An unconventional method for measuring the Tc L3‐edge of technetium compounds

Tc L₃‐edge XANES spectra have been collected on powder samples of SrTcO₃ (octahedral Tc⁴⁺) and NH₄TcO₄ (tetrahedral Tc⁷⁺) immobilized in an epoxy resin. Features in the Tc L₃‐edge XANES spectra are compared with the pre‐edge feature of the Tc K‐edge as well as other 4d transition metal L₃‐edges. Evidence of crystal field splitting is obvious in the Tc L₃‐edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L₃ absorption edge energy difference between SrTcO₃ (Tc⁴⁺) and NH₄TcO₄ (Tc⁷⁺) shows that the energy shift at the Tc L₃‐edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L₃‐edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L₃‐edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K‐edge.     

Phosphorus L2,3‐edge XANES: overview of reference compounds

Synchrotron‐based X‐ray absorption near‐edge structure (XANES) spectroscopy is becoming an increasingly used tool for the element speciation in complex samples. For phosphorus (P) almost all XANES measurements have been carried out at the K‐edge. The small number of distinctive features at the P K‐edge makes in some cases the identification of different P forms difficult or impossible. As indicated by a few previous studies, the P L_2,3‐edge spectra were richer in spectral features than those of the P K‐edge. However, experimentally consistent spectra of a wide range of reference compounds have not been published so far. In this study a library of spectral features is presented for a number of mineral P, organic P and P‐bearing minerals for fingerprinting identification. Furthermore, the effect of radiation damage is shown for three compounds and measures are proposed to reduce it. The spectra library provided lays a basis for the identification of individual P forms in samples of unknown composition for a variety of scientific areas.     

Muffin‐tin potentials in EXAFS analysis

Muffin‐tin potentials are the standard tool for calculating the potential surface of a cluster of atoms for use in the analysis of extended X‐ray absorption fine‐structure (EXAFS) data. The set of Cartesian coordinates used to define the positions of atoms in the cluster and to calculate the muffin‐tin potentials is commonly also used to enumerate the scattering paths used in the EXAFS data analysis. In this paper, it is shown that these muffin‐tin potentials are sufficiently robust to be used to examine quantitatively contributions to the EXAFS data from scattering geometries not represented in the original cluster.     

Langmuir Probe in Non‐Maxwellian Plasma: Correlation Between the Electron Energy Distribution Function,the Ion Energy at the Sheath Edge and the Floating Voltage

This work is devoted to the study of the Langmuir probe in non‐Maxwellian plasma, assuming mono‐energetic singly charged ions and a collisionless sheath. Using a general analytical equation for the Electron Energy Distribution Function (EEDF), we study the effect of the EEDF profile on: a) The ion energy at the sheath edge of a negatively biased collector, b) the I‐V probe characteristic and c) the floating voltage (V_p‐Vf). Different methods are used and compared to determine these parameters or characteristics. A correlation is given between the floating voltage, the ion energy at the sheath edge and the EEDF profile. The study is also extended to distribution functions with several components. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

K‐edge XANES analysis of sulfur compounds: an investigation of the relative intensities using internal calibration

Sulfur K‐edge XANES (X‐ray absorption near‐edge structure) spectroscopy is an excellent tool for determining the speciation of sulfur compounds in complex matrices. This paper presents a method to quantitatively determine the kinds of sulfur species in natural samples using internally calibrated reference spectra of model compounds. Owing to significant self‐absorption of formed fluorescence radiation in the sample itself the fluorescence signal displays a non‐linear correlation with the sulfur content over a wide concentration range. Self‐absorption is also a problem at low total absorption of the sample when the sulfur compounds are present as particles. The post‐edge intensity patterns of the sulfur K‐edge XANES spectra vary with the type of sulfur compound, with reducing sulfur compounds often having a higher post‐edge intensity than the oxidized forms. In dilute solutions (less than 0.3–0.5%) it is possible to use sulfur K‐edge XANES reference data for quantitative analysis of the contribution from different species. The results show that it is essential to use an internal calibration system when performing quantitative XANES analysis. Preparation of unknown samples must take both the total absorption and possible presence of self‐absorbing particles into consideration.     

