High-resolution X-ray Emission Research Articles

Towards a comprehensive X-ray approach for studying the photosynthetic manganese complex–XANES, Kα/Kβ/Kβ-satellite emission lines, RIXS, and comparative computational approaches for selected model complexes

Advanced X-ray spectroscopy experiments can contribute to elucidation of the mechanism of water oxidation in biological (tetra-manganese complex of Photosystem II) and artificial systems. Although the electronic structure of the catalytic metal site is of high interest, it is experimentally not easily accessible. Therefore, we and other researchers are working towards a comprehensive approach involving a combination of methods, namely (1) quantitative analysis of X-ray absorption near-edge structure (XANES) spectra collected at the K-edge and, in the long run, at the L-edge of manganese; (2) high-resolution X-ray emission spectroscopy (XES) of Kα and Kβ lines, (3) two-dimensional resonant inelastic X-ray scattering (RIXS) spectra. Collection of these spectroscopic data sets requires state-of-the-art synchrotron radiation facilities as well as experimental strategies to minimize the radiation-induced modifications of the samples. Data analysis requires the use and development of appropriate theoretical tools. Here, we present exemplary data collected for three multi-nuclear synthetic Mn complexes with the Mn ions in the oxidation states II, III, and IV, and for MnVII of the permanganate ion. Emission spectra are calculated for the MnVII ion using both multiple-scattering (MS) approach and time-dependent density functional theory (TDDFT).

Electronic Structure of Sulfur Studied by X-ray Absorption and Emission Spectroscopy

An X-ray spectroscopy and theoretical study of the chemical state of several sulfur bearing minerals and a synthetic sodium sulfite sample was performed. X-ray absorption and high-resolution Kα X-ray emission spectra were recorded and compared to ab initio quantum chemical calculations. A consistent interpretation of the chemical shift in the Kα emission spectra is obtained based on three different theoretical approaches (density functional theory, multiple scattering theory, and atomic multiplet theory). An analysis of the theoretical sulfur orbital population and valence bond is in agreement with the fluorescence energy position of the Kα lines even within the sulfide (S2−) series. It is shown that the Kα energy shifts can be used for a quantitative determination of the proportion of different sulfur species in heterogeneous samples.

Root uptake and reduction of hexavalent chromium by aquatic macrophytes as assessed by high-resolution X-ray emission

Aquatic macrophytes Salvinia auriculata, Pistia stratiotes and Eichhornia crassipes were chosen to investigate the Cr(VI) reduced by root-based biosorption in a chromium uptake experiment, using a high-resolution XRF technique. These plants were grown in hydroponics medium supplied with non-toxic Cr concentrations during a 27-day metal uptake experiment. The high-resolution Cr-Kβ fluorescence spectra for dried root tissues and Cr reference material (100% Cr, Cr 2O 3, and CrO 3) were measured using an XRF spectrometer. For all species of aquatic plant treated with Cr(VI), the energy of the Cr-Kβ 2,5 line was shifted around 8 eV below the same spectral line identified for the Cr(VI) reference, but it was also near to the line identified for the Cr(III) reference. Moreover, there was a lack of the strong Cr-Kβ″ line assigned to the Cr(VI) reference material within the Cr(VI)-treated plant spectra, suggesting the reduction of Cr(VI) for other less toxic oxidation states of Cr. As all Cr-Kβ spectra of root tissue species were compared, the peak energies and lineshape patterns of the Cr-Kβ 2,5 line are coincident for the same aquatic plant species, when they were treated with Cr(III) and Cr(VI). Based on the experimental evidence, the Cr(VI) reduction process has happened during metal biosorption by these plants.

Evaluation of High-Resolution X-ray Absorption and Emission Spectroscopy for the Chemical Speciation of Binary Titanium Compounds

For the chemical speciation of binary compounds of tri- and tetravalent titanium, high-resolution X-ray absorption and emission spectra were recorded in different energy regimes in order to evaluate and to qualify both near-edge X-ray absorption fine structure (NEXAFS or XANES) spectroscopy and wavelength-dispersive X-ray emission spectroscopy (WDXES) as spectroscopic methods for this analytical task. A high resolving power in the excitation channel was ensured by use of monochromatic synchrotron radiation provided by BESSY II, where the soft X-ray emission spectra were recorded as well. In the hard X-ray range, emission measurements were performed at SPring-8. For a comparison of the information gained from the various methods, the titanium compounds were classified according to the bonded titanium's oxidation state. Thus, it was possible to distinguish between inner atomic effects due to different oxidation states and external effects related to the respective ligand and the surrounding structure. It becomes evident, that certain compounds, while hardly distinguishable in their Ti-K XANES spectra, still show significant differences in their emission characteristics. On the other hand, some compounds with little difference in their emission spectra are easily distinguished by their NEXAFS structures. Only the combined use of the complementary methods both in the soft and the hard X-ray range allows for a reliable speciation of tri- and tetravalent titanium compounds.

