High-quality X-ray Absorption Research Articles

QUATI beamline: QUick x-ray Absorption spectroscopy for TIme and space-resolved experiments at the Brazilian Synchrotron Light Laboratory

The QUATI beamline is now under assembly at the Brazilian Synchrotron Laboratory and will be dedicated to high-quality X-ray absorption spectroscopy experiments, with temporal and spatial resolution on a millisecond scale for in situ and operando studies performing: XANES (X-ray Absorption Near Edge Structure), EXAFS (Extended X-ray Absorption Fine Structure) and XES (X-ray Emission Spectroscopy). Furthermore, it will combine X-ray absorption techniques with X-ray microscopy, Raman or Infrared spectrometry, X-ray Diffraction, and Mass Spectrometry to investigate different aspects of the sample in a single experiment to characterize complex materials under real working conditions.

Design of Tender X-ray Absorption Spectroscopy Beamline in Taiwan Photon Source

A new tender X-ray absorption spectroscopy beamline for operating over the photon energy range from 1.7 keV to 11 keV is under construction at port 32 of the Taiwan Photon Source (TPS). The TPS 32A beamline uses two back-to-back water-cooled double crystal monochromators (DCMs), InSb(111) and Si(111), for achieving the wide energy tunability. The beamline design takes advantage of the low emittance of the TPS accelerator and a bending magnet (BM) to obtain a high-brightness focussed beam. In order to overcome the issue of poor thermal conductivity of InSb(111) crystals while facing high thermal load, we have implemented the following features : (i) we have controlled the flux and heat load by optimizing the source collection angle of 2 × 0.15 mrad2 (h × v), and (ii) we have installed water cooled double-bounce high harmonic rejection mirrors (HHRMs) with a variable incidence angle upstream of the DCM to reduce the heat load on the first InSb crystal. The DCM is followed by a 2:1 toroidal focusing mirror (TFM) which focuses the beam parallel to the floor of the experimental endstation. The simulated focus beam size is 0.9 × 0.9 mm2 (h × v, FWHM) using the InSb(111) DCM at 1.7 keV, and 0.2 × 0.45 mm2 (h × v, FWHM) using the Si(111) DCM at 4 keV, respectively. The photon flux is 1010 -1012 photons s−1 in the energy range of 1.7 to 11 keV at the sample stage after passing through the Be windows. The design parameters of photon flux and beam spot size will facilitate a tender X-ray source for carrying out high-quality X-ray absorption spectroscopy experiments.

Origin of the isostructural electronic states of the topological insulator Bi2Te3

The novel physics, such as the pressure-induced electronic topological transition (ETT), topological superconductivity and Majorana fermions in the isostructural $R\text{\ensuremath{-}}3m$ phase of three-dimensional topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, holds considerable interest in condensed-matter physics. We carried out a combined investigation of single-crystal x-ray diffraction, high-quality x-ray absorption fine structure, and first-principles theoretical calculations to decipher the puzzling origin of the intriguing electronic states in the isostructural $R\text{\ensuremath{-}}3m$ phase of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ at high pressure. Three distinct regions with two isostructural phase transitions (IPTs) in the $R\text{\ensuremath{-}}3m$ phase have been identified. The first IPT, which is known as the ETT, occurs at the boundary of region I (0--2 GPa) and region II (2--5 GPa) with a sharp minimum in the $c/a$ ratio of $R\text{\ensuremath{-}}3m$ structure, while the second IPT happens as pressure increases from region II (2--5 GPa) to region III (5--7 GPa). The positions of the Bi ($6c$) and Se ($6c$) sites in the unit cell change rapidly in region II (2--5 GPa), but there is little change at these sites in region III (5--7 GPa). The band-gap closure in region I reflects the pressure-induced metallization. At higher pressures, the band gap opens in region II but remains almost constant after the second IPT in region III, which agrees well with the topological superconductivity of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. Our results demonstrate that the combination of local structure, long-range crystal structure, and first-principles calculation is critically important for understanding the isostructural electronic states and the connection between the structure and function as in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ at high pressure.

Distinct intermediate states in the isostructural R−3m phase of the topological insulator Bi2Se3 at high pressure

The electronic state of layered three-dimensional topological insulators at high pressure is a compelling but a puzzling hot topic. The open question is the structural origins for the pressure-induced novel physics of electronic topological transition (ETT), topological superconductivity, and Majorana fermions in the same isostructural $R\text{\ensuremath{-}}3m$ phase. Here, we report a combined investigation on the local structure and bulk electronic state of topological insulator $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ using x-ray diffraction, high quality x-ray absorption fine-structure spectroscopies both at the $\mathrm{Bi}\phantom{\rule{0.16em}{0ex}}{L}_{3}$ edge and at the Se K edge, and first-principles theoretical calculations. We have found three pressure-induced distinct intermediate states in the isostructural rhombohedral phase of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$. The bulk electronic structure of the $R\text{\ensuremath{-}}3m$ phase was calculated based on the experimental structure. The corresponding distinct electronic states provide the origins of ETT at \ensuremath{\sim}3 GPa and the metallization at 7--9.5 GPa in the $R\text{\ensuremath{-}}3m$ phase of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$. Our results demonstrate that the local structure plays a critical role in the electronic states of topological insulator $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$.