Polycapillary‐optics‐based micro‐XANES and micro‐EXAFS at a third‐generation bending‐magnet beamline

A focusing system based on a polycapillary half‐lens optic has been successfully tested for transmission and fluorescence µ‐X‐ray absorption spectroscopy at a third‐generation bending‐magnet beamline equipped with a non‐fixed‐exit Si(111) monochromator. The vertical positional variations of the X‐ray beam owing to the use of a non‐fixed‐exit monochromator were shown to pose only a limited problem by using the polycapillary optic. The expected height variation for an EXAFS scan around the Fe K‐edge is approximately 200 µm on the lens input side and this was reduced to ～1 µm for the focused beam. Beam sizes (FWHM) of 12–16 µm, transmission efficiencies of 25–45% and intensity gain factors, compared with the non‐focused beam, of about 2000 were obtained in the 7–14 keV energy range for an incoming beam of 0.5 × 2 mm (vertical × horizontal). As a practical application, an As K‐edge µ‐XANES study of cucumber root and hypocotyl was performed to determine the As oxidation state in the different plant parts and to identify a possible metabolic conversion by the plant.     

Solid energy calibration standards for P K‐edge XANES: electronic structure analysis of PPh4Br

P K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy is a powerful method for analyzing the electronic structure of organic and inorganic phosphorus compounds. Like all XANES experiments, P K‐edge XANES requires well defined and readily accessible calibration standards for energy referencing so that spectra collected at different beamlines or under different conditions can be compared. This is especially true for ligand K‐edge X‐ray absorption spectroscopy, which has well established energy calibration standards for Cl (Cs₂CuCl₄) and S (Na₂S₂O₃·5H₂O), but not neighboring P. This paper presents a review of common P K‐edge XANES energy calibration standards and analysis of PPh₄Br as a potential alternative. The P K‐edge XANES region of commercially available PPh₄Br revealed a single, highly resolved pre‐edge feature with a maximum at 2146.96 eV. PPh₄Br also showed no evidence of photodecomposition when repeatedly scanned over the course of several days. In contrast, we found that PPh₃ rapidly decomposes under identical conditions. Density functional theory calculations performed on PPh₃ and PPh₄⁺ revealed large differences in the molecular orbital energies that were ascribed to differences in the phosphorus oxidation state (III versus V) and molecular charge (neutral versus +1). Time‐dependent density functional theory calculations corroborated the experimental data and allowed the spectral features to be assigned. The first pre‐edge feature in the P K‐edge XANES spectrum of PPh₄Br was assigned to P 1s → P‐C π* transitions, whereas those at higher energy were P 1s → P‐C σ*. Overall, the analysis suggests that PPh₄Br is an excellent alternative to other solid energy calibration standards commonly used in P K‐edge XANES experiments.     

Extended X‐ray absorption fine structure and multiple‐scattering simulation of nickel dithiolene complexes Ni[S2C2(CF3)2]2n (n = −2, −1, 0) and an olefin adduct Ni[S2C2(CF3)2]2(1‐hexene)

A series of Ni dithiolene complexes Ni[S₂C₂(CF₃)]₂ⁿ (n = ?2, ?1, 0) ( 1 , 2 , 3 ) and a 1‐hexene adduct Ni[S₂C₂(CF₃)₂]₂(C₆H₁₂) ( 4 ) have been examined by Ni K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine‐structure (EXAFS) spectroscopies. Ni XANES for 1 – 3 reveals clear pre‐edge features and approximately +0.7 eV shift in the Ni K‐edge position for `one‐electron' oxidation. EXAFS simulation shows that the Ni—S bond distances for 1 , 2 and 3 (2.11–2.16 Å) are within the typical values for square planar complexes and decrease by ～0.022 Å for each `one‐electron' oxidation. The changes in Ni K‐edge energy positions and Ni—S distances are consistent with the `non‐innocent' character of the dithiolene ligand. The Ni—C interactions at ～3.0 Å are analyzed and the multiple‐scattering parameters are also determined, leading to a better simulation for the overall EXAFS spectra. The 1‐hexene adduct 4 presents no pre‐edge feature, and its Ni K‐edge position shifts by ?0.8 eV in comparison with its starting dithiolene complex 3 . Consistently, EXAFS also showed that the Ni—S distances in 4 elongate by ～0.046 Å in comparison with 3 . The evidence confirms that the neutral complex is `reduced' upon addition of olefin, presumably by olefin donating the π‐electron density to the LUMO of 3 as suggested by UV/visible spectroscopy in the literature.     

