EXAFS Spectra Research Articles

Bioinspired Design of a Cu–Zn–Imidazolate Mesoporous Silica Catalyst System for Superoxide Dismutation

An imidazolate-bridged Cu(II)–Zn(II) model compound, Cu(II)-diethylenetriamino-μ-imidazolato-Zn(II)-tris(aminoethyl)amine perchlorate (CZS), was synthesized and encapsulated into various mesoporous silicas (MPSs) to mimic the structure and functionalities of copper zinc superoxide dismutase (CuZnSOD). Two encapsulation methods were studied: covalent bonding and ionic exchange. The encapsulated CZS yielded good thermal stability and enhanced superoxide disproportionation activity in various designed MPS solids mimicking the environment and functionality of native CuZnSOD enzyme. Ionic exchange method generally gave better SOD-like catalytic activities than covalent bonding because the local attraction of SiO– to Cu(II) opens up the active axial position. We employed several spectroscopic techniques: UV–vis, EPR, and EXAFS to characterize the active site-Cu(II) ion of the immobilized CZS and to obtain the structural information of Cu(II) and Zn(II) centers confined in MPS. In the analyses of EPR and EXAFS spectra, we established the geometry of confined copper center changed to a more distorted square planar symmetry than free-form CZS, which can accommodate superoxide anion radical (O2–•) coordinating to copper center in the proper manner. As the result of confinement in MPS, the active site of Cu(II) center gives rise to much increased SOD activities. Inspired by the local environments of guanidinium group in the native CuZnSOD enzyme in facilitating the superoxide dismutation, we further modified the silanol surface of MPS with N-trimethoxysilylpropyl-N,N,N-trimethyl-ammonium chloride (TMAC) preparing MPS-N+ to facilitate the transport of superoxide anion radicals. Optimized activities are associated with fine-tuning of pore size, surface acidity, and positively charged functional group. The spectroscopic studies allow us to establish the structure–reactivity relationship between the nanostructure of MPS and the efficacy of the superoxide dismutation in MPS and to elucidate the mechanism of the SOD-like activity of immobilized model compound in MPS.

Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite

Due to their redox reactivity, surface sorption characteristics, and ubiquity as corrosion products or as minerals in natural sediments, iron(II)-bearing minerals control to a large extent the environmental fate of actinides. Pu-L(III)-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous (242)Pu(III) and (242)Pu(V) reacted with magnetite, mackinawite, and chukanovite under anoxic conditions. As Pu concentrations in the liquid phase were rapidly below detection limit, oxidation state and local structure of Pu were determined for Pu associated with the solid mineral phase. Pu(V) was reduced in the presence of all three minerals. A newly identified, highly specific Pu(III)-sorption complex formed with magnetite. Solid PuO(2) phases formed in the presence of mackinawite and chukanovite; in the case of chukanovite, up to one-third of plutonium was also present as Pu(III). This highlights the necessity to consider, under reducing anoxic conditions, Pu(III) species in addition to tetravalent PuO(2) for environmental risk assessment. Our results also demonstrate the necessity to support thermodynamic calculations with spectroscopic data.

Pyrovanadolysis, a Pyrophosphorolysis-like Reaction Mediated by Pyrovanadate, Mn2+, and DNA Polymerase of Bacteriophage T7

DNA polymerases catalyze the 3'-5'-pyrophosphorolysis of a DNA primer annealed to a DNA template in the presence of pyrophosphate (PP(i)). In this reversal of the polymerization reaction, deoxynucleotides in DNA are converted to deoxynucleoside 5'-triphosphates. Based on the charge, size, and geometry of the oxygen connecting the two phosphorus atoms of PP(i), a variety of compounds was examined for their ability to carry out a reaction similar to pyrophosphorolysis. We describe a manganese-mediated pyrophosphorolysis-like activity using pyrovanadate (VV) catalyzed by the DNA polymerase of bacteriophage T7. We designate this reaction pyrovanadolysis. X-ray absorption spectroscopy reveals a shorter Mn-V distance of the polymerase-VV complex than the Mn-P distance of the polymerase-PP(i) complex. This structural arrangement at the active site accounts for the enzymatic activation by Mn-VV. We propose that the Mn(2+), larger than Mg(2+), fits the polymerase active site to mediate binding of VV into the active site of the polymerase. Our results may be the first documentation that vanadium can substitute for phosphorus in biological processes.