Structural response of Ni/ZrO2 to feed modulations during CH4 reforming reactions

Time resolved oxidation and reduction cycles of ZrO2-supported Ni catalysts for dry reforming of CH4 (CO2 + CH4 ↔ 2 CO + 2 H2) during feed modulations have been studied. Under reaction conditions (1073 K) Ni remains fully reduced, whereas switching the feed gas to pure CO2 results in a slow (25 sec) formation of a stable NiO phase. When switching back to reaction conditions the NiO phase is rapidly reduced (∼ 1 sec) to metallic Ni. In the context of this study, a novel capillary cell has been built, allowing the parallel treatment of 5 catalyst samples with different gas compositions and different pressures. A comparison of the capillary cell to conventional systems regarding the spectral quality and the kinetic data shows that the capillary cell can be used to obtain identical kinetic data and high quality X-ray absorption spectra.

Detailed Characterization of a Nanosecond-Lived Excited State: X-ray and Theoretical Investigation of the Quintet State in Photoexcited [Fe(terpy)2]2.

Theoretical predictions show that depending on the populations of the Fe 3dxy, 3dxz, and 3dyz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)2]2+. The differences in the structure and molecular properties of these 5B2 and 5E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)2]2+ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)2]2+ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe–ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)–high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as 5E, in agreement with our theoretical expectations.

The multi-purpose hard X-ray beamline BL10 at the DELTA storage ring.

The layout and the characteristics of the hard X-ray beamline BL10 at the superconducting asymmetric wiggler at the 1.5 GeV Dortmund Electron Accelerator DELTA are described. This beamline is equipped with a Si(111) channel-cut monochromator and is dedicated to X-ray studies in the spectral range from ∼4 keV to ∼16 keV photon energy. There are two different endstations available. While X-ray absorption studies in different detection modes (transmission, fluorescence, reflectivity) can be performed on a designated table, a six-axis kappa diffractometer is installed for X-ray scattering and reflectivity experiments. Different detector set-ups are integrated into the beamline control software, i.e. gas-filled ionization chambers, different photodiodes, as well as a Pilatus 2D-detector are permanently available. The performance of the beamline is illustrated by high-quality X-ray absorption spectra from several reference compounds. First applications include temperature-dependent EXAFS experiments from liquid-nitrogen temperature in a bath cryostat up to ∼660 K by using a dedicated furnace. Besides transmission measurements, fluorescence detection for dilute sample systems as well as surface-sensitive reflection-mode experiments are presented.

Femtosecond X-ray Absorption Spectroscopy at a Hard X-ray Free Electron Laser: Application to Spin Crossover Dynamics

X-ray free electron lasers (XFELs) deliver short (<100 fs) and intense (∼10(12) photons) pulses of hard X-rays, making them excellent sources for time-resolved studies. Here we show that, despite the inherent instabilities of current (SASE based) XFELs, they can be used for measuring high-quality X-ray absorption data and we report femtosecond time-resolved X-ray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2'-bipyridine) in water. The data indicate that the low-spin to high-spin transition can be modeled by single-exponential kinetics convoluted with the overall time resolution. The resulting time constant is ∼160 fs.

A sample holder for soft x-ray absorption spectroscopy of liquids in transmission mode

A novel sample holder for soft x-ray absorption spectroscopy of liquids in transmission mode based on sample cells with x-ray transparent silicon nitride membranes is introduced. The sample holder allows for a reliable preparation of ultrathin liquid films with an adjustable thickness in the nm-μm range. This enables measurements of high quality x-ray absorption spectra of liquids in transmission mode, as will be shown for the example of liquid H(2)O, aqueous solutions of 3d-transition metal ions and alcohol-water mixtures. The fine structure of the x-ray absorption spectra is not affected by the sample thickness. No effects of the silicon nitride membranes were observed in the spectra. It is shown how an inhomogeneous thickness of the sample affects the spectra and how this can be avoided.

High quality x-ray absorption spectroscopy measurements with long energy range at high pressure using diamond anvil cell.