Quantitative local structure determination in mica crystals: ab initio simulations of polarization XANES at the potassium K‐edge

An attempt to refine the local structure of a layered structure such as mica is made by combining angle‐resolved XANES (AXANES) and single‐crystal X‐ray diffraction (SC‐XRD) experiments. Ab initio calculations of AXANES spectra of several tri‐octahedral micas have been used to further interpolate experimental data and to deduce physico/chemical effects. Structural distortions have been found highly correlated with the compositional disordering that arises from electronic interactions between anions and cations, and extend the interlayer entering deep into nearby tetrahedral and octahedral sheets. Multiple occupations at the same atomic site have been investigated in detail both in the parallel and perpendicular components of AXANES spectra. Finally, the best fit obtained, computed in the framework of the multiple‐scattering theory, is presented and the limitations of the muffin‐tin potential in layered systems are briefly discussed.     

Magnesium K‐edge XANES spectroscopy of geological standards

Magnesium K‐edge X‐ray absorption near‐edge structure (XANES) spectra have been investigated to develop a systematic understanding of a suite of Mg‐bearing geological materials such as silicate and carbonate minerals, sediments, rocks and chemical reagents. For the model compounds the Mg XANES was found to vary widely between compounds and to provide a fingerprint for the form of Mg involved in geologic materials. The energy positions and resonance features obtained from these spectra can be used to specify the dominant molecular host site of Mg, thus shedding light on Mg partitioning and isotope fractionation in geologic materials and providing a valuable complement to existing knowledge of Mg geochemistry.     

Iron speciation in ancient Attic pottery pigments: a non‐destructive SR‐XAS investigation

The present work reports a detailed investigation on the speciation of iron in the pigments of decorated pottery fragments of cultural heritage relevance. The fragments come from the Gioiosa Guardia archaeological site in the area of the `Strait of Messina' (Sicily, Southern Italy), and date back to VI–V century BC. The purpose of this study is to characterize the main pigmenting agents responsible for the dark‐red coloration of the specimens using non‐destructive analytical techniques such as synchrotron radiation X‐ray absorption spectroscopy (SR‐XAS), a well established technique for cultural heritage and environmental subjects. Absorption spectra were collected at the Fe K‐edge on the Italian beamline for absorption and diffraction (BM8‐GILDA) at the European Synchrotron Radiation Facility in Grenoble (France). In order to determine the speciation of Fe in the samples, principal component analysis and least‐squares fitting procedures were applied to the near‐edge part of the absorption spectra (XANES). Details on the local structure around the Fe sites were obtained by analyzing the extended part of the spectra (EXAFS). Furthermore, an accurate determination of the average Fe oxidation state was carried out through analysis of the pre‐edge peaks of the absorption spectra. Samples resulted composed of an admixture of Fe₂O₃ (hematite or maghemite) and magnetite (Fe₃O₄), occurring in different relative abundance in the dark‐ and light‐colored areas of the specimens. The results obtained are complementary to information previously obtained by means of instrumental neutron activation analysis, Fourier transform infrared absorbance and time‐of‐flight neutron diffraction.     

Ion‐ and positron‐acoustic solitons in magnetized dusty plasma with q‐non‐extensive electron and positron velocity distribution

In this study, the properties of ion‐ and positron‐acoustic solitons are investigated in a magnetized multi‐component plasma system consisting of warm fluid ions, warm fluid positrons, q‐non‐extensive distributed positrons, q‐non‐extensive distributed electrons, and immobile dust particles. To drive the Korteweg–de Vries (KdV) equation, the reductive perturbation method is used. The effects of the ratio of the density of positrons to ions, the temperature of the positrons, and ions to electrons, the non‐extensivity parameters q_e and q_p , and the angle of the propagation of the wave with the magnetic field on the potential of ion‐ and positron‐acoustic solitons are also studied. The present investigation is applicable to solitons in fusion plasmas in the edge of tokamak.     