High-spin-low-spin transition and the sequence of the phase transformations in the BiFeO3 crystal at high pressures

The transition of Fe3+ ions from the high-spin (HS) state (S = 5/2) to the low-spin (LS) state (S = 1/2) has been observed in the BiFeO3 multiferroic crystal at high pressures in the range 45–55 GPa. This effect has been studied in high-pressure diamond-anvil cells by means of two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS or synchrotron Mossbauer spectroscopy) and FeKβ high-resolution X-ray emission spectroscopy (XES). The HS-LS transition correlates with anomalies in the magnetic, optical, transport, and structural properties of the crystal. At room temperature, the transition is not stepwise, but is extended in a pressure range of about 10 GPa due to thermal fluctuations between the high-spin and low-spin states. It has been found that the transition of the BiFeO3 insulator to the metal occurs only in the low-spin phase and the cause of all phase transitions is the HS-LS crossover.

A short working distance multiple crystal x-ray spectrometer

For x-ray spot sizes of a few tens of microns or smaller, a millimeter-sized flat analyzer crystal placed approximately 1 cm from the sample will exhibit high energy resolution while subtending a collection solid angle comparable to that of a typical spherically bent crystal analyzer (SBCA) at much larger working distances. Based on this observation and a nonfocusing geometry for the analyzer optic, we have constructed and tested a short working distance (SWD) multicrystal x-ray spectrometer. This prototype instrument has a maximum effective collection solid angle of 0.14 sr, comparable to that of 17 SBCA at 1 m working distance. We find good agreement with prior work for measurements of the Mn Kbeta x-ray emission and resonant inelastic x-ray scattering for MnO, and also for measurements of the x-ray absorption near-edge structure for Dy metal using Lalpha(2) partial-fluorescence yield detection. We discuss future applications at third- and fourth-generation light sources. For concentrated samples, the extremely large collection angle of SWD spectrometers will permit collection of high-resolution x-ray emission spectra with a single pulse of the Linac Coherent Light Source. The range of applications of SWD spectrometers and traditional multi-SBCA instruments has some overlap, but also is significantly complementary.

Intrashell Electron-Interaction-Mediated Photoformation of Hollow Atoms near Threshold

Double photoionization (DPI) of an atom by a single photon is a direct consequence of electron-electron interactions within the atom. We have measured the evolution of the K-shell DPI from threshold up in transition metals by high-resolution x-ray emission spectroscopy of the Kh alpha hypersatellites, photoexcited by monochromatized synchrotron radiation. The measured evolution of the single-to-double photoionization cross-section ratio with excitation energy was found to be universal. Theoretical fits suggest that near threshold DPI is predominantly a semiclassical knockout effect, rather than the purely quantum-mechanical shake-off observed at the infinite photon energy limit.

High-resolution X-ray emission Mα and Mβ spectra of Bi atoms

The structure and relative intensity of x-ray fluorescence Mα and Mβ spectra of Bi atoms have been studied experimentally under excitation by Cr and Cu Kα1,2 radiation. It has been found that observed short-wavelength satellites are mostly caused by the radiative transitions M 5 N-N 6,7 N (M 5 N satellites) and M 4 N-N 6 N (M 4 N satellites), whereas the transitions M 5 O-N 6,7 O and M 4 O-N 6 O are overlapped with the profiles of x-ray diagram Mα1,2 and Mβ lines and are not observed as individual structures. The M 5 N and M 4 N satellites have been separated from total spectral profiles, and the relative intensities of such groups of lines have been determined. A model has been proposed for calculation of emission cross sections of the M 4 N and M 4 N satellites, as well as the Mα1,2 lines along with the M 5 O satellites and the Mβ line with the M 4 O satellites. The model takes into account main channels of generation and migration of vacancies in M subshells connected with the Coster-Kronig transitions M i -M j N and M i -M j O, radiative transitions M i -M j and M i -N j , and shaking processes upon production of the M i vacancy, as well as with cascades of such processes. Comparison of experimental values of the relative intensity of the separated satellites with calculated results indicates the correctness of the model used.