Applying angle resolved EXAFS spectroscopy to the study of layers and interfaces in metallic multilayer nanoheterostructures

A new technique is proposed for determining concentration profiles and local atomic structure at various depths in metallic multilayer films, using the dependence of the EXAFS spectra on the angle of incidence. The abilities of this technique are demonstrated via modelling calculations for the three-layered Cr/Fe/Cr system. The first experimental results for thin Cr films on sapphire substrates are obtained.

Ge-modified Pt/SiO2 catalysts used in preferential CO oxidation (CO-PROX)

A bimetallic PtGe catalyst was prepared by a controlled surface reaction and studied for the PROX reaction. The good activity of the bimetallic catalyst can be assigned to the presence of a “noble metal-oxidized metal promoter” ensemble site in close contact, the noble metal (Pt) being the CO adsorption site and the oxidized metal promoter (Ge) the O 2 adsorption site. The stability of the PtGe clusters is confirmed by the EXAFS spectra and activity measurements in the preferential oxidation reaction that show similar values for samples subjected to different oxidation–reduction cycles. ► PtGe catalyst presents a "noble metal-oxidized metal promoter" ensemble site. ► Pt is the CO adsorption site and the GeOx the O 2 adsorption site. ► The stability of the PtGe clusters is confirmed by EXAFS and activity measurements. ► PtGe systems, unlike PtSn systems, do not show alloy formation. ► Electronic effect with charge transfer from Pt to Ge is evidenced by XANES spectra.

Identification of the Active Species Generated from Supported Pd Catalysts in Heck Reactions: An in situ Quick Scanning EXAFS Investigation

Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 °C was reached. At the same time, the conversion of PhBr started, and during the following "active phase" of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd(0) clusters with a size of ∼2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr(4)](2-), [Pd(2)Br(6)](2-)). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions.

Products of abiotic U(VI) reduction by biogenic magnetite and vivianite

Reductive immobilization of uranium by the stimulation of dissimilatory metal-reducing bacteria (DMRB) has been investigated as a remediation strategy for subsurface U(VI) contamination. In those environments, DMRB may utilize a variety of electron acceptors, such as ferric iron which can lead to the formation of reactive biogenic Fe(II) phases. These biogenic phases could potentially mediate abiotic U(VI) reduction. In this work, the DMRB Shewanella putrefaciens strain CN32 was used to synthesize two biogenic Fe(II)-bearing minerals: magnetite (a mixed Fe(II)–Fe(III) oxide) and vivianite (an Fe(II)-phosphate). Analysis of abiotic redox interactions between these biogenic minerals and U(VI) showed that both biogenic minerals reduced U(VI) completely. XAS analysis indicates significant differences in speciation of the reduced uranium after reaction with the two biogenic Fe(II)-bearing minerals. While biogenic magnetite favored the formation of structurally ordered, crystalline UO 2, biogenic vivianite led to the formation of a monomeric U(IV) species lacking U–U associations in the corresponding EXAFS spectrum. To investigate the role of phosphate in the formation of monomeric U(IV) such as sorbed U(IV) species complexed by mineral surfaces, versus a U(IV) mineral, uranium was reduced by biogenic magnetite that was pre-sorbed with phosphate. XAS analysis of this sample also revealed the formation of monomeric U(IV) species suggesting that the presence of phosphate hinders formation of UO 2. This work shows that U(VI) reduction products formed during in situ biostimulation can be influenced by the mineralogical and geochemical composition of the surrounding environment, as well as by the interfacial solute–solid chemistry of the solid-phase reductant.

