High-resolution X-ray Absorption Spectra Research Articles

Deconvolution of the X-ray absorption spectrum of trans-1,3-butadiene with resonant Auger spectroscopy.

High-resolution carbon K-edge X-ray photoelectron, X-ray absorption, non-resonant and resonant Auger spectra are presented of gas phase trans-1,3-butadiene alongside a detailed theoretical analysis utilising nuclear ensemble approaches and vibronic models to simulate the spectroscopic observables. The resonant Auger spectra recorded across the first pre-edge band reveal a complex evolution of different electronic states which remain relatively well-localised on the edge or central carbon sites. The results demonstrate the sensitivity of the resonant Auger observables to the weighted contributions from multiple electronic states. The gradually evolving spectral features can be accurately and feasibly simulated within nuclear ensemble methods and interpreted with the population analysis.

Operando High Energy Resolution Fluorescence Detected (HERFD) X-Ray Absorption Spectroscopic Study of Toyota Mirai Gen.1 Catalysts

The development of proton exchange membrane fuel cells (PEMFCs) have received increasing attention for its potential to get rid of dependence on fossil fuels and curb carbon emissions. However, the application of PEMFC has been limited by the consumption of platinum due to the sluggish kinetics of the cathode reaction and limiting platinum resources. It is well-known that the oxygen species generated on the Pt surface hiders ORR reaction at high potential region. Therefore, it is very important to evaluate the interaction between the oxygen species and the catalyst based on the electronic structure. However, the differences in XAFS spectra due to the presence of oxygen species are relatively small, and conventional detection methods, such as transmittance and total fluorescence yield, have difficulty distinguishing differences in electronic structure because the fine structure of the spectra is filled in by the lifetime width of the Pt 2p 3/2 core hole. In contrast, the new high-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) monochromatized fluorescence lines (e.g., Pt Lα1), which are broadened only by Pt 3d 5/2 (~2.4 eV) rather than by Pt 2p 3/2 (~5.2 eV) core hole lifetime, are used to monitor core High-resolution X-ray absorption spectra with extended hole lifetimes can now be extracted. The present study covers conventional platinum catalysts supported on Vulcan carbon and TOYOTA MIRAI Gen.1 catalysts. The HERFD-XAS method was applied to elucidate the cause of the superior electrochemical properties of the TOYOTA MIRAI Gen.1 catalyst. o perando Pt LIII edge HERFD-XAS measurements were performed on Pt/C and TOYOTA MIRAI Gen.1 catalysts under oxygen reduction conditions, and the high energy resolution revealed the interaction of oxygen species with the Pt surface.Pt/C (29.1 wt %; TEC10V30E, average particle size of 3 nm, observed by TEM) catalyst was purchased from TKK Co. Ltd., Japan. The Pt/C catalyst ink contained water and 2-propanol in a 3:2 ratio (99.99%, FUJIFILM Wako Pure Chemical Corporation, Japan), and the amount of Pt/C was adjusted to form a 20 μg carbon cm–2 catalyst layer after dropping 10 μL on a glassy carbon RDE (GC RDE, HOKUTO DENKO, Japan; 0.196 cm2 area). TOYOTA MIRAI Gen.1 catalyst was also prepared by the above-mentioned preparation method. operando XAS measurements for Pt LΙΙΙ-edge of the catalysts were carried out at BL39XU of SPring-8, Hyogo, Japan. Prior to the experiments, the samples were electrochemically cleaned by CV in the range 0.0–1.1 V (vs. RHE). The electrode potential was swept cathodically from 0.5 to 1.1 V (vs. RHE).Figure 1 shows the results for in situ HERFD-XAS spectra of Pt and MIRAI Gen.1 catalyst. For both samples, the white line intensity changes depending on the adsorption conditions. The obtained HERFD-XAS spectra were decomposed into oxidized (Pt-OH) and reduced (Pt metal) components. The ratio of the peak area intensity of the two components is proportional to the surface coverage assuming similar charge transfer among the catalysts. It is noted that enhancement of oxidation is significantly suppressed on MIRAI Gen.1 catalysts. Figure 1. operando HERFD-XAS spectra for (a) TOYOTA MIRAI Gen.1 catalyst. (b) Pt/C (TEC10V30E) Acknowledgement This work was supported by a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Irreversible Transition from GaO6 Octahedra to GaO4 Tetrahedra for Improved Electrochemical Stability in Ga-Doped Li(Ni0.9Co0.1)O2