High resolution X-ray emission spectroscopy of liquid water: The observation of two structural motifs

The structure of liquid water is presently under intense debate. We summarize the historical development of the present description of water and the controversy surrounding the X-ray absorption spectroscopy (XAS) study of Wernet et al. [Ph. Wernet et al., Science 304 (2004) 995]. In order to provide further insight into the connection between X-ray spectroscopy and the structure of the hydrogen bonding network, we report new data comprising high resolution oxygen K-edge X-ray emission spectra (XES) of liquid water. We observe two distinct narrow lone-pair derived peaks, assigned, respectively, to tetrahedral and strongly distorted hydrogen-bonded species; the assignment is based on comparison with ice and gas phase spectra, temperature dependent measurements, excitation energy dependence and theoretical simulations. We estimate a 2:1 ratio between distorted and tetrahedral species at room temperature and discuss what this implies in connection to the previous XAS study and in terms of liquid water structure.

Evolution of charge and spin state of transition metals in the LaMn1−xCoxO3 perovskite series

Charge and spin state evolution of the transition metals in the LaMn1−xCoxO3 perovskite series was studied by means of high resolution x-ray absorption and emission spectroscopy. The gradual charge transfer from Mn to Co sites was observed upon increasing Co doping. The high spin of the Mn3+∕4+ and Co2+ ions is preserved in the entire solid solution. Co3+ ions reveal low spin state at low doping, while the gradual increase of the average Co3+ spin state is observed at higher cobalt concentrations. The results plausibly explain the observed magnetization anomalies.

Experiment investigation of La 1− xSr xMnO 3 by high-resolution X-Ray emission and spin-polarized X-ray absorption spectroscopy

Big changes in resistivity along with the changing of local structure in some oxide systems, such as high-temperature superconductors and colossal magnetoresistance system, strongly suggest the need of a systematic investigation of their local electronic and atomic structures. In this work we present the high-resolution X-ray emission spectra and the spin-polarized X-ray absorption near-edge spectroscopy (SPXANES) data at the Mn K-edge in the La 1− x Sr x MnO 3. This experiment is based on a high-resolution large-acceptance crystal analyzer based on Si (1 1 1) and optimized for X-ray fluorescence spectroscopy. With a spherical bent crystal monochromator, a Mn Kβ emission spectra with high resolution was obtained with a short collection time and SPXANES spectra of La 1− x Sr x MnO 3 at room temperature were also measured at high temperature.

Isotope and Temperature Effects in Liquid Water Probed by X-Ray Absorption and Resonant X-Ray Emission Spectroscopy

High-resolution x-ray absorption and emission spectra of liquid water exhibit a strong isotope effect. Further, the emission spectra show a splitting of the 1b1 emission line, a weak temperature effect, and a pronounced excitation-energy dependence. They can be described as a superposition of two independent contributions. By comparing with gas phase, ice, and NaOH/NaOD, we propose that the two components are governed by the initial state hydrogen bonding configuration and ultrafast dissociation on the time scale of the O 1s core hole decay.

Erratum: “Development of a high-efficiency high-resolution particle-induced x-ray emission system for chemical-state analysis of environmental samples” [Rev. Sci. Instrum. 78, 073105 (2007)

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Development of a high-efficiency high-resolution particle-induced x-ray emission system for chemical state analysis of environmental samples

We have developed a high-efficiency high-resolution particle-induced x-ray emission (PIXE) system employing a von Hamos-type crystal spectrometer for a chemical state identification of trace elements in environmental samples. The energy resolution of the system was determined to be about 0.05% through the observation of Si Kalpha(1,2) x rays (1.74 keV) from elemental silicon. The throughput efficiency of the system was also evaluated quasitheoretically to be 1.6x10(-7) counts/incident proton for Si Kalpha(1,2) emission. To demonstrate a chemical state analysis using the high-resolution PIXE system, Si Kalpha(1,2) and Kbeta x-ray spectra for SiC, Si(3)N(4), and SiO(2) were measured and compared. The observed chemical shifts of the Si Kalpha(1,2) peaks for SiC, Si(3)N(4), and SiO(2) relative to elemental silicon were 0.20, 0.40, and 0.55 eV, respectively. The tendency of these shifts were well explained by the effective charges of the silicon atoms calculated by a molecular orbital method.

High resolution X-ray emission spectra from picosecond laser irradiated Ge targets

Investigations of a high resolution X-ray emission spectrum in the range 0.66–0.75 nm obtained by irradiating a Germanium target with high-power p-polarized, 40 picosecond laser radiation at 532 nm wavelength was done. Spectra in the wavelength region of 2l-4l′ and 2l-5l′ L-shell transitions in F-like, Ne-like and Na-like germanium ions were recorded using the FSSR-2D spectrometer equipped with a spherically bent quartz crystal with a spectral resolution λ/Δλ better than 5000. Spectral lines were compared with theoretical values obtained using the LANL plasma kinetic code ATOMIC. Fair agreement between experimental and theoretical spectral lines has been observed, which allowed to measure enough high bulk electron temperature values of 560 eV and electron density of ∼1021 cm−3 in Ge plasma irradiated by rather small commercial high repetition rate Nd:YAG laser system.

