Pre-edge Region Research Articles

Distribution of mixed-valence ions in Mn1+xFe2-xO4ferrites

Mn ferrite has a spinel structure to show the ferrimagnetism, where the magnetic moments show a collinear ferrimagnetic ordering between tetrahedral A and octahedral B sites. Since Mn2+ and Fe3+ have the same electronic configuration, it is not easy to determine the cation distribution of Mn ferrite from usual magnetization measurements. Especially in Mn ferrite, both Mn2+ and Fe3+ have a large spin polarization to give strong magnetic moments through the super exchange interaction between the two sites. Replacing Fe3+ by Mn2+ and Mn4+, the ferrimagnetic property weakens through magnetic balance between the sites. Since Mn and Fe ions may have multiple valences in the oxide structure, the scheme of site preference, based on careful study of various valence states, has been investigated for Mn1+xFe2-xO4. Single crystals for Mn1+xFe2-xO4 (x = 0.05, 0.20, 1.36 and 1.50) were synthesized from stoichiometric proportions of Mn3O4 and Fe3O4 in an evacuated silica capsule at 1353 K for 96 h. Each of spherical or parallel-piped crystals, ranging 0.05 to 0.08 mm, was mounted on the glass fiber. Conventional intensity measurements were made using a Rigaku AFC-5S four-circle diffractometer with a graphite (002) monochromator for Mo Kα radiation. Least-squares refinements were made to obtain atomic parameters, converged with R factors ranging 0.023 to 0.029. The site occupancy of Mn and Fe atoms was then determined on the basis of the resonant scattering effect at the Fe K absorption edge (λ = 1.7535 Å), by using a vertical-type four-circle diffractometer at PF-BL-10A. The results show that 89, 82, 100 and 100 percent of Mn atoms occupy the A site for the four samples, respectively. In the third step of analyses, absorption experiments were performed at PF-BL-6C. XANES and XMCD spectra were used at Mn and Fe K edges for determining the valence states of Mn and Mn ions. Finally, the distribution of mixed-valence ions for Mn1.20Fe1.80O4 was determined by the valence-difference contrast method, where the intensity data were collected for a spherical single crystal of 0.08 mm in a diameter at both threshold and pre-edge regions of Mn K edge, by using an AFC-5u four-circle diffractometer installed in PF-BL-6C. The site occupancy with the valence state will be discussed in the presentation, compared with the other type of transition-metal ferrites.

Valence-to-core-detected X-ray absorption spectroscopy: targeting ligand selectivity.

X-ray absorption spectroscopy (XAS) can provide detailed insight into the electronic and geometric structures of transition-metal active sites in metalloproteins and chemical catalysts. However, standard XAS spectra inherently represent an average contribution from the entire coordination environment with limited ligand selectivity. To address this limitation, we have investigated the enhancement of XAS features using valence-to-core (VtC)-detected XAS, whereby XAS spectra are measured by monitoring fluorescence from valence-to-core X-ray emission (VtC XES) events. VtC emission corresponds to transitions from filled ligand orbitals to the metal 1s core hole, with distinct energetic shifts for ligands of differing ionization potentials. VtC-detected XAS data were obtained from multiple valence emission features for a series of well-characterized Mn model compounds; taken together, these data correspond to a VtC resonant XES (VtC RXES) plane. For comparison, standard total fluorescence yield (TFY) XAS and nonresonant XES data were obtained. Dramatic intensity variations and the appearance of new features were observed in the pre-edge region by detecting at different VtC emission energies. The TFY XAS, nonresonant XES, and VtC RXES data were all modeled within a density functional theory approach. While the TFY XAS and nonresonant XES data are readily interpreted by theory, the VtC RXES cannot be reproduced within such a simplified model. Nonetheless, dramatic changes in the experimental spectra are observed that have the potential to further the information content and selectivity of XAS. Potential applications and required theoretical developments are discussed.

