High-resolution X-ray Emission Spectroscopy Research Articles

Magnetic microcalorimeter with paramagnetic temperature sensors and integrated dc-SQUID readout for high-resolution x-ray emission spectroscopy

We present two variants of a magnetic microcalorimeter with paramagnetic temperature sensors and integrated dc-superconducting quantum interference device readout for high-resolution x-ray emission spectroscopy. Each variant employs two overhanging gold absorbers with a sensitive area of 150 × 150 μm2 and a thickness of 3 μm, thus providing a thickness related quantum efficiency of >98% for photons up to 5 keV and >50% for photons up to 10 keV. The first variant operated nominally but suffered from Joule power dissipation of the Josephson junction shunt resistors, athermal phonon loss, and slew rate limitations of the overall setup. Overall, it only achieved an energy resolution of ΔEFWHM=8.9 eV for 5.9 keV photons. In the second variant, we introduced an innovative tetrapod absorber geometry as well as a membrane technique for diverting dissipated heat away from the temperature sensors. When all mitigations are applied optimally, the second variant achieves an energy resolution of ΔEFWHM=1.25(18) eV for 5.9 keV photons and hence provides the present best energy resolving power E/ΔEFWHM among all existing energy-dispersive detectors for soft and tender x-rays.

Hydrogen-Bonded Structures of Water Molecules in Hydroxy-Functionalized Nanochannels of Columnar Liquid Crystalline Nanostructured Membranes Studied by Soft X-ray Emission Spectroscopy.

Here, we report a synchrotron-based high-resolution soft X-ray emission spectroscopy study on hydrogen-bonded structures of water molecules in the self-organized, hydroxy-group-functionalized one-dimensional nanochannels of liquid crystalline nanostructured membranes. The water molecules confined in the uncharged hydroxy-functionalized nanochannels (which have a diameter of about 1.5 nm) exhibit hydrogen-bonded structures close to those of bulk liquid water, even directly interacting with diol groups. These hydrogen-bonded structures contrast with the more distorted hydrogen bonding of water molecules confined in self-organized channels with a diameter of 0.6 nm formed by an analogous nanostructured membrane with a cationic moiety, which was explained by the ability of the channel functional groups to donate and accept hydrogen bonds in a confined space and the nanochannel diameter.

High-Resolution X-ray Absorption and Emission Spectroscopy for Detailed Analysis of New CO2 Methanation Catalysts.

A new approach for the characterization of CO2 methanation catalysts prepared by thermal decomposition of a nickel MOF by hard X-ray photon-in/photon-out spectroscopy in form of high energy resolution fluorescence detected X-ray absorption near edge structure spectroscopy (HERFD-XANES) and valence-to-core X-ray emission (VtC-XES) is presented. In contrast to conventional X-ray absorption spectroscopy, the increased resolution of both methods allows a more precise phase determination of the final catalyst, which is influenced by the conditions during MOF decomposition.

Laser energy absorption and x-ray generation in nanowire arrays irradiated by relativistically intense ultra-high contrast femtosecond laser pulses

We report here on the results of comparative experimental measurements of laser energy absorption in a bulk and different morphology nanowire arrays interacting with relativistically intense, ultra-high temporal contrast femtosecond laser pulses. We compare polished, flat bulk samples with vertically and randomly oriented nanowires made of ZnO semiconductor material. The optical absorption of the 45° incident laser pulses of ∼40 fs duration with a central wavelength of 400 nm at intensities above 1019Wcm2 was determined using an integrating Ulbricht sphere. We demonstrate an almost twofold enhancement of absorption in both nanowire morphologies with an average of (79.6±1.9)% in comparison to the flat bulk sample of (45.8±1.9)%. The observed substantially enhanced absorption in nanowire arrays is also confirmed by high-resolution x-ray emission spectroscopy. The spectral analysis of the K-shell x-ray emission lines revealed that the He-like resonance line emission from highly ionized Zn (Zn28+) is only present in the case of nanowire arrays, whereas, for the flat bulk samples, only neutral and low charge states were observed. Our numerical simulations, based on radiative-collisional kinetic code FLYCHK, well reproduce the measured He-like emission spectrum and suggest that high charge state observed in nanowire arrays is due to substantially higher plasma temperature. Our results, which were measured for the first time with femtosecond laser pulses, can be used to benchmark theoretical models and numerical codes for the relativistic interaction of ultrashort laser pulses with nanowires.