Pressure-induced electron spin transition in the paramagnetic phase of the GdFe3(BO3)4 Heisenberg magnet

The HS → LS spin crossover effect (high-spin → low-spin transition) induced by high pressure in the range 45–53 GPa is observed in trivalent Fe3+ ions in the paramagnetic phase of a Gd57Fe3(BO3)4 gadolinium iron borate crystal. This effect is studied in high-pressure diamond-anvil cells by two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS) and Fe K β high-resolution x-ray emission spectroscopy (XES). The manifestation of the crossover in the paramagnetic phase, which has no order parameter to distinguish between the HS and LS states, correlates with the optical-gap jump and with the insulator-semiconductor transition in the crystal. Based on a theoretical many-electron model, an explanation of this effect at high pressures is proposed.

Electronic Origins of Structural Distortions in Post-Transition Metal Oxides: Experimental and Theoretical Evidence for a Revision of the Lone Pair Model

Structural distortions in post-transition metal oxides are often explained in terms of the influence of sp hybrid "lone pairs." Evidence is presented here showing that this model must be revised. The electronic structures of prototypically distorted alpha-PbO and alpha-Bi2O3 have been measured by high-resolution x-ray photoemission and soft x-ray emission spectroscopies. In contrast with the expectations of the lone pair model, a high density of metal 6s states is observed at the bottom of the valence band. The measurements are consistent with the results of density functional theory calculations.

High-resolution soft X-ray emission spectrograph at advanced light source

Abstract The progress of valence electronic spectroscopy with X-rays has been largely limited by the challenge of fine instrumentation. Relatively weak scattering cross-section and/or limited (coarse) energy resolution restrict X-ray spectroscopy to address fundamental issues in condensed-matter electron physics. Depending on the nature of excitations, high brightness photon beam from third-generation synchrotron facilities helps raise the count rates to a detectable limit over the noise level. Over the past decade, it has been realized that high-resolution resonant inelastic soft X-ray scattering has the potential to play an important role in understanding complex phenomena observed in highly correlated systems. Driven by such demand, we have developed a soft X-ray emission spectrograph based on variable line spacing (VLS) gratings to work in the soft X-ray and deep UV (the M edge of transition metals) regime. The slit-less design coupled with high quality optical elements and a high quantum efficiency (QE) in-vacuum CCD detector greatly improves the overall throughput. The M edge spectrograph has been demonstrated to have a resolving power better than 10,000 and will be used in conjunction with meV-resolution beamline (MERLIN) at the Advanced Light Source at Lawrence Berkeley National Laboratory.

Momentum selectivity and anisotropy effects in the nitrogenK-edge resonant inelastic x-ray scattering from GaN

High-resolution soft x-ray emission and absorption spectra near the N $K$-edge of wurtzite GaN are presented. The experimental data are interpreted in terms of full-potential electronic structure calculations. The absorption spectra, compared with calculations including core hole screening, indicate partial core hole screening in the absorption process. The resonantly excited x-ray emission spectra show pronounced dispersion of spectral structures, which is attributed to effects of momentum conservation in the resonant inelastic x-ray scattering (RIXS) process. In development of GaN based optoelectronics, momentum selectivity in RIXS can be utilized to control development of band structure in GaN nanostructures.

High‐Resolution X‐Ray Emission Spectroscopy of Molybdenum Compounds.

High-resolution molybdenum K-edge X-ray emission spectroscopy (XES) was used to characterize the Kβ4 and Kβ‘ ‘ valence-to-core transition bands in the oxo-Mo compounds K2MoO4, MoO(S2CNEt2)2, and MoO2(S2CNEt2)2. The Kβ4 and Kβ‘ ‘ emission bands are attributed to transitions to the Mo 1s core hole from molecular orbitals possessing primarily molybdenum 4d and oxygen 2s character, respectively. This communication describes the first assignment of the Kβ‘ ‘ interatomic band in the emission spectra of molybdenum complexes. Additionally, the Kβ4 and Kβ‘ ‘ transitions are shown to be sensitive to the chemical and electronic environment of the metal, suggesting that high-resolution XES might be an effective method for elucidating the nature of the molybdenum centers in biological systems.