Role of Local and Electronic Structural Changes with Partially Anion substitution Lithium Manganese Spinel Oxides on Their Electrochemical Properties: X-ray Absorption Spectroscopy Study

The electronic and local structures of partially anion-substituted lithium manganese spinel oxides as positive electrodes for lithium-ion batteries were investigated using X-ray absorption spectroscopy (XAS). LiMn(1.8)Li(0.1)Ni(0.1)O(4-η)F(η) (η = 0, 0.018, 0.036, 0.055, 0.073, 0.110, 0.180) were synthesized by the reaction between LiMn(1.8)Li(0.1)Ni(0.1)O(4) and NH(4)HF(2). The shift of the absorption edge energy in the XANES spectra represented the valence change of Mn ion with the substitution of the low valent cation as Li(+), Ni(2+), or F(-) anion. The local structural change at each compound with the amount of a Jahn-Teller Mn(3+) ion could be observed by EXAFS spectra. The discharge capacity of the tested electrode was in the order of LiMn(2)O(4) > LiMn(1.8)Li(0.1)Ni(0.1)O(4-η)F(η) (η = 0.036) > LiMn(1.8)Li(0.1)Ni(0.1)O(4) while the cycleability was in the order of LiMn(1.8)Li(0.1)Ni(0.1)O(4-η)F(η) (η = 0.036) ≈ LiMn(1.8)Li(0.1)Ni(0.1)O(4) > LiMn(2)O(4). It was clarified that LiMn(1.8)Li(0.1)Ni(0.1)O(4-η)F(η) has a good cycleability because of the anion doping effect and simultaneously shows acceptable rechargeable capacity because of the large amount of the Jahn-Teller Mn(3+) ions in the pristine material.

Rational design, synthesis and evaluation of first generation inhibitors of the Giardia lamblia fructose-1,6-biphosphate aldolase

Inhibitors of the Giardia lamblia fructose 1,6-bisphosphate aldolase ( GlFBPA), which transforms fructose 1,6-bisphosphate (FBP) to dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, were designed based on 3-hydroxy-2-pyridone and 1,2-dihydroxypyridine scaffolds that position two negatively charged tetrahedral groups for interaction with substrate phosphate binding residues, a hydrogen bond donor to the catalytic Asp83, and a Zn 2+ binding group. The inhibition activities for the GlFBPA catalyzed reaction of FBP of the prepared alkyl phosphonate/phosphate substituted 3-hydroxy-2-pyridinones and a dihydroxypyridine were determined. The 3-hydroxy-2-pyridone inhibitor 8 was found to bind to GlFBPA with an affinity (K i = 14 μM) that is comparable to that of FBP (K m = 2 μM) or its inert analog TBP (K i = 1 μM). The X-ray structure of the GlFBPA–inhibitor 8 complex (2.3 Å) shows that 8 binds to the active site in the manner predicted by in silico docking with the exception of coordination with Zn 2+. The observed distances and orientation of the pyridone ring O=C–C–OH relative to Zn 2+ are not consistent with a strong interaction. To determine if Zn 2+coordination occurs in the GlFBPA–inhibitor 8 complex in solution, EXAFS spectra were measured. A four coordinate geometry comprised of the three enzyme histidine ligands and an oxygen atom from the pyridone ring O=C–C–OH was indicated. Analysis of the Zn 2+ coordination geometries in recently reported structures of class II FBPAs suggests that strong Zn 2+ coordination is reserved for the enediolate-like transition state, accounting for minimal contribution of Zn 2+ coordination to binding of 8 to GlFBPA.

Capabilities of coupling dissimilar computer codes

A method for automatically controlling commercial computer codes is described. The coupling of the automation system of the EXAFS spectrometer controlled by a DOS-operated computer that we developed and the commercial program of a semiconductor-detector control installed on a Windows-operated computer is given as the example. The described complex system is used for the automation of the intermediate processing of amplitude spectra while measuring EXAFS spectra at the Kurchatov Synchrotron Radiation Source.