Ion Selectivity of Water Molecules in Subnanoporous Liquid-Crystalline Water-Treatment Membranes: A Structural Study of Hydrogen Bonding.

We demonstrate hydrogen‐bonded structures of water in self‐organized subnanoporous water treatment membranes obtained using synchrotron‐based high‐resolution soft X‐ray emission spectroscopy. The ion selectivity of these water treatment membranes is usually understood by the size compatibility of nanochannels in the membrane with the Stokes radius of hydrated ions, or by electrostatic interaction between charges inside the nanochannels and such ions. However, based on a comparison between the hydrogen‐bonded structures of water molecules in the nanochannels of the water treatment membrane and those surrounding the ions, we propose a definite contribution of structural consistency among the associated hydrogen‐bonded water molecules to the ion selectivity. Our observation delivers a novel concept to the design of water treatment membranes where water molecules in the nanochannel can be regarded as a part of the material that controls the ion selectivity.

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).

Multiple ionization X-ray satellites of Ti and its binary oxides in alpha particle PIXE

High energy resolution X-ray emission spectroscopy was used to study the multiple ionization satellites in the K X-ray spectra of titanium induced by target irradiation with MeV alpha particles. Beam energies of 3–5 MeV were used to explore the beam energy dependence. In addition, several titanium binary oxides excited with 5 MeV beam were measured to study also the chemical bonding effects on the multiple ionization satellites. Energy shifts and intensities of the satellites corresponding to one and two L-shell vacancies were determined from the measured spectra. The results will be used to refine the existing experimental alpha particle induced multiple ionization satellite database which is needed to improve the accuracy of PIXE analysis using alpha particle beams, including the PIXE mode of the Mars alpha particle X-ray spectrometers.

X-ray emission spectroscopy optimization for chemical speciation in laboratory

K X-ray emission spectra emitted from carbon, aluminum, silicon, phosphorus, titanium, chromium, and manganese compounds were measured, using a compact lab-scale flat crystal X-ray fluorescence spectrometer. The sensitivity of K X-ray transitions line-shape, relative to the atom's chemical speciation, is considered. Spectrometer's optimization concerning high energy resolution takes place for each measured fluorescence transition. The detected changes on the measured X-ray line-shapes allow extracting the chemical speciation of the emitting atom. In-lab high-resolution X-ray emission spectroscopy is shown to be a highly versatile technique for materials chemical characterization.

Determination of Hexavalent Chromium Fractions in Plastics Using Laboratory-Based, High-Resolution X-ray Emission Spectroscopy.

Cr(VI) is a well-known human carcinogen with many water-soluble moieties. Its presence in both natural and man-made substances poses a risk to public health, especially when contamination of groundwater is possible. This has led the European Union and other jurisdictions to include Cr(VI) in restriction of hazardous substances regulations. However, for several important industrial and commercial purposes, analytical capability to characterize Cr(VI) is known to be insufficient for regulatory purposes. For example, advanced X-ray spectroscopies, particularly synchrotron-based X-ray absorption fine structure (XAFS) studies, have shown that species interconversion and under-extraction can be difficult to prevent in many existing liquid extraction protocols when applied to plastics, mining ores and tailings, and paint sludges. Here, we report that wavelength dispersive X-ray fluorescence spectroscopy taken at energy resolutions close to the theoretical limit imposed by the core-hole lifetime, generally called X-ray emission spectroscopy (XES) in the synchrotron community, can be used in the laboratory setting for noninvasive, analytical characterization of the Cr(VI)/Cr ratio in plastics. The selected samples have been part of ongoing efforts by standards development organizations to create improved Cr(VI) testing protocols, and the present work provides a direct proof-of-principle for the use of such extremely high-resolution laboratory WDXRF as an alternative to liquid extraction methods for regulatory compliance testing of Cr(VI) content. As a practical application of this work, we report a value for the Cr(VI) mass fraction of the new NIST Standard Reference Material 2859 Restricted Elements in Polyvinyl Chloride.