Trace doping is an efficient way to improve the stability of nickel-rich layered cathodes for lithium-ion batteries, but the structural origin of such improvement, rather than a simple replacement, has been rarely explored. Herein, Ga dopants have been introduced into a nickel-rich host, LiNi0.9Co0.1O2, by a combination of coprecipitation and the solid-state sintering method. Structural analyses based on high-resolution synchrotron powder diffraction data and X-ray absorption spectra suggest that Ga preferentially sits in the NiO6 slabs, resulting in reduced Ni/Li cationic mixing and depressed lattice vibration. Due to the smaller dissociation energy of Ga-O bonds, some Ga3+ cations migrate first toward Li layers upon delithiation and form the GaO4 tetrahedral symmetry irreversibly during the electrochemical process, which acts as a pillar in the subsequent electrochemical processes. As a result, the doped material exhibits both improved cycling performance and rate capability under a high operating voltage (4.5 V) due to the stabilized structure in the lithiation/delithiation process. This study illustrates how a dopant enhances the electrochemical stability in views of both pristine and charged structure and suggests that a positive effect is expected from the dopant favoring the tetrahedral symmetry (e.g., Al).

Hierarchy-induced X-ray linear dichroism in cortical bone

The high-resolution X-ray absorption spectra of rat tibia cortex have been measured for two different orientations of the bone long axis in respect to the electric field vector of linear polarized synchrotron radiation. Our measurements performed near the Ca 2p and O 1s absorption edges have revealed the substantial element-specific orientation dependence of the X-ray absorption coefficient. Strong changes in intensity of core-to-valence transitions are detected near the Ca L2,3 absorption edges, whereas minor changes occur near the O K absorption edge. The specific behavior is attributed to the emergent properties of X-ray linear dichroism in cortex. This optical phenomenon is assigned with relationship between the chemical Ca–O bonds in bone and hierarchical organization of the skeleton. The hierarchy-induced X-ray linear dichroism is discussed in the framework of the 3D superlattice model and Wolff paradigm. Prospects for using this phenomenon for medical diagnosis are also being discussed.

Theoretical Modelling of High-Resolution X-Ray Absorption Spectra at Uranium M4 Edge

ABSTRACTWe investigate the origin of satellite features that appear in the high-resolution x-ray absorption spectra measured at the uranium M4 edge in compounds where the uranium atoms are in the U6+ oxidation state. We employ a material-specific Anderson impurity model derived from the electronic structure obtained by the density-functional theory.

The fluctuating population of Sm 4f configurations in topological Kondo insulator SmB6 explored with high-resolution X-ray absorption and emission spectra.

High-resolution partial-fluorescence-yield X-ray absorption and resonant X-ray emission spectra were used to characterize the temperature dependence of Sm 4f configurations and orbital/charge degree of freedom in SmB6. The variation of Sm 4f configurations responds well to the formed Kondo gap, below 140 K, and an in-gap state, below 40 K. The topological in-gap state is correlated with the fluctuating population of Sm 4f configurations that arises via carrier transfer between 3d94f6 and 3d94f5 states; both states are partially delocalized, and the mediating 5d orbital plays the role of a transfer path. Complementary results shown in this work thus manifest the importance of configuration fluctuations and orbital delocalization in the topological surface state of SmB6.

