ago. Identifying quadrupole features in X-ray absorption spectroscopy is central to the interpretation of 'pre-edge' spectra, which are often taken to be indicators of local symmetry, valence and atomic environment. Quadrupolar absorption isolates states of different symmetries to that of the dominant dipole spectrum, and typically reveals orbitals that dominate the electronic ground-state properties of lanthanides and 3d transition metals, including magnetism. Results from our Borrmann spectroscopy technique feed into contemporary discussions regarding resonant X-ray diffraction and the nature of pre-edge lines identified by inelastic X-ray scattering. Furthermore, because the Borrmann effect has been observed in photonic materials, it seems likely that the quadrupole enhancement reported here will play an important role in modern optics.
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