Benchtop, High-Resolution XAFS and Xes Spectrometers As Tools for Electrochemical Research

Advanced x-ray spectroscopies allow the direct and element-specific interrogation of local electronic structure, yet their scientific impact is necessarily constrained by access limitations. This is particularly evident for x-ray absorption fine structure (XAFS) and x-ray emission spectroscopy (XES) techniques. Here, the technical capability afforded by a synchrotron x-ray facility allows researchers to answer basic or applied research questions and push the forefront of science. These national-lab scale sources often provide the only means to perform these techniques, consequently slowing progress on critical projects, and also largely excluding routine analytical characterization. Over the last four years, our group at the University of Washington has been developing several new families of lab-based instruments to expand the accessibility of advanced x-ray spectroscopies. [1-5] It is now possible to identify the electroactive element in a material and assess its oxidation state, spin state, and coordination chemistry – all with benchtop XAFS and XES. The feasibility of these studies is established by comparing performance of our benchtop instruments with various synchrotrons in the context of several electrochemical systems. From the standpoint of materials inquiry, we will present a variety of measurements of lithium ion batteries, valence-to-core XES results from a series of oxides, and preliminary efforts toward solution-phase measurements will be presented. The latter will be done with an eye toward future electrolyte analyses. William M. Holden, et al. “A Compact Dispersive Refocusing Rowland Circle X-ray Emission Spectrometer for Laboratory, Synchrotron and XFEL Applications”, Rev. Sci. Instrum 88, 073904 (2017). [DOI: 10.1063/1.4994739]T. Seidler, et al., “A Laboratory-based Hard X-ray Monochromator for High-Resolution X-ray Emission Spectroscopy and X-ray Absorption Near Edge Structure Measurements,” Review of Scientific Instruments 85, 113906 (2014). [DOI: 10.1063/1.4901599]T. Seidler, et al., “A Modern Laboratory XAFS Cookbook,” Journal of Physics: Conference Series 712, 012015 (2016). [ http://iopscience.iop.org/article/10.1088/1742-6596/712/1/012015]R. Mortensen, et al., “Benchtop Nonresonant X-ray Emission Spectroscopy: Coming Soon to Laboratories and Beamlines Near You,” Journal of Physics: Conference Series 712, 012036. (2016) [http://iopscience.iop.org/article/10.1088/1742-6596/712/1/012036]G.T. Seidler, “The Case for Analytical XAFS”, https://www.linkedin.com/pulse/case-benchtop-analytical-xafs-gerald-seidler

Extracting chemical information from high-resolution Kβ X-ray emission spectroscopy

High-resolution X-ray emission spectroscopy allows studying the chemical environment of a wide variety of materials. Chemical information can be obtained by fitting the X-ray spectra and observing the behavior of some spectral features. Spectral changes can also be quantified by means of statistical parameters calculated by considering the spectrum as a probability distribution. Another possibility is to perform statistical multivariate analysis, such as principal component analysis. In this work the performance of these procedures for extracting chemical information in X-ray emission spectroscopy spectra for mixtures of Mn2+ and Mn4+ oxides are studied. A detail analysis of the parameters obtained, as well as the associated uncertainties is shown. The methodologies are also applied for Mn oxidation state characterization of double perovskite oxides Ba1+xLa1−xMnSbO6 (with 0≤x≤0.7). The results show that statistical parameters and multivariate analysis are the most suitable for the analysis of this kind of spectra.

Chemical speciation using high energy resolution PIXE spectroscopy in the tender X-ray range

High energy resolution X-ray emission spectroscopy employing wavelength dispersive (WDS) crystal spectrometers can provide energy resolution on the level of core-hole lifetime broadening of the characteristic emission lines. While crystal spectrometers have been traditionally used in combination with electron excitation for major and minor element analysis, they have been rarely considered in proton induced X-ray emission (PIXE) trace element analysis mainly due to low detection efficiency. Compared to the simplest flat crystal WDS spectrometer the efficiency can be improved by employing cylindrically or even spherically curved crystals in combination with position sensitive X-ray detectors. When such spectrometer is coupled to MeV proton excitation, chemical bonding effects are revealed in the high energy resolution spectra yielding opportunity to extend the analytical capabilities of PIXE technique also towards chemical state analysis. In this contribution we will focus on the high energy resolution PIXE (HR-PIXE) spectroscopy in the tender X-ray range performed in our laboratory with our home-built tender X-ray emission spectrometer. Some general properties of high energy resolution PIXE spectroscopy in the tender X-ray range are presented followed by an example of sulfur speciation in biological tissue illustrating the capabilities as well as limitations of HR-PIXE method used for chemical speciation in the tender X-ray range.