Oxygen-enhanced water gas shift on ceria-supported Pd–Cu and Pt–Cu bimetallic catalysts

Aiming at enhancing H 2 production in water gas shift (WGS) for fuel cell application, a small amount of oxygen was added to WGS reaction toward oxygen-enhanced water gas shift (OWGS) on ceria-supported bimetallic Pd–Cu and Pt–Cu catalysts. Both CO conversion and H 2 yield were found to increase by the oxygen addition. The remarkable enhancement of H 2 production by O 2 addition in short contact time was attributed to the enhanced shift reaction, rather than the oxidation of CO on catalyst surface. The strong dependence of H 2 production rate on CO concentration in OWGS kinetic study suggested O 2 lowers the CO surface coverage. It was proposed that O 2 breaks down the domain structure of chemisorbed CO into smaller domains to increase the chance for coreactant (H 2O) to participate in the reaction and the heat of exothermic surface reaction helping to enhance WGS kinetics. Pt–Cu and Pd–Cu bimetallic catalysts were found to be superior to monometallic catalysts for both CO conversion and H 2 production for OWGS at 300 °C or lower, while the superiority of bimetallic catalysts was not as pronounced in WGS. These catalytic properties were correlated with the structure of the bimetallic catalysts. EXAFS spectra indicated that Cu forms alloys with Pt and with Pd. TPR demonstrated the strong interaction between the two metals causing the reduction temperature of Cu to decrease upon Pd or Pt addition. The transient pulse desorption rate of CO 2 from Pd–Cu supported on CeO 2 is faster than that of Pd, suggesting the presence of Cu in Pd–Cu facilitate CO 2 desorption from Pd catalyst. The oxygen storage capacity (OSC) of CeO 2 in the bimetallic catalysts indicates that Cu is much less pyrophoric in the bimetallic catalysts due to lower O 2 uptake compared to monometallic Cu. These significant changes in structure and electronic properties of the bimetallic catalysts are the result of highly dispersed Pt or Pd in the Cu nanoparticles.

Fischer−Tropsch Synthesis: An In-Situ TPR-EXAFS/XANES Investigation of the Influence of Group I Alkali Promoters on the Local Atomic and Electronic Structure of Carburized Iron/Silica Catalysts

The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomic structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K = Rb = Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Hägg carbide and Fe3O4. After 10 h of CO/He treatment at 290 °C, the main component observed was Hägg carbide. A model containing Hägg and ε-carbides, and Fe3O4, was also investigated. However, the r-factor was not significantly impacted by including ε-carbide in the fitting, and the resulting contribution of ε-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests.

Structure and stability of cadmium chloride complexes in hydrothermal fluids

The identity and stability of aqueous species formed by cadmium in H 2O–Na/LiCl–HCl–HNO 3 solutions were investigated using in situ X-ray absorption spectroscopy (XANES and EXAFS) at 20–450 °C and 1–600 bar, combined with solubility measurements of CdO and CdS solids at 350–400 °C and 300–600 bar. Results show that aqueous Cd speciation is dominated by the cation Cd(H 2O) 6 2+ in acidic Cl-free solutions, and by chloride species CdCl m (H 2O) n 2 − m over a wide range of temperatures (20 ≤ T ≤ 450 °C), acidities (1 ≤ pH ≤ 8), and chloride concentrations (0.04 ≤ m Cl ≤ 18 mol/kg H 2O). EXAFS spectra from chloride solutions show that with increasing T and m Cl the average number of Cl atoms increases from 1 to ∼ 3 (± 0.6), accompanied by a decrease of the number of O from 6 to ∼ 1 (± 0.7), in the nearest coordination sphere around Cd. Average Cd–Cl distances in Cd complexes vary in the range 2.41–2.52 (± 0.02) Å, increasing with increasing m Cl and decreasing with increasing T, while Cd–O distances remain constant within errors at 2.30 (± 0.05) Å in the whole T and m Cl range investigated. This evolution of the Cd atomic environment is accompanied by changes in geometry of the dominant Cd–O/Cl species from octahedral-like to tetrahedral-like with increasing T and m Cl, as shown by XANES spectra analyses. These structural data are consistent with the formation of octahedral Cd(H 2O) 6 2+ and CdCl(H 2O) 5 + and tetrahedral CdCl 2(H 2O) 2 0, CdCl 3(H 2O) − and CdCl 4 − species at T ≤ 200–300 °C and m Cl ≤ 18. At T > 300 °C, aqueous Cd speciation is dominated by two complexes, CdCl 2(H 2O) 2 0 and CdCl 3(H 2O) − in a wide m Cl range (0.04–5 m). The stability constants of these complexes, derived from solubility measurements at 400 °C and 600 bar are ∼ 6 orders of magnitude higher than available HKF-model predictions based on low- T data. The new data on aqueous Cd complexes may be used for interpretation of Cd/Zn ratios measured in natural hydrothermal saline fluids and volcanic vapors in which the greater stability of Cd chloride species as compared to their Zn analogs is likely to be responsible for the elevated Cd/Zn ratios in comparison to crustal rocks and Cl-poor waters.

