Kα X-ray Emission Spectroscopy Research Articles

Multicolor single-analyzer high-energy-resolution XES spectrometer for simultaneous examination of different elements.

The present work demonstrates the performance of a von Hámos high-energy-resolution X-ray spectrometer based on a non-conventional conical Si single-crystal analyzer. The analyzer is tested with different primary and secondary X-ray sources as well as a hard X-ray sensitive CCD camera. The spectrometer setup is also characterized with ray-tracing simulations. Both experimental and simulated results affirm that the conical spectrometer can efficiently detect and resolve the two pairs of two elements (Ni and Cu) Kα X-ray emission spectroscopy (XES) peaks simultaneously, requiring a less than 2 cm-wide array on a single position-sensitive detector. The possible applications of this simple yet broad-energy-spectrum crystal spectrometer range from quickly adapting it as another probe for complex experiments at synchrotron beamlines to analyzing X-ray emission from plasma generated by ultrashort laser pulses at modern laser facilities.

Chemical Sensitivity of Kβ and Kα X-ray Emission from a Systematic Investigation of Iron Compounds.

K-fluorescence X-ray emission spectroscopy (XES) is receiving growing interest in all fields of natural sciences to investigate the local spin. The spin sensitivity in Kβ (Kα) XES stems from the exchange interaction between the unpaired 3p (2p) and the 3d electrons, which is greater for Kβ than for Kα. We present a thorough investigation of a large number of iron-bearing compounds. The experimental spectra were analyzed in terms of commonly used quantitative parameters (Kβ1,3-first moment, Kα1-full width at half-maximum, and integrated absolute difference -IAD-), and we carefully examined the difference spectra. Multiplet calculations were also performed to elucidate the underlying mechanisms that lead to the chemical sensitivity. Our results confirm a strong influence of covalency on both Kβ and Kα lines. We establish a reliable spin sensitivity of Kβ XES as it is dominated by the exchange interaction, whose variations can be quantified by either Kβ1,3-first moment or Kβ-IAD and result in a systematic difference signal line shape. We find an exception in the Kβ XES of Fe3+ and Fe2+ in water solution, where a new difference spectrum is identified that cannot be reproduced by scaling the exchange integrals. We explain this by strong differences in orbital mixing between the valence orbitals. This result calls for caution in the interpretation of Kβ XES spectral changes as due to spin variations without a careful analysis of the line shape. For Kα XES, the smaller exchange interaction and the influence of other electron-electron interactions make it difficult to extract a quantity that directly relates to the spin.

Probing Sulfur Chemical and Electronic Structure with Experimental Observation and Quantitative Theoretical Prediction of Kα and Valence-to-Core Kβ X-ray Emission Spectroscopy.

An extensive experimental and theoretical study of the Kα and Kβ high-resolution X-ray emission spectroscopy (XES) of sulfur-bearing systems is presented. This study encompasses a wide range of organic and inorganic compounds, including numerous experimental spectra from both prior published work and new measurements. Employing a linear-response time-dependent density functional theory (LR-TDDFT) approach, strong quantitative agreement is found in the calculation of energy shifts of the core-to-core Kα as well as the full range of spectral features in the valence-to-core Kβ spectrum. The ability to accurately calculate the sulfur Kα energy shift supports the use of sulfur Kα XES as a bulk-sensitive tool for assessing sulfur speciation. The fine structure of the sulfur Kβ spectrum, in conjunction with the theoretical results, is shown to be sensitive to the local electronic structure including effects of symmetry, ligand type and number, and, in the case of organosulfur compounds, to the nature of the bonded organic moiety. This agreement between theory and experiment, augmented by the potential for high-access XES measurements with the latest generation of laboratory-based spectrometers, demonstrates the possibility of broad analytical use of XES for sulfur and nearby third-row elements. The effective solution of the forward problem, i.e., successful prediction of detailed spectra from known molecular structure, also suggests future use of supervised machine learning approaches to experimental inference, as has seen recent interest for interpretation of X-ray absorption near-edge structure (XANES).

Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering

The non-equilibrium dynamics of electrons and nuclei govern the function of photoactive materials. Disentangling these dynamics remains a critical goal for understanding photoactive materials. Here we investigate the photoinduced dynamics of the [Fe(bmip)2]2+ photosensitizer, where bmip = 2,6-bis(3-methyl-imidazole-1-ylidine)-pyridine, with simultaneous femtosecond-resolution Fe Kα and Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS). This measurement shows temporal oscillations in the XES and XSS difference signals with the same 278 fs period oscillation. These oscillations originate from an Fe-ligand stretching vibrational wavepacket on a triplet metal-centered (3MC) excited state surface. This 3MC state is populated with a 110 fs time constant by 40% of the excited molecules while the rest relax to a 3MLCT excited state. The sensitivity of the Kα XES to molecular structure results from a 0.7% average Fe-ligand bond length shift between the 1 s and 2p core-ionized states surfaces.