Pressure dependence of the electronic structure of 4f and 3d electron systems studied by X-ray emission spectroscopy

ABSTRACTTechniques based on X-ray emission spectroscopy (XES) have been utilized to investigate pressure-induced changes in the electronic structure in samples subject to high pressure in the diamond anvil cell. High-resolution X-ray absorption spectra can be obtained with partial fluorescence yield mode (PFY-XAS) measurement. As typical examples, we show results of measurements of the valence fluctuating systems of CeCuGe, Ce115 systems, and YbNiGe. The pre-edge feature of the XAS spectra at K-absorption edge of 3d-elements correlates to the spin states. We also show the results of the Fe XES and PFY-XAS of KFeSe, which has two pressure-induced superconducting (SC) domes.

21pPSA-3 Ce化合物における高分解能2pXASと2p-3d共鳴X線発光スペクトルの理論解析

21pPSA-3 Ce化合物における高分解能2pXASと2p-3d共鳴X線発光スペクトルの理論解析

Identification of the iron oxidation state and coordination geometry in iron oxide- and zeolite-based catalysts using pre-edge XAS analysis.

Analysis of the oxidation state and coordination geometry using pre-edge analysis is attractive for heterogeneous catalysis and materials science, especially for in situ and time-resolved studies or highly diluted systems. In the present study, focus is laid on iron-based catalysts. First a systematic investigation of the pre-edge region of the Fe K-edge using staurolite, FePO4, FeO and α-Fe2O3 as reference compounds for tetrahedral Fe(2+), tetrahedral Fe(3+), octahedral Fe(2+) and octahedral Fe(3+), respectively, is reported. In particular, high-resolution and conventional X-ray absorption spectra are compared, considering that in heterogeneous catalysis and material science a compromise between high-quality spectroscopic data acquisition and simultaneous analysis of functional properties is required. Results, which were obtained from reference spectra acquired with different resolution and quality, demonstrate that this analysis is also applicable to conventionally recorded pre-edge data. For this purpose, subtraction of the edge onset is preferentially carried out using an arctangent and a first-degree polynomial, independent of the resolution and quality of the data. For both standard and high-resolution data, multiplet analysis of pre-edge features has limitations due to weak transitions that cannot be identified. On the other hand, an arbitrary empirical peak fitting assists the analysis in that non-local transitions can be isolated. The analysis of the oxidation state and coordination geometry of the Fe sites using a variogram-based method is shown to be effective for standard-resolution data and leads to the same results as for high-resolution spectra. This method, validated by analysing spectra of reference compounds and their well defined mixtures, is finally applied to track structural changes in a 1% Fe/Al2O3 and a 0.5% Fe/BEA zeolite catalyst during reduction in 5% H2/He. The results, hardly accessible by other techniques, show that Fe(3+) is transformed into Fe(2+), while the local Fe-O coordination number of 4-5 is maintained, suggesting that the reduction involves a rearrangement of the oxygen neighbours rather than their removal. In conclusion, the variogram-based analysis of Fe K-edge spectra proves to be very useful in catalysis research.

Site-selective high-resolution X-ray absorption spectroscopy and high-resolution X-ray emission spectroscopy of cobalt nanoparticles.

The special (macroscopic) properties of nanoparticles are mainly due to their large surface-to-volume ratio. Thus, the separate characterization of geometric and electronic properties of surface and bulk would be favorable for a better understanding of the properties of nanoparticles. Because of the chemical sensitivity of X-ray fluorescence lines, in particular those involving higher lying electronic states, high-resolution fluorescence-detected X-ray absorption spectra (HRFD-XAS) offer these opportunities. In this study, three types of wet-chemically synthesized Co nanoparticles, ∼6 nm in diameter with varying thicknesses of a protective shell, were investigated at the ID26 beamline of the European Synchrotron Radiation Facility. HRFD-XAS spectra at the Co K-edge, that is, X-ray absorption near-edge structure (HRFD-XANES) and extended X-ray absorption fine structure (HRFD-EXAFS) spectra, were recorded via detection of the Kβ1,3 fluorescence at specific energies. As these spectra are only partly site-selective due to a strong overlap of the emission lines, a numerical procedure was applied based on a least-squares fitting procedure, realized by singular value decomposition. The detailed analysis of the obtained site-selective spectra, regarding chemical composition and crystallographic phase, using measured and simulated FEFF9-based reference spectra, showed that the metallic core had mainly hexagonal close-packed structure with lattice constants matching bulk Co; the spectra for the shell could be satisfactorily fitted by a mixture of CoO and CoCO3; however, with an obvious need for at least a third compound. To obtain additional information about ligands attached to Co, valence-to-core X-ray emission spectra (VTC-XES) using the Kβ2,5 and the satellite structure Kβ″ and VTC-XANES spectra thereof were also recorded, by which the former results are confirmed. Further on, FEFF simulations indicate that a Co-N compound is a very likely candidate for the third component. The presented results clearly show that VTC-XES and HRFD-XAS are suitable tools for the detailed specification of the core and the surface of nanoparticles, in particular upon realizing "real" site-selectivity for XANES and EXAFS with a general strategy applicable to a wide range of systems.