CopK from Cupriavidus metallidurans CH34 Binds Cu(I) in a Tetrathioether Site: Characterization by X-ray Absorption and NMR Spectroscopy

Cupriavidus metallidurans CH34 is a bacterium that is resistant to high metal concentrations in the environment. Increased copper resistance is associated with the cop cluster on the large plasmid pMOL30 that is composed of at least 21 genes. The copK gene encodes a 74 residue periplasmic protein whose expression is strongly upregulated in the presence of copper. CopK was previously shown to cooperatively bind Cu(I) and Cu(II) in distinct, specific sites. The solution structure of Cu(I)-CopK and the characterization of the Cu(I) site by X-ray absorption spectroscopy and NMR are reported here. EXAFS spectra are in agreement with a tetrathioether Cu(I) site, providing so far unique spectral information on a 4S-coordinated Cu(I) in a protein. The methionine residues forming the Cu(I) site, M28, M38, M44, and M54, are identified by NMR. We propose the chemical shift of the methionine C(epsilon) as a new and sensitive probe for the detection of Cu(I) bound to thioether groups. The solution structure of Cu(I)-CopK demonstrates that Cu(I) binding induces a complete structural modification with the disruption of the second beta-sheet and a rotation of the C-terminal part of nearly 180 degrees around a hinge formed by asparagine 57. This conformational change is directly related to the loss of the dimer interface and most probably to the formation of the Cu(II) site involving histidine 70. The solution structure of Cu(I)-CopK therefore provides the molecular basis for the understanding of the Cu(I)/Cu(II) binding cooperativity.

TiO2 doping by hydroxyurea at the nucleation stage: towards a new photocatalyst in the visible spectral range

We report an original method of preparation of OCN-doped TiO(2) for photocatalysis in the visible spectral range. The preparation is achieved by a sol-gel route using titanium tetraisopropoxide precursor. Special attention was paid to fluid micromixing, which enables homogeneous reaction conditions in the reactor bulk and monodispersity of the produced clusters/nanoparticles. The dopant hydroxyurea (HyU, CH(4)N(2)O(2)) is injected into the reactive fluid at the nucleation stage, which lasts tens of milliseconds. The doping results in a strong yellow coloration of the nanocolloids due to the absorption band in the spectral range 380-550 nm and accelerates the aggregation kinetics of both nuclei at the induction stage and sub-nuclei units (clusters) at the nucleation stage. FTIR, Raman and UV-visible absorption analyses show the formation of a stable HyU-TiO(2) complex. EXAFS spectra indicate no appreciable changes of the first-shell Ti atom environment. The doping agent takes available surface sites of TiO(2) clusters/nanoparticles attaining ∼10% molar loading. The reaction kinetics then accelerates due to a longer collisional lifetime between nanoparticles induced by the formation of a weak [double bond, length as m-dash]OTi bond. The OCN-group bonding to titanium atoms produces a weakening of the C[double bond, length as m-dash]O double bond and a strengthening of the C-N and N-O bonds.