Monitoring synchrotron X‐ray‐induced radiolysis effects on metal (Fe,W) ions in high‐temperature aqueous fluids

Radiolysis‐induced effects on aqueous tungsten ions are observed to form a precipitate within seconds upon exposure to a synchrotron X‐ray micro‐beam in a WO₃ + H₂O system at 873 K and 200 MPa. In situ Fe K‐edge energy‐dispersive X‐ray absorption spectroscopy (ED‐XAS) measurements were made on Fe(II)Cl₂ aqueous solutions to 773 K in order to study the kinetics of high‐temperature reactions of Fe²⁺ and Fe³⁺ ions with transient radiolysis species. The radiolytic reactions in a fluid sample within a hydrothermal diamond anvil cell result in oxidation of the Fe²⁺ ion at 573 K and reduction of Fe³⁺ at temperatures between 673 and 773 K and of the Fe²⁺ ion at 773 K. The edge‐energy drift evident in the ED‐XAS data directly reflects the kinetics of reactions resulting in oxidation and/or reduction of the Fe²⁺ and Fe³⁺ ions in the aqueous solutions at high temperatures. The oxidation and reduction trends are found to be highly consistent, making reliable determinations of reaction kinetics possible.     

Origin of the X‐ray magnetic circular dichroism at the L‐edges of the rare‐earths in RxR1−x′Al2 systems

An X‐ray magnetic circular dichroism (XMCD) study performed at the rare‐earth L_2,3‐edges in the R_xR_1?x′Al₂ compounds is presented. It is shown that both R and R′ atoms contribute to the XMCD recorded at the L‐edges of the selected rare‐earth, either R or R′. The amplitude of the XMCD signal is not directly correlated to the magnetization or to the value of the individual (R, R′) magnetic moments, but it is related to the molecular field acting on the rare‐earth tuned in the photoabsorption process. This result closes a longstanding study of the origin of the XMCD at the L‐edge of the rare‐earths in multi‐component systems, allowing a full understanding of the exact nature of these signals.     

XAFS study of aqua (diethylenetriamine)(isonicotinato)‐copper(II) complex – inference of square‐pyramidal geometry

X‐ray absorption fine structure (XAFS) of aqua (diethylenetriamine) (isonicotinato) copper(II) complex has been investigated. The crystal structure of the complex is unavailable. The X‐ray absorption near edge structure (XANES) spectrum of the complex has been compared with those of the standard compounds to estimate the coordination geometry and oxidation state of copper. This study indicates that the copper center has +2 oxidation state and that it may be penta or hexa‐coordinated. The spectrum has been further compared with those of the earlier studied complexes having square‐pyramidal geometry. Similarity has been observed between them indicating that the complex should have penta‐coordinated square‐pyramidal geometry. The same inference has been obtained from the study of the characteristic pre‐edge and XANES features. This inference has been further examined by analysis of the extended X‐ray absorption fine structure (EXAFS) spectrum. The experimental EXAFS data has been fitted with three different theoretical models for tetra‐coordination, penta‐coordination, and hexa‐coordination. The tetra model is unable to provide a satisfactory fit so it has been ignored. Although the penta and hexa models give comparable results, the parameters obtained indicate that the penta‐coordinated model fits better to the experimental data than the hexa‐coordinated model. This penta‐coordinated model has been further fitted to the experimental data by allowing the coordination number to vary. Such fitting also shows that the penta‐coordinated model is a satisfactory model for the studied complex. The obtained values of mean‐square disorder indicate that the force constants are different for the equatorial and axial bonds. Copyright © 2014 John Wiley & Sons, Ltd.     

Solid‐phase cadmium speciation in soil using L3‐edge XANES spectroscopy with partial least‐squares regression

Cadmium (Cd) has a high toxicity and resolving its speciation in soil is challenging but essential for estimating the environmental risk. In this study partial least‐square (PLS) regression was tested for its capability to deconvolute Cd L₃‐edge X‐ray absorption near‐edge structure (XANES) spectra of multi‐compound mixtures. For this, a library of Cd reference compound spectra and a spectrum of a soil sample were acquired. A good coefficient of determination (R²) of Cd compounds in mixtures was obtained for the PLS model using binary and ternary mixtures of various Cd reference compounds proving the validity of this approach. In order to describe complex systems like soil, multi‐compound mixtures of a variety of Cd compounds must be included in the PLS model. The obtained PLS regression model was then applied to a highly Cd‐contaminated soil revealing Cd₃(PO₄)₂ (36.1%), Cd(NO₃)₂·4H₂O (24.5%), Cd(OH)₂ (21.7%), CdCO₃ (17.1%) and CdCl₂ (0.4%). These preliminary results proved that PLS regression is a promising approach for a direct determination of Cd speciation in the solid phase of a soil sample.     