Occupancy of 4d-states in T (T: Mo, Nb) from NEXAFS L 3,2 Spectra of Nd-Fe-B-T Alloys

Introduction. Neodymium-iron-boron nanocomposite permanent magnets were developed in the early 1990s [1, 2]. Their low neodymium content, less than the stoichiometric amount of 12 at.%, made them cheaper and more corrosion resistant. Moreover, their higher boron content improves the amorphization ability. These systems, called exchange-spring magnets, exhibit interesting magnetic properties due to the exchange coupling between the hard and soft magnetic phases at nanoscale. The coexistence of both types of magnetic phases results in rema nence and magnetic energy product ((BH)max) enhancement. However, the presence of the soft phases reduces the coercivity. The use of alloying components such as Mn, Cu, Nb, Ti, and Mo, among others, helps to control the grain growth during the melt-spinning production of these kind of magnets, improving in this way the magnetic response of the material [3–5]. In particular, the addition of Mo and Nb was reported to be very effective for grain refi nement of nanocomposites, leading to the improvement of the coercivity after annealing [6]. However, the mechanisms that cause the suppression of α-Fe formation are not fully understood, as well as the role of Mo or Nb as an additive component that helps to stabilize the Nd2Fe14B phase (ϕ-phase). Urse et al. explained the coercivity enhancement by the pinning of domain walls at grain boundaries where the additive precipitates, in the case of Modoped Nd-Fe-B thin fi lms [7]. For Nb-doped ribbons, it was found that a higher concentration of this additive is required than in the case of Mo in order to obtain higher coercivities and (BH)max [8]. Thus, a study of the electronic state of these elements becomes appropriate to elucidate the role they play as additives in Nd-Fe-B magnets. Synchrotron radiation is a powerful tool that helps in this study. The information on the electronic structure of occupied/unoccupied 4d-electron states in these alloys was derived from near-edge X-ray absorption fine structure (NEXAFS) measurements. The purpose of the present work is to quantify, by X-ray absorption spectroscopy, variations in occupancy of the 4d-states of molybdenum and niobium added to NdFeB alloys produced by the melt spinning technique, both as quenched (AQ) and subsequently annealed (TT), and to identify correlations between these results and magnetic features of the alloys. Experimental. The starting ingots with nominal compositions of NdyFe(86–y–x)B14Tx (T: Mo, Nb; x = 2, 4; y = 7, 8) were prepared by arc melting of pure elements in argon atmosphere. The ribbons were produced by rapid solidifi cation of molten alloy (melt-spinning) with a roll speed of 20 m/s. Ribbons were sealed in quartz ampoules in a protective atmosphere of argon. Then, the overquenched ribbons were annealed at 973 K for 20 min. The thickness of the ribbons was about 20 μm. From here, the samples are identifi ed by their x and y values, namely, TxNdy (T: Mo, Nb). The measurements (fl uorescence mode) were carried out at the SXS beamline (source: D04A bending magnet) of the Laboratorio Nacional de Luz Sincrotron, Campinas, Brazil. X-ray absorption data were analyzed using standards procedures: a linear background was fi tted at the pre-edge region and then subtracted from the entire spectrum; the jump of the spectrum

A near edge X-ray absorption fine structure study of oxidized single crystal and polycrystalline diamond surfaces

The influence of oxidizing environments on single crystal diamond and polycrystalline chemical vapor deposited CVD diamond films was studied using the near edge X-ray absorption fine structure (NEXAFS) pre-edge region in both bulk and surface sensitive modes. The NEXAFS of (100) oriented single crystal diamond was measured following (i) exposure to a microwave (MW) hydrogen plasma, (ii) annealing to 1000 °C, (iii) exposure of the as annealed surface to H2O, and (iv) exposure of the as annealed surface to O2. From these measurements particular surface bonding configurations have been assigned to features in the pre-edge structure. The NEXAFS of microcrystalline CVD diamond films was studied following different oxidative treatments using (i) a thermal atomic oxygen (AO) environment, (ii) a hyperthermal (5 eV) AO source, and (iii) an RF oxygen plasma exposure. The nature of the surface layer was found to be different for differently oxidized surfaces. These treatments were carried out as part of a study of CVD diamond durability in the low Earth orbit space environment.