Processing of amplitude spectra in measurements with ionizing radiation by low energy resolution detectors

A method using intermediate processing of amplitude (energy) spectra is described. The method is an alternative to the known differential discrimination method, which is widely applied in measurements of EXAFS spectra and in Mossbauer and diffraction measurements. The method is efficient in cases in which the radiation spectrum contains lines poorly resolved in the used detector. For example, if weak fluorescence of a low-concentration impurity is detected in a sample against the background of intense fluorescence of the matrix, the described method allows one to noticeably improve the quality of the obtained information.

Quantum relativistic investigation about the coordination and bonding effects of different ligands on uranyl complexes

The coordination and bonding effects of equatorial ligands such as fluoride (F −), chloride (Cl −), cyanide (CN −), isocyanide (NC −), and carbonate (CO 3 −2) on uranyl dication (UO 2 2+) has been studied using relativistic density functional theory. The ZORA Hamiltonian was applied for the inclusion of relativistic effects taking into account all the electrons for the optimization and the explicit inclusion of spin–orbit coupling effects. Geometry optimizations including the counterions and frequencies analysis were carried out with PW91 and PBE functional. Solvents effects were considered by using the conductor like screening model (COSMO) for water and acetonitrile. The Time-Dependent Density Functional Theory (TDDFT) was used to calculate the excitation energies with GGA SAOP functional and the electronic transitions were analyzed using double group irreducible representations. The theoretical results are in a good agreement with experimental IR, Raman and EXAFS spectra and previous theoretical results. New information about the effect of different (donor and acceptors) ligands on the bonding of uranyl ion and on the electronic transitions involved in these complexes is provided with a possible impact on the understanding of the uranyl coordination chemistry.

Confirmation of standard error analysis techniques applied to EXAFS using simulations

Proper application of error analysis techniques remains uncommon in most EXAFS analyses. Consequently, many researchers in the community remain distrustful of parameter-error estimates. Here, we demonstrate the accuracy of conventional methods through r-space fits to simulated data. Error estimates are determined as a function of r by averaging many scan simulations in r-space. The statistical-χ2 value can then be calculated. Since ⟨χ2⟩ corresponds to the degrees of freedom in a fit, we check Stern's rule for the number of independent data points in an EXAFS spectra. Finally, we apply these simple methods to real data from a Cu foil, highlighting the overwhelming role of systematic errors in theoretical backscattering functions and pointing to the ultimate power of the EXAFS technique if such errors could be removed.

Polydispersity and EXAFS simulations

EXAFS is an important experimental technique for determining the local atomic structure of nanoclusters embedded in a bulk material. In practical cases, nanocluster samples do not contain homogeneous clusters of just one size, and the average cluster size is strongly influenced by the specific distribution of cluster sizes. Combinations of different cluster sizes might provide very similar results; this issue is called polydispersity. The goal of this study is to understand if there are any principal limitations for EXAFS studies related to polydispersity. Here a new approach based on EXAFS simulations followed by linear combination (LC) on EXAFS spectra is presented. The simulations were performed on pure Cu and binary Cu-Fe clusters. The main result of this study concerns the proof that polydispersity does not affect XAFS studies on nano-clusters within a size of up to 140 atoms.

High resolution in EXAFS data analysis of multilayer nanostuctures

The regularization method of solving ill-posed problem is used to determine three partial interatomic distances from one EXAFS spectrum. The mathematical procedure and the experimental results of the EXAFS analysis for multilayer FeNi/V nanostructures are discussed. The measurements were performed using ESRF facilities (BM line 26A, Grenoble, France). The Fe, Ni and V edge absorption spectra were recorded in fluorescence and transmission modes at room temperature for foils Fe, Ni, V and for multilayer nanostructures [Fe 82Ni 18(5ML)/V(6ML)] 25, [Fe 82Ni 18(10ML)/V(6ML)] 25, films Fe 82Ni 18, Fe, Ni, V (1000 Å thickness) in fluorescence mode only. All film samples were prepared at University Uppsala (Sweden). Peculiar features of the method are high resolution for overlapping in r-space shells and high accuracy in the determination of partial interatomic distances.