Developments in Benchtop Spectrometers: A High Access XANES User Facility and a Compact Spectrometer for Energy Storage Research

Until recently, advanced x-ray spectroscopy was largely the sole domain of synchrotron facilities. While national-lab scale sources provide the ultimate in technical capability, the present access model is necessarily restrictive to the point of slowing progress on critical projects and prohibiting more routine analytical characterization. Over the last three years, our group at the University of Washington has been developing several new families of lab-based instruments for x-ray absorption fine structure (XAFS) and high-resolution x-ray emission spectroscopy (XES). This poster will first report on the installation of a hard x-ray XAFS user facility for battery research in the University of Washington Molecular Analysis Facility (MAF) before detailing the development of a small-scale spectrometer for benchtop XES measurements at energies down to a few keV.The UW-MAF’s recent installment of a laboratory XANES spectrometer provides users with a valuable tool for high-throughput transition metal and lanthanide speciation studies. The instrument is available as a mail-in service for academic, industry, and national lab scientists, making it ideal for system prototypes in need of rapid feedback. The spectrometer is further recommended for such studies by its sub-eV energy resolution and high flux provided in transmission-mode. Selected applications reported here include redox flow batteries, Li-ion batteries, and a low absorption pouch cell design for in operando studies. This latter-most capability was developed in collaboration with the Pacific Northwest National Laboratory.Another area of active research within our group is the development of XES spectrometers for lower energy applications, such as x-ray emission from sulfur and phosphorous. Our new instrument is surprisingly small, yet provides synchrotron-level performance in both energy resolution and count rates. Ongoing developments include its integration into a glove box system to facilitate the study of air-sensitive systems such as are relevant for Li-S battery research and development

High resolution x-ray absorption and emission spectroscopy of LixCoO2 single crystals as a function delithiation

The effect of delithiation in LixCoO2 is studied by high resolution Co K-edge x-ray absorption and x-ray emission spectroscopy. Polarization dependence of the x-ray absorption spectra on single crystal samples is exploited to reveal information on the anisotropic electronic structure. We find that the electronic structure of LixCoO2 is significantly affected by delithiation in which the Co ions oxidation state tending to change from 3+ to 4+. The Co intersite (intrasite) 4p–3d hybridization suffers a decrease (increase) by delithiation. The unoccupied 3d t2g orbitals with a1g symmetry, containing substantial O 2p character, hybridize isotropically with Co 4p orbitals and likely to have itinerant character unlike anisotropically hybridized 3d eg orbitals. Such a peculiar electronic structure could have significant effect on the mobility of Li in LixCoO2 cathode and hence the battery characteristics.

Robust optic alignment in a tilt-free implementation of the Rowland circle spectrometer

High-resolution x-ray emission spectroscopy (XES) has recently been demonstrated in the laboratory setting, achieving nearly synchrotron-level count rates despite the use of only conventional x-ray tube sources. This development holds high potential for expanding the reach of x-ray spectroscopies beyond the specialist community of synchrotron users, but comes with its own unique technical challenges for instrument performance and also, just as importantly, for ease of use by non-experts in x-ray science. Here, we address spectrometer design and operations in the context of the imperfect parallelness between the desired crystal plane and the wafer surface in spherically bent crystal analyser (SBCAs), an effect usually called “wafer miscut”. This introduces an ambiguous re-focusing error that typically requires a motorized two-axis tilt stage for fine alignment of the SBCA optic onto the ideal Rowland circle configuration. We instead demonstrate an asymmetric Rowland geometry that eliminates all need for motorized fine-tilt adjustment. We find rapid, extremely reproducible re-insertion of any aligned SBCA, i.e., without the need for any subsequent reoptimization. These improvements strongly benefit the ease of use of laboratory based spectrometers, taking them an important step closer to the level of turnkey operations needed for wide adoption outside of the existing specialist community.