Inhomogeneous Electronic Structure of FeTe1−x Se x Revealed by X-ray Absorption Near Edge Structure

Ab initio Fe K-edge X-ray absorption near edge structure (XANES) calculations on FeTe1−x Se x , based on its crystallographic structure, are compared with experimental data. The calculated XANES spectra are inconsistent with the observed increase of spectral weight in the preedge region of the experimental spectra on the ternary system in which Te is substituted by Se. However, the weighted average of the XANES of FeTe and FeSe binaries reproduce well the observed trend in the experimental spectra of the ternary systems. This suggests that the FeTe1−x Se x is characterized by an inhomogeneous local electronic structure, resulting from a random-alloy like local atomic structure. The calculated unoccupied Fe d-density of states at the Fermi level increases monotonically with Te content unlike the observed change in T c with Te content. T c increases with increasing Te content up to x=0.5; however, the superconductivity is suppressed in the FeTe due to magnetic order, which is not considered in these calculations.

The protonation states of oxo-bridged Mn(IV) dimers resolved by experimental and computational Mn K pre-edge X-ray absorption spectroscopy.

In nature, the protonation of oxo bridges is a commonly encountered mechanism for fine-tuning chemical properties and reaction pathways. Often, however, the protonation states are difficult to establish experimentally. This is of particular importance in the oxygen evolving complex of photosystem II, where identification of the bridging oxo protonation states is one of the essential requirements toward unraveling the mechanism. In order to establish a combined experimental and theoretical protocol for the determination of protonation states, we have systematically investigated a series of Mn model complexes by Mn K pre-edge X-ray absorption spectroscopy. An ideal test case for selective bis-μ-oxo-bridge protonation in a Mn dimer is represented by the system [Mn(IV)2(salpn)2(μ-OHn)2](n+). Although the three species [Mn(IV)2(salpn)2(μ-O)2], [Mn(IV)2(salpn)2(μ-O)(μ-OH)](+) and [Mn(IV)2(salpn)2(μ-OH)2](2+) differ only in the protonation of the oxo bridges, they exhibit distinct differences in the pre-edge region while maintaining the same edge energy. The experimental spectra are correlated in detail to theoretically calculated spectra. A time-dependent density functional theory approach for calculating the pre-edge spectra of molecules with multiple metal centers is presented, using both high spin (HS) and broken symmetry (BS) electronic structure solutions. The most intense pre-edge transitions correspond to an excitation of the Mn 1s core electrons into the unoccupied orbitals of local e(g) character (d(z)(2) and d(xy) based in the chosen coordinate system). The lowest energy experimental feature is dominated by excitations of 1s-α electrons, and the second observed feature is primarily attributed to 1s-β electron excitations. The observed energetic separation is due to spin polarization effects in spin-unrestricted density functional theory and models final state multiplet effects. The effects of spin polarization on the calculated Mn K pre-edge spectra, in both the HS and BS solutions, are discussed in terms of the strength of the antiferromagnetic coupling and associated changes in the covalency of Mn-O bonds. The information presented in this paper is complemented with the X-ray emission spectra of the same compounds published in an accompanying paper. Taken together, the two studies provide the foundation for a better understanding of the X-ray spectroscopic data of the oxygen evolving complex (OEC) in photosystem II.

Structural Characterization of Supported RhI(CO)2/γ-Al2O3Catalysts by Periodic DFT Calculations

Microscopic structures of monodispersed rhodium dicarbonyl species chemisorbed on a ceramic metal-oxide support (?-alumina) have been obtained by density functional theory (DFT) calculations with periodic boundary conditions applied. Several minimum energy structures of species were obtained and their relative energies indicate that, in the most energetically stable geometry, the rhodium atom is coordinated in a square-planar environment and forms a four-membered Rh–O–Al–O ring, with one Al atom octahedrally coordinated. Another docking geometry, close lying in energy, also has a square-planar coordination for the rhodium atom and involves a six-membered Rh–O–Al–O–Al–O ring with one Al octahedrally coordinated and one Al tetrahedrally coordinated. Computed bond lengths were found to be in reasonable agreement with experimental bond lengths as determined by EXAFS spectroscopy. Theoretical Rh K-edge XANES spectra suggest that the pre-edge region probes electronic states localized on the RhI(CO)2 unit, while postedge features probe the electronic states arising from the RhI(CO)2 interaction with the support, which partly depends on the docking geometry of the RhI(CO)2 units.