We describe an approach for acquiring high quality x-ray absorption fine structure (XAFS) spectroscopy spectra with wide energy range at high pressure using diamond anvil cell (DAC). Overcoming the serious interference of diamond Bragg peaks is essential for combining XAFS and DAC techniques in high pressure research, yet an effective method to obtain accurate XAFS spectrum free from DAC induced glitches has been lacking. It was found that these glitches, whose energy positions are very sensitive to the relative orientation between DAC and incident x-ray beam, can be effectively eliminated using an iterative algorithm based on repeated measurements over a small angular range of DAC orientation, e.g., within +/-3 degrees relative to the x-ray beam direction. Demonstration XAFS spectra are reported for rutile-type GeO2 recorded by traditional ambient pressure and high pressure DAC methods, showing similar quality at 440 eV above the absorption edge. Accurate XAFS spectra of GeO2 glass were obtained at high pressure up to 53 GPa, providing important insight into the structural polymorphism of GeO2 glass at high pressure. This method is expected be applicable for in situ XAFS measurements using a diamond anvil cell up to ultrahigh pressures.

Chemical Modification in Wear Tracks of Chemical Vapor-Deposited Diamond Surfaces Studied with X-ray Absorption Spectroscopy

We present high-quality X-ray absorption near edge structure spectra of chemical vapor-deposited diamond at the C K-edge recorded with high spatial resolution. We compare unworn surfaces with surfaces worn in Ar-atmosphere, in air, and in water, respectively. Strikingly, the degree of chemical modification in the wear tracks is strongest for wear in an inert Ar-atmosphere which we attribute to the massive creation of unsaturated bonds in the surface vicinity due to high friction forces. We discuss the nature of chemical modification, in particular, whether and to what degree graphitization, amorphization, and ex situ reactions take place.

Direct EXAFS evidence for incorporation of As5+ in the tetrahedral site of natural andraditic garnet

Exceptionally high-quality X-ray absorption spectroscopy (XAS) spectra of the As K-edge measured on a garnet from the Central Oslo Rift (Jamtveit et al. 1993) are presented and interpreted. The best fit to the extended X-ray absorption fine structure (EXAFS) spectrum indicates that arsenic occurs predominantly as As5+ in the tetrahedral Si4+ site. The first shell As-O bond lengths of 1.69 angstrom are identical to those observed for As5+-O in sodium arsenate and the edge position of 11872 eV is also characteristic of As5+. Determination of oxidized As within the structure places an important constraint on fluid chemistry, but perhaps even more importantly, EXAFS fitting of outer shells (As-Ca 3.07,3.75, 5.71, 6.11 angstrom; As-Fe/Al 3.47, 5.35 angstrom; As-Si 3.88, 5.60, 5.91 angstrom) in this high-quality data set gives unequivocal evidence that AsO4-3 substitutes for SiO4-4 in the garnet tetrahedral site. This is, to the best of our knowledge, the first Such XAS study yielding direct evidence, through outer shell fitting, for both the coordination environment and oxidation state of arsenic within (as opposed to sorbed onto) a silicate mineral.

The development of X-ray photo-emission electron microscopy (XPEEM) for valence-state imaging of mineral intergrowths

AbstractWe demonstrate the potential of X-ray photo-emission electron microscopy (XPEEM) to reveal valence-state images of the spatial distribution and relative concentration of metals in specific oxidation states. Additionally, XPEEM allows X-ray absorption spectra to be extracted from chosen pixel areas of the images. Using an in-house-built XPEEM instrument we show an application of the method in providing valence-state images of complex mineral intergrowths. The image resolution achieved with this instrument of simple design was ∼5 μm and reasonable quality X-ray absorption spectra were extracted from areas of ∼5×5 μm. These initial results suggest that by using commercial XPEEM instruments on 3rd generation, high-brightness synchrotron sources a spatial resolution of 100 nm or better could be achieved, with the ability to extract high-quality X-ray absorption spectra from areas of 0.1 μm2. Given that standard thin sections or polished blocks can be studied by XPEEM, and that each XPEEM image records ∼1000 μm2, XPEEM can be used in conjunction with other analytical methods such as EMPA and TEM-EELS/PEELS.

Liquid Rb micrometric droplets confined in paraffin wax: an X-ray absorption spectroscopy study.

We have performed high-quality X-ray absorption measurements on crystalline (c-Rb) and liquid (l-Rb) Rubidium in the range from 15 K to 320 K. Performing a consistent analysis that takes into account the contribution of the medium range structure, we observe that the l-Rb spectrum is compatible with pair correlation function g(r) previously determined by neutron diffraction experiments. Due to the micrometric size of the liquid droplets we were able to observe a slight undercooling down to 290 K. We were also able to study the details of the very strong multielectron excitations channels in terms of resonances, edges and shake-off features at proper theoretical energy values.