Kα X-ray Emission Spectroscopy on the Photosynthetic Oxygen-Evolving Complex Supports Manganese Oxidation and Water Binding in the S3 State.

The unique manganese-calcium catalyst in photosystem II (PSII) is the natural paragon for efficient light-driven water oxidation to yield O2. The oxygen-evolving complex (OEC) in the dark-stable state (S1) comprises a Mn4CaO4 core with five metal-bound water species. Binding and modification of the water molecules that are substrates of the water-oxidation reaction is mechanistically crucial but controversially debated. Two recent crystal structures of the OEC in its highest oxidation state (S3) show either a vacant Mn coordination site or a bound peroxide species. For purified PSII at room temperature, we collected Mn Kα X-ray emission spectra of the S0, S1, S2, and S3 intermediates in the OEC cycle, which were analyzed by comparison to synthetic Mn compounds, spectral simulations, and OEC models from density functional theory. Our results contrast both crystallographic structures. They indicate Mn oxidation in three S-transitions and suggest additional water binding at a previously open Mn coordination site. These findings exclude Mn reduction and render peroxide formation in S3 unlikely.

Sulfur Speciation in Biochars by Very High Resolution Benchtop Kα X-ray Emission Spectroscopy.

The analytical chemistry of sulfur-containing materials poses substantial technical challenges, especially due to the limitations of 33S NMR and the time-intensive preparations required for wet-chemistry analyses. A number of prior studies have found that synchrotron-based X-ray absorption near edge structure (XANES) measurements can give detailed speciation of sulfur chemistry in such cases. However, due to the obvious access limitations, synchrotron XANES of sulfur cannot be part of routine analytical practice across the chemical sciences community. Here, in a study of the sulfur chemistry in biochars, we compare and contrast the chemical inferences available from synchrotron XANES with that given by benchtop, extremely high resolution wavelength-dispersive X-ray fluorescence (WD-XRF) spectroscopy, also often called X-ray emission spectroscopy (XES). While the XANES spectra have higher total information content, often giving differentiation between different moieties having the same oxidation state, the lower sensitivity of the S Kα XES to coordination and local structure provides pragmatic benefit for the more limited goal of quantifying the S oxidation state distribution. Within that constrained metric, we find good agreement between the two methods. As the sulfur concentrations were as low as 150 ppm, these measurements provide proof-of-principle for characterization of the sulfur chemistry of biochars and potential applications to other areas such as soils, batteries, catalysts, and fossil fuels and their combustion products.

The effect of thermal reduction on the photoluminescence and electronic structures of graphene oxides.

Electronic structures of graphene oxide (GO) and hydro-thermally reduced graphene oxides (rGOs) processed at low temperatures (120–180°C) were studied using X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). C K-edge XANES spectra of rGOs reveal that thermal reduction restores C = C sp2 bonds and removes some of the oxygen and hydroxyl groups of GO, which initiates the evolution of carbonaceous species. The combination of C K-edge XANES and Kα XES spectra shows that the overlapping π and π* orbitals in rGOs and GO are similar to that of highly ordered pyrolytic graphite (HOPG), which has no band-gap. C Kα RIXS spectra provide evidence that thermal reduction changes the density of states (DOSs) that is generated in the π-region and/or in the gap between the π and π* levels of the GO and rGOs. Two-dimensional C Kα RIXS mapping of the heavy reduction of rGOs further confirms that the residual oxygen and/or oxygen-containing functional groups modify the π and σ features, which are dispersed by the photon excitation energy. The dispersion behavior near the K point is approximately linear and differs from the parabolic-like dispersion observed in HOPG.

Energy band structure and X-ray spectra of phenakite Be2SiO4

The electronic structure of crystalline phenakite Be2SiO4 is investigated using x-ray emission spectroscopy (XES) (Be K α XES, Si L 2, 3 XES, O K α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be2SiO4 compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be2SiO4 compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m e ) and holes (3.0m e ) for the Be2SiO4) compound are estimated.

X-ray absorption and emission spectroscopy of ZnO nanoparticle and highly oriented ZnO microrod arrays

The electronic structures of ZnO nanoparticles and microrod arrays are studied by O 1s X-ray absorption spectroscopy (XAS) and O Kα X-ray emission spectroscopy (XES). We show that the present LDA+USIC calculation approach is suitable to correct the LDA self-interaction error of the cation d-states. The atomic eigenstates of 3d in zinc and 2p in oxygen are energetically close, which induces strong Zn-3d-O-2p hybridization. This anomalous valence band cation-d-anion-p hybridization is affected when the localization of the Zn 3d-states is taken into account. Experimentally, the XES spectra show energy dependence in the spectral shape revealing selected excitations to the Zn 3d, 4s and 4p states, hybridized with O 2p states. Strong anisotropic effects are observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles. The nanostructured ZnO has primarily bulk XAS and XES properties.