The role of Hartree–Fock exchange in the simulation of X-ray absorption spectra: A study of photoexcited [formula omitted

We present a theoretical analysis for the K- and L2/3-edge static and picosecond X-ray absorption spectra of [Fe(bpy)3]2+. Simulations of the pre-edge region at the Fe K-edge using time-dependent density functional theory demonstrate the importance of Hartree–Fock exchange within the exchange–correlation functional, especially when describing charge transfer and 1s→3d transitions. This becomes particularly relevant for range separated functionals, for which the incorporation of Hartree–Fock exchange at short range decreases the absolute error in the excitation energy usually observed using TD-DFT. Finally, we compute and interpret the L2/3-edge spectrum using the Restricted Open-Shell Configuration Interaction Singles method.

Temperature dependence of pre-edge features in TiK-edge XANES spectra forATiO3(A= Ca and Sr),A2TiO4(A= Mg and Fe), TiO2rutile and TiO2anatase

XANES (X-ray absorption near-edge structure) spectra of the Ti K-edges of ATiO3 (A = Ca and Sr), A2TiO4 (A = Mg and Fe), TiO2 rutile and TiO2 anatase were measured in the temperature range 20-900 K. Ti atoms for all samples were located in TiO6 octahedral sites. The absorption intensity invariant point (AIIP) was found to be between the pre-edge and post-edge. After the AIIP, amplitudes damped due to Debye-Waller factor effects with temperature. Amplitudes in the pre-edge region increased with temperature normally by thermal vibration. Use of the AIIP peak intensity as a standard point enables a quantitative comparison of the intensity of the pre-edge peaks in various titanium compounds over a wide temperature range.

Communication: X-ray excited optical luminescence from TbCl3 at the giant resonance of terbium

We have studied the optical recombination channels of TbCl(3) using x-ray excited optical luminescence at the N(4,5) absorption edge of Tb (giant resonance) in both the energy and time domain. The luminescence exhibits a relatively fast (5)D(3), and a slow (5)D(4) decay channel in the blue and green, respectively. The rather short lifetime of the (5)D(3) state indicates that the decay is mainly driven by Tb-Tb ion interaction via non-radiative energy transfer (cross-relaxation). At the giant resonance the X-ray Absorption Near Edge Structure (XANES) recorded using partial photoluminescence yield is inverted. In the pre-edge region the contrast of the spectral feature is significantly better in optical XANES than in total electron yield. Changes in the intensity of (5)D(3)-(7)F(5) (544 nm) and (5)D(4)-(7)F(6) (382 nm) optical transitions as the excitation energy is tuned across the giant resonance are also noted. The results provide detailed insight into the dynamics of the optical recombination channels and an alternative method to obtain high sensitivity, high energy resolution XANES at the giant resonance of light emitting rare-earth materials.