An experimental station for advanced research on condensed matter under extreme conditions at the European Synchrotron Radiation Facility - BM29 beamline

We describe state-of-the-art experimental techniques using the beamline BM29 of the European Synchrotron Radiation Facility (ESRF). This station exploits the unique characteristics of an ESRF bending magnet source to provide a tunable, collimated, x-ray beam to perform high quality x-ray absorption spectroscopy within the energy range of E=5–75 keV using Si(111), Si(311), and Si(511) crystal pairs. Energy scans can be performed over this wide energy range with excellent reproducibility, stability and resolution, usually better than ΔE/E≃5×10−5. The experimental setup has been exploited to study condensed matter under extreme conditions. We describe here two sample environment devices; the L’ Aquila–Camerino oven for high-temperature studies up to 3000 K in high vacuum and the Paris–Edinburgh press suitable for high-pressure high-temperature studies in the range 0.1–7 GPa and temperatures up to 1500 K. These devices can be integrated in an experimental setup which combines various control and detection systems suitable to perform x-ray absorption spectroscopy, x-ray absorption temperature scans, and energy scanning x-ray diffraction (ESXD). The ESXD setup is based on a scintillator detector behind a fixed angle collimator aligned to the sample. The combination of these three measurements, which can be performed in rapid sequence on the sample during the experiment, provides an essential tool for structural investigations and in situ sample characterization.

Accurate determination of molecular structures by x-ray absorption spectroscopy

High-quality x-ray absorption fine structure (XAFS) spectra of the Br2, GeCl4, and BBr3 molecules, collected at a third generation synchrotron radiation source above the Br or Ge K-edges, are presented. Excellent fits are obtained using model spectra calculated in the muffin-tin approximation assuming Gaussian atomic distributions. The extended energy ranges of the spectra (up to 24 Å−1 for Br2) contribute to the reduction of the statistical errors in the structural parameters. We show that the potential accuracy of present XAFS determinations is 0.001 Å in bond lengths and 0.0001 Å2 in vibrational amplitudes. These results demonstrate that XAFS is nowadays competitive with electron diffraction in the determination of simple molecular structures in the presence of heavy atomic species.

Multielectron excitation in XAFS on Br in organic solvents

High-quality X-ray absorption spectra near the K-edge of Br in organic solvents have been observed using synchrotron radiation. Several structures assigned to multielectron excitations are detected in the liquid phase. They are analyzed by the use of the shakeup and shakeoff probabilities including their dependence on the photon energy. An additional absorption edge is found between [1s3p] and [1s3d] double electron excitation. The possible assignment for this absorption edge is discussed.

Time-resolved x-ray absorption spectroscopy on microsecond timescales: Implications for the examination of structural motions

We demonstrate a system for collecting high quality x-ray absorption spectroscopy data on microsecond and longer timescales. The method uses a fast-multiscaling board to partition output pulses from a germanium energy resolving detector into a microsecond timebase. Full spectra accumulation and kinetic spectrophotometry are demonstrated for 1–4 mM biological samples. The use of an energy resolving detector promises that the dynamic behavior of a wide range of chemical elements (in conjunction with photochemical, rapid mixing, and temperature jump perturbations) can be observed with this method. We have successfully tested the system by examining the structural evolution of the iron atom of a myoglobin-carbon monoxide complex subsequent to photolysis at low temperature.

Structural characterization of bismuth molybdates by x-ray absorption spectroscopy and powder neutron diffraction profile analysis

By use of synchrotron radiation, high-quality X-ray absorption data have been recorded for three bismuth molybdate phases with the general composition Bi/sub 2/O/sub 3/ x nMoO/sub 3/, for n = 3 (..cap alpha..-phase) n = 2 (..beta..-phase), and n = 1 (..gamma..-phase). The results of these bismuth L/sub 3/-edge and molybdenum K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies provide structural insights about the local environments of the Bi/sup 3 +/ and Mo/sup 6 +/ ions. Also, the structure of ..beta..-Bi/sub 2/O/sub 3/ x 2MoO/sub 3/ was investigated by the use of time-of-flight powder neutron diffraction. The profile analysis of the neutron data for ..beta..-bismuth molybdate (monoclinic, P2/sub 1//n, a = 11.9515 (4) A, b = 10.7993 (4) A, c = 11.8805 (4) A, ..beta.. = 90.142 (5)/sup 0/, and Z = 8) revealed that the bismuth and molybdenum cations are present in a fluorite-like sublattice.

Laboratory EXAFS spectrometer for catalyst studies

A laboratory x-ray absorption spectrometer is described which allows very high quality x-ray absorption edge and extended x-ray absorption fine structure (EXAFS) data to be obtained. The spectrometer utilizes a channel-cut crystal configuration monochromator for absorption edge studies and a fully focusing Johansson cut bent crystal monchromator operated on the Rowland circle for high flux, high resolution EXAFS studies. X-ray generation is by a rotating anode generator. Photon fluxes are about 104 higher than for a sealed-tube flat-crystal spectrometer; it is shown that data of quality comparable to that obtained from a synchrotron source (SSRL) can be obtained in comparable time.