Soft X‐ray absorption spectroscopy and resonant inelastic X‐ray scattering spectroscopy below 100 eV: probing first‐row transition‐metal M‐edges in chemical complexes

X‐ray absorption and scattering spectroscopies involving the 3d transition‐metal K‐ and L‐edges have a long history in studying inorganic and bioinorganic molecules. However, there have been very few studies using the M‐edges, which are below 100 eV. Synchrotron‐based X‐ray sources can have higher energy resolution at M‐edges. M‐edge X‐ray absorption spectroscopy (XAS) and resonant inelastic X‐ray scattering (RIXS) could therefore provide complementary information to K‐ and L‐edge spectroscopies. In this study, M_2,3‐edge XAS on several Co, Ni and Cu complexes are measured and their spectral information, such as chemical shifts and covalency effects, are analyzed and discussed. In addition, M_2,3‐edge RIXS on NiO, NiF₂ and two other covalent complexes have been performed and different d–d transition patterns have been observed. Although still preliminary, this work on 3d metal complexes demonstrates the potential to use M‐edge XAS and RIXS on more complicated 3d metal complexes in the future. The potential for using high‐sensitivity and high‐resolution superconducting tunnel junction X‐ray detectors below 100 eV is also illustrated and discussed.     

Improved charge transfer multiplet method to simulate M‐ and L‐edge X‐ray absorption spectra of metal‐centered excited states

Charge transfer multiplet (CTM) theory is a computationally undemanding and highly mature method for simulating the soft X‐ray spectra of first‐row transition metal complexes. However, CTM theory has seldom been applied to the simulation of excited‐state spectra. In this article, the CTM4XAS software package is extended to simulate M_2,3‐ and L_2,3‐edge spectra for the excited states of first‐row transition metals and also interpret CTM eigenfunctions in terms of Russell–Saunders term symbols. These new programs are used to reinterpret the recently reported excited‐state M_2,3‐edge difference spectra of photogenerated ferrocenium cations and to propose alternative assignments for the electronic state of these cations responsible for the spectroscopic features. These new programs were also used to model the L_2,3‐edge spectra of Fe^II compounds during nuclear relaxation following photoinduced spin crossover and to propose spectroscopic signatures for their vibrationally hot states.     

Application of glancing‐emergent‐angle fluorescence for polarized XAFS studies of single crystals

X‐ray absorption fine‐structure (XAFS) data were obtained for the V K‐edge for a series of anisotropic single crystals of (Cr_xV_1–x)₂O₃. The data and the results were compared for the as‐prepared bulk single crystals (measured in fluorescence in two different orientations) and those ground to powder (measured in transmission). For the bulk single crystals, the glancing‐emergent‐angle (GEA) method was used to minimize fluorescence distortion. The reliability of the GEA technique was tested by comparing the polarization‐weighted single‐crystal XAFS data with the experimental powder data. These data were found to be in excellent agreement throughout the entire energy range. Thus, it was possible to reliably measure individual V–V contributions parallel and perpendicular to the c axis of the single crystals, i.e. those unavailable by powder data XAFS analysis. These experiments demonstrate that GEA is a premiere method for non‐destructive high‐photon‐count in situ studies of local structure in bulk single crystals.     

Beam‐induced redox transformation of arsenic during As K‐edge XAS measurements: availability of reducing or oxidizing agents and As speciation

During X‐ray absorption spectroscopy (XAS) measurements of arsenic (As), beam‐induced redox transformation is often observed. In this study, the As species immobilized by poorly crystallized mackinawite (FeS) was assessed for the susceptibility to beam‐induced redox reactions as a function of sample properties including the redox state of FeS and the solid‐phase As speciation. The beam‐induced oxidation of reduced As species was found to be mediated by the atmospheric O₂ and the oxidation products of FeS [e.g. Fe(III) (oxyhydr)oxides and intermediate sulfurs]. Regardless of the redox state of FeS, both arsenic sulfide and surface‐complexed As(III) readily underwent the photo‐oxidation upon exposure to the atmospheric O₂ during XAS measurements. With strict O₂ exclusion, however, both As(0) and arsenic sulfide were less prone to the photo‐oxidation by Fe(III) (oxyhydr)oxides than NaAsO₂ and/or surface‐complexed As(III). In case of unaerated As(V)‐reacted FeS samples, surface‐complexed As(V) was photocatalytically reduced during XAS measurements, but arsenic sulfide did not undergo the photo‐reduction.