An investigation of pyrochlore-type oxides (Yb2Ti2−xFexO7-δ) by XANES

Yb2Ti2O7, which adopts the pyrochlore-type structure, has a large number of applications due to its remarkable chemical and physical properties. Some of these properties (e.g., ionic conduction) can be improved by introducing oxygen vacancies. The effect of composition on the structural and electronic properties needs to be understood to further develop these materials. The Yb2Ti2−xFexO7-δ (0≤x≤0.2) system was synthesized by conventional solid-state reaction methods and investigated by X-ray absorption near edge spectroscopy (XANES), which allows for the examination of the effects of substitution on metal charge and coordination number (CN). Analysis of the transition metal K-edge and L2,3-edge spectra from the Yb2Ti2−xFexO7-δ system allowed for the elucidation of the oxidation state and CN of Fe and Ti metal with increasing Fe content. Fe3+, rather than Fe4+, substitutes for Ti4+ in Yb2Ti2−xFexO7-δ. As x increases, a greater oxygen deficiency lowers the metal CN, as observed by the increase in intensity of the pre-edge region of the Fe K- and Ti K-edge XANES spectra, and by analysis of the Fe L3- and Ti L3-edge XANES spectra. The presence of oxygen defects may make this material appropriate for fuel cell applications, as has been suggested previously.

High-resolution molybdenum K-edge X-ray absorption spectroscopy analyzed with time-dependent density functional theory

X-ray absorption spectroscopy (XAS) is a widely used experimental technique capable of selectively probing the local structure around an absorbing atomic species in molecules and materials. When applied to heavy elements, however, the quantitative interpretation can be challenging due to the intrinsic spectral broadening arising from the decrease in the core-hole lifetime. In this work we have used high-energy resolution fluorescence detected XAS (HERFD-XAS) to investigate a series of molybdenum complexes. The sharper spectral features obtained by HERFD-XAS measurements enable a clear assignment of the features present in the pre-edge region. Time-dependent density functional theory (TDDFT) has been previously shown to predict K-pre-edge XAS spectra of first row transition metal compounds with a reasonable degree of accuracy. Here we extend this approach to molybdenum K-edge HERFD-XAS and present the necessary calibration. Modern pure and hybrid functionals are utilized and relativistic effects are accounted for using either the Zeroth Order Regular Approximation (ZORA) or the second order Douglas-Kroll-Hess (DKH2) scalar relativistic approximations. We have found that both the predicted energies and intensities are in excellent agreement with experiment, independent of the functional used. The model chosen to account for relativistic effects also has little impact on the calculated spectra. This study provides an important calibration set for future applications of molybdenum HERFD-XAS to complex catalytic systems.

An X-ray absorption spectroscopic study of the effect of bond covalency on the electronic structure of Gd2Ti2−xSnxO7

The titanate and stannate pyrochlore-type oxides have been investigated because of their potential applications in different fields. Pyrochlore-type oxides exhibit a wide variety of properties such as fast ionic conduction, resistance to radiation induced structural damage, and ferro- and antiferro-magnetism. These properties mainly depend on the metal-oxygen bonding interactions and electronic structure of the materials, both of which can change with composition. The Gd2Ti(2-x)Sn(x)O7 (0 ≤ x ≤ 2) system was synthesized by the ceramic method and investigated by X-ray absorption near edge spectroscopy (XANES), which allows for the examination of the effect of substitution on bonding and the electronic structure of materials. Examination of metal K- and L3-edge XANES spectra from the Gd2Ti(2-x)Sn(x)O7 system allowed for the elucidation of how the metal-oxygen bond covalency effects the electronic structure of these materials with increasing Sn content. The ionic character of the Ti-O and Gd-O bonds increases while the Sn-O bond becomes more covalent as x increases in the formula, and resulted in changes in energy and/or line shape of the spectra. This study shows that the bonding interactions between metal and oxygen vary strongly with composition, which may affect the fast ionic conduction and resistance to radiation induced structural damage that has been previously reported for these materials. It was also observed in this study that the intensity of the intersite-hybrid peaks found in the pre-edge region of the Ti K-edge XANES spectra resulting from excitations of 1s electrons into next-nearest neighbour Ti 3d states decreased significantly with increasing Sn incorporation.