Electronic Structure and Crystal Structure of Multiferroic o-YMnO3 at High Temperature

The dependence of the electronic structure and crystal structure of multiferroic YMnO3 in an orthorhombic phase (o-YMnO3) on temperature up to 873 K was investigated on combining the measurements of synchrotron x-ray powder diffraction (SXRD) and high-resolution x-ray absorption spectra measured in the partial fluorescence yield. Based on SXRD refinement, the small equatorial bond distance (Mn–O2)s of o-YMnO3 increases progressively, whereas the large equatorial bond distance (Mn–O2)l decreases with increasing temperature. The Mn–O–Mn angle of two adjacent MnO6 octahedra increases monotonically with increasing temperature. Based on these SXRD results, the local Jahn–Teller (JT) distortion of Mn sites within MnO6 octahedra of o-YMnO3 became suppressed, and the adjacent JT-distorted MnO6 octahedra became progressively symmetric with increasing temperature. The white-line profile of Mn \(K\)-edge spectra becomes more symmetric, originating from the suppression of the Jahn–Teller distortion of MnO6 octahedra ...

Vibronic fine structure in high-resolution x-ray absorption spectra from ion-bombarded boron nitride nanotubes

The authors have applied high-resolution near-edge x-ray absorption fine structure measurements around the nitrogen K-edge to study the effects of ion-bombardment on near-surface properties of boron nitride nanotubes. A notable difference has been observed between surface sensitive partial electron yield (PEY) and bulk sensitive total electron yield (TEY) fine-structure measurements. The authors assign the PEY fine structure to the coupling of excited molecular vibrational modes to electronic transitions in NO molecules trapped just below the surface. Oxidation resistance of the boron nitride nanotubes is significantly reduced by low energy ion bombardment, as broken B-N bonds are replaced by N-O bonds involving oxygen present in the surface region. In contrast to the PEY spectra, the bulk sensitive TEY measurements on as-grown samples do not exhibit any fine structure while the ion-bombarded samples show a clear vibronic signature of molecular nitrogen.

Growth and characterization of SmxY1−xCa4O(BO3)3 single crystals for nonlinear optical applications

YCa4O(BO3)3 (YCOB) is a good candidate nonlinear optical crystal for optical parametric chirped-pulse application. In order to generate the suitable absorption of idler light near 1588 nm, Sm3+ ions with various concentrations are doped into YCOB crystals. SmxY1−xCa4O(BO3)3 (x = 0.05, 0.3, 0.5, 0.75, 1) crystals of high quality have been grown by the Czochralski method. The X-ray powder diffraction, high resolution X-ray diffraction, ICP-AES, density, absorption spectra, specific heat and thermal diffusion coefficient of as-grown crystals were measured. The results show that the as-grown crystals are isostructural with YCOB; the full width at half maximum (FWHM) of the rocking curve of the wafers with different Sm3+ concentrations are all lower than 30′′; the effective segregation coefficient of Sm3+ ion in this crystal is 0.773 with a Sm concentration of 5 at%; the crystals have absorption peaks in the 1400–1600 nm range.