Electronic Structure of Third-Row Elements in Different Local Symmetries Studied by Valence-to-Core X-ray Emission Spectroscopy.

The electronic structure of phosphorus, sulfur, and chlorine in compounds with Td and C3v local symmetries was studied with high-resolution Kβ X-ray emission spectroscopy (XES) in the tender X-ray range. Measured spectra are compared to the results of ab initio quantum chemical calculations based on density functional theory (DFT). The spectral structure is reproduced by the model spectra of isolated XO4n– and XO3n– (X = P, S, or Cl) anions incorporating only the first coordination sphere around the central atom. The main spectral components can be explained by the molecular orbital theory. Finally, the potential of XES spectroscopy combined with DFT calculations to study the electronic structure of third-row elements in a slightly larger molecule is investigated.

A Modern Laboratory XAFS Cookbook

We have recently demonstrated a very favorable, inexpensive modernization of lab- based x-ray absorption fine structure (XAFS) and high-resolution x-ray emission spectroscopy (XES) using only commercially-available optics and x-ray tube sources. Here, we survey several proven instrument designs that can be readily implemented in any laboratory setting to achieve synchrotron-quality XAFS and XES for many systems in the 5 keV to 10 keV energy range. These approaches are based on our immediate experience with the development of: (1) an inexpensive, low-powered monochromator capable of performing either XAFS or XES, (2) a mid-scale XAFS user facility having 106/sec flux with sub-eV bandwidth on each of two independent beamlines, and (3) multiple XES spectrometers having outstanding performance that can complement synchrotron XAFS studies in battery and actinide research.

Chemical State Analysis of Phosphorus Performed by X-ray Emission Spectroscopy

An experimental and theoretical study of phosphorus electronic structure based on high energy resolution X-ray emission spectroscopy was performed. The Kα and Kβ emission spectra of several phosphorus compounds were recorded using monochromatic synchrotron radiation and megaelectronvolt (MeV) proton beam for target excitation. Measured spectra are compared to the results of ab initio quantum chemical calculations based on density functional theory (DFT). Clear correlation between energy position of the Kα emission line and the phosphorus formal oxidation state as well as DFT-calculated number of valence electrons is obtained; measured energy shifts are reproduced by the calculations. Chemical sensitivity is increased further by looking at the Kβ emission spectra probing directly the structure of occupied molecular orbitals. Energies and relative intensities of main components are given together with the calculated average atomic character of the corresponding molecular orbitals involved in transitions.

High-resolution X-ray emission spectroscopy with transition-edge sensors: present performance and future potential.

X-ray emission spectroscopy (XES) is a powerful element-selective tool to analyze the oxidation states of atoms in complex compounds, determine their electronic configuration, and identify unknown compounds in challenging environments. Until now the low efficiency of wavelength-dispersive X-ray spectrometer technology has limited the use of XES, especially in combination with weaker laboratory X-ray sources. More efficient energy-dispersive detectors have either insufficient energy resolution because of the statistical limits described by Fano or too low counting rates to be of practical use. This paper updates an approach to high-resolution X-ray emission spectroscopy that uses a microcalorimeter detector array of superconducting transition-edge sensors (TESs). TES arrays are discussed and compared with conventional methods, and shown under which circumstances they are superior. It is also shown that a TES array can be integrated into a table-top time-resolved X-ray source and a soft X-ray synchrotron beamline to perform emission spectroscopy with good chemical sensitivity over a very wide range of energies.

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X-ray Emission Spectroscopy.

We combine Mn L2,3-edge X-ray absorption, high resolution Mn 2p-3d-2p resonant X-ray emission, and configuration-interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn(2+) low-spin state separately from that of the Mn(2+) high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn(2+) low spin state having a stronger ligand-to-metal charge-transfer effect between Mn t2g and CN π orbitals than the Mn(2+) high spin state. The d-d-excitation peak nearest to the elastic scattering was ascribed to the Mn(2+) LS state by the CIFM calculation, indicating that the Mn(2+) LS state with a hole on the t2g orbital locates near the Fermi level.