Nano-structured titanium and aluminium nitride coatings: Study by grazing incidence X-ray diffraction and X-ray absorption and anomalous diffraction

Titanium and aluminium nitride thin films, Ti1−xAlxN (x=0, x=0.5, x=0.68), deposited by reactive magnetron sputtering on silicon substrates are investigated by combining two different X-ray diffraction experiments carried out using synchrotron radiation. Grazing-incidence X-ray diffraction and Ti K-edge diffraction anomalous near edge structure spectroscopy provide information on the micro- and nano-structure of the films respectively, which play a crucial role in the functionality of coatings. The spectroscopic data of Ti0.50Al0.50N film show that Ti atoms in crystallized domains and grain boundaries are all in octahedral cubic local order, but their growth mode is quite different. It is found that the crystallized part of the Ti0.50Al0.50N film has a single-crystalline nature, whereas the TiN one presents a fibrillar microstructure. For Ti0.32Al0.68N film, grazing-incidence X-ray diffraction provides information on the uniaxial texture along the [001] direction of the hexagonal lattice. A sharp Ti K pre-edge peak is observed in diffraction anomalous near edge spectrum that definitely shows that Ti atoms are incorporated in the hexagonal lattice of those fibrillar domains. Moreover, the difference observed between Ti K-edge diffraction anomalous and X-ray absorption pre-edge regions proves that a significant part of Ti atoms is located in nanocrystallites with cubic symmetry outside of the crystallized domains.

Interplay of ballistic and chemical effects in the formation of structural defects for Sn and Pb implanted silica

The electronic structures of Sn and Pb implanted SiO2 are studied using soft X-ray absorption (XAS) and emission (XES) spectroscopy. We show, using reference compounds and ab initio calculations, that the presence of PbO and SnO interactions can be detected in the pre-edge region of the oxygen K-edge XAS. Via analysis of this interaction-sensitive pre-edge region, we find that Pb implantation results primarily in the clustering of Pb atoms. Conversely, with Sn implantation using identical conditions, strong SnO interactions are present, showing that Sn is coordinated with oxygen. The varying results between the two ion types are explained using both ballistic considerations and density functional theory calculations. We find that the substitution of Pb into Si sites in SiO2 requires much more energy than substituting Sn in these same sites, primarily due to the larger size of the Pb ions. From these calculated formation energies it is evident that Pb requires far higher temperatures than Sn to be soluble in SiO2. These results help explain the complex processes which take place upon implantation and determine the final products.

Spin-state studies with XES and RIXS: From static to ultrafast

We report on extending hard X-ray emission spectroscopy (XES) along with resonant inelastic X-ray scattering (RIXS) to study ultrafast phenomena in a pump-probe scheme at MHz repetition rates. The investigated systems include low-spin (LS) FeII complex compounds, where optical pulses induce a spin-state transition to their (sub)nanosecond-lived high-spin (HS) state. Time-resolved XES clearly reflects the spin-state variations with very high signal-to-noise ratio, in agreement with HS–LS difference spectra measured at thermal spin crossover, and reference HS–LS systems in static experiments, next to multiplet calculations. The 1s2p RIXS, measured at the Fe 1s pre-edge region, shows variations after laser excitation, which are consistent with the formation of the HS state. Our results demonstrate that X-ray spectroscopy experiments with overall rather weak signals, such as RIXS, can now be reliably exploited to study chemical and physical transformations on ultrafast time scales.

Spin-ordering mediated orbital hybridization in CoO at high pressures

We demonstrate a strong dependence of high-resolution 1$s$3$p$ (or ${K}_{\ensuremath{\beta}}$) resonant x-ray emission spectroscopy (RXES) on local spin correlations at high pressure. We show that the pre-edge region in ${K}_{\ensuremath{\beta}}$ RXES of CoO can be separated into a local quadrupolar contribution and nonlocal dipolar intensity, which arises from the mixing of cobalt 4$p$ states with neighboring cobalt 3$d$ orbitals. For pressures of 0--12 GPa, we observe a twofold increase in nonlocal contributions with respect to local contributions. For pressures greater than 12 GPa, the ratio remains nearly constant. The observed pressure dependence and strong intensity changes cannot be explained by a conventional picture of bond-length shortening and is ascribed to increased intersite 4$p$-3$d$ mixing due to changes in magnetic ordering. These results are supported by theoretical calculations.