DENSITY PROFILES IN SEYFERT OUTFLOWS

For the past decade, ionized outflows of a few 100 km/s from nearby Seyfert galaxies have been studied in great detail using high resolution X-ray absorption spectra. A recurring feature of these outflows is their broad ionization distribution including essentially ions (e.g., of Fe) from neutral to fully ionized. The absorption measure distribution (AMD) is defined as the distribution of column density with ionization parameter |d N_H/d (log xi)|. AMDs of Seyfert outflows can span up to five orders of magnitude in xi. We present the AMD of five outflows and show that they are all rather flat, perhaps slightly rising towards high ionization. More quantitatively, a power-law fit for log AMD ~ (log xi)^a yields slopes of 0 < a < 0.4. These slopes tightly constrain the density profiles of the wind, which until now could be addressed only by theory. If the wind is distributed on large scales, the measured slopes imply a generic density radial profile of n ~ r^{-alpha} with 1 < alpha < 1.3. This scaling rules out a mass conserving radial flow of n ~ r^{-2}, or a constant density absorber, but is consistent with a non-spherical MHD outflow model in which n ~ r^{-1} along any given line of sight. On the other hand, if ionization variations are a result of local (delta r) density gradients, e.g. as in the turbulent interstellar medium (ISM), the AMD slopes imply density scaling of n ~ delta r^{-alpha} with 0.7 < alpha < 1.0, which is quite different from the scaling of approximately n ~ delta r^{0.4} found in the Milky Way ISM and typical of incompressible turbulence.

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.

Experimental estimate of electron escape depth in Fe

Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is a powerful experimental tool to investigate the electronic and atomic structure of materials. The great power of NEXAFS derives from its elemental specificity and symmetry selection rules. Because the various elements have different core level energies, NEXAFS permits extraction of the signal from a surface monolayer or even a single buried layer in the presence of a huge background signal. We have calculated electron escape depth ( λ e ) in Fe at the L 2,3 edge using pure Fe thin films of different thickness by soft x-ray absorption spectroscopy measurements. We have recorded high-resolution x-ray absorption spectra in transmission of Fe thin films resolving the near edge x-ray absorption fine structure. As a conclusion we found the mean escape depth of emitted electrons to be λ e = 22 ± 2 Å . We believe our results provide important information for the improved understanding of theory of the photo absorption mechanism.

Electronic structure ofFeSi1−xGexandFeGa3investigated by soft x-ray photoelectron spectroscopy complementary to x-ray emission spectroscopy

Soft x-ray photoelectron spectroscopy for ${\text{FeSi}}_{1\ensuremath{-}x}{\text{Ge}}_{x}$ ($x=0$, 0.11, 0.19, 0.26, and 0.44) and ${\text{FeGa}}_{3}$ was performed at $\text{Fe}\text{ }L$ absorption edge complementary to the x-ray emission spectroscopy. The absorption and valence-band spectra show the phase transition of the electronic state between $x=0.26$ and 0.44 in ${\text{FeSi}}_{1\ensuremath{-}x}{\text{Ge}}_{x}$ accompanying the narrowing of the density of state near the Fermi edge. High-resolution x-ray absorption spectra (total electron yields) at $\text{Fe}\text{ }2{p}_{3/2}$ absorption edge resolve the fine structure, originated from the valence and Auger component of electrons. Resonant valence-band and constant initial-state spectra show Fano-type profile at $\text{Fe}\text{ }2{p}_{3/2}$ absorption edge only for the $x=0.44$ sample but not for the $x=0$ sample and ${\text{FeGa}}_{3}$. The crossover point, where the Raman and Auger features mix, is near the $\text{Fe}\text{ }2p$ absorption edge for both of the $x=0$ and 0.44 samples, showing the delocalized $3d$ states.

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.

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.

X-ray photoabsorption and total electron yield of Fe thin films at the L 2,3 edge

We have studied thin Fe films by soft X-ray absorption spectroscopy to determine the total electron yield (TEY) sampling depth ( λ e) at the Fe L 2,3 edge. For this, we have recorded high-resolution X-ray absorption spectra in TEY resolving the near edge X-ray absorption fine structure (NEXAFS). Our analysis yields a value of λ e = 21 ± 2 Å for the TEY sampling depth.