Nonresonant inelastic x-ray scattering from actinides and rare earths

Calculated multiplet spectra are presented for the electric-multipole transitions in nonresonant inelastic x-ray scattering (NIXS) at the ${O}_{4,5}$ edges of actinides and the ${N}_{4,5}$ edges of rare earths. The multiplet structure is characteristic for the $f$ count as well as for the angular momentum coupling. As a remarkable peculiarity, the calculations for the actinide ${O}_{4,5}$ edge show that the higher multipole spectra resemble $jj$ coupling, whereas the dipole spectrum is close to $LS$ coupling. The exchange integral ${G}^{1}(5d,5f)$ is responsible for a shift of $\ensuremath{\sim}$15 eV of the dipole spectrum to higher energy. This high energy is conserved by a sum rule for the average energy of the accessible final states. In the higher multipole spectra, where the allowed final states are different, the splitting by the $5d$ spin-orbit interaction exceeds that of the electrostatic interaction. This leads to a distinct spin-orbit-split doublet structure, with a broad first peak and a narrower second peak. According to the spin-orbit sum rule, the $5f$ spin-orbit interaction per hole is linearly related to the core-level branching ratio, where the proportionality factor is opposite in sign for the $k=3$ and 5 spectra. Compared to the $k=3$ spectrum, the $k=5$ spectrum has overall a lower intensity in the low-energy region but a higher intensity in the high-energy region, especially for the heavier elements in the series. The NIXS at the rare-earth ${N}_{4,5}$ edge is quite different since the electrostatic and core spin-orbit splitting in the $k=3$ and 5 spectra are of similar size. The pre-edge region of the rare-earth spectra, where the linewidth is narrow, shows a rich multiplet structure that depends strongly on the initial-state $J$ value.

Experimental evidence of thermal fluctuations on the x-ray absorption near-edge structure at the aluminumKedge

After a review of temperature-dependent experimental x-ray absorption near-edge structure (XANES) and related theoretical developments, we present the Al $K$-edge XANES spectra of corundum and beryl for temperature ranging from 300 to 930 K. These experimental results provide a first evidence of the role of thermal fluctuation in XANES at the Al $K$ edge, especially, in the pre-edge region. The study is carried out by polarized XANES measurements of single crystals. For any orientation of the sample with respect to the x-ray beam, the pre-edge peak grows and shifts to lower energy with temperature. In addition, temperature induces modifications in the position and intensity of the main XANES features. First-principles DFT calculations are performed for both compounds. They show that the pre-edge peak originates from forbidden 1$s$$\ensuremath{\rightarrow}$$3s$ transitions induced by vibrations. Three existing theoretical models are used to take vibrations into account in the absorption cross-section calculations: (i) an average of the XANES spectra over the thermal displacements of the absorbing atom around its equilibrium position, (ii) a method based on the crude Born-Oppenheimer approximation where only the initial state is averaged over thermal displacements, and (iii) a convolution of the spectra obtained for the atoms at the equilibrium positions with an approximate phonon spectral function. The theoretical spectra so obtained permit to qualitatively understand the origin of the spectral modifications induced by temperature. However, the correct treatment of thermal fluctuation in XANES spectroscopy requires more sophisticated theoretical tools.

Interference phenomena of synchrotron radiation in TEY spectra for silicon-on-insulator structure

The general matrix theory of the photoelectron/fluorescence excitation in anisotropic multilayer films at the total reflection condition of X-rays has been developed. In a particular case the theory has been applied to explain the oscillation structure of L(2,3) XANES spectra for a SiO(2)/Si/SiO(2)/c-Si sample in the pre-edge region which has been observed by a sample current technique at glancing angles of synchrotron radiation. Remarkably the phase of the oscillations is reversed by a ∼2° angle variation. The observed spectral features are found to be a consequence of waveguide mode creation in the middle layer of strained Si, which changes the radiation field amplitude in the top SiO(2) layer. The fit of the data required the correction of the optical constants for Si and SiO(2) near the Si L(2,3)-edges.