The effect of self‐consistent potentials on EXAFS analysis

A theory program intended for use with extended X‐ray‐absorption fine structure (EXAFS) spectroscopy and based on the popular FEFF8 is presented. It provides an application programming interface designed to make it easy to integrate high‐quality theory into EXAFS analysis software. This new code is then used to examine the impact of self‐consistent scattering potentials on EXAFS data analysis by methodical testing of theoretical fitting standards against a curated suite of measured EXAFS data. For each data set, the results of a fit are compared using a well characterized structural model and theoretical fitting standards computed both with and without self‐consistent potentials. It is demonstrated that the use of self‐consistent potentials has scant impact on the results of the EXAFS analysis.     

An investigation of the use of the Hedin-Lundqvist exchange and correlation potential in EXAFS data analysis

In real systems, inelastic processes remove photoelectrons from the elastic scattering channel. This reduces the amplitude of the EXAFS. Traditionally the discrepancies between experimental and theoretical amplitudes were treated by including two semi-empirical reduction factors in the data analysis. Some inelastic effects may, however, be modelled more rigorously using a complex exchange and correlation potential, for example the Hedin-Lundqvist (HL) potential used in most EXAFS data-analysis programs. In this paper a systematic study of the effects of the HL potential on the calculated EXAFS amplitudes is presented. Expressions are derived whereby the EXAFS amplitudes may be examined in the presence of an arbitrary complex potential independently to the rest of the EXAFS signal. These results are used to study the effects of the HL potential on EXAFS data analysis in detail.     

扩展X射线吸收精细结构(EXAFS)谱

本文评述了近年来发展起来的扩展X射线吸收精细结构(EXAFS)谱的理论、实验及数据分析方法,对于EXAFS的应用作了介绍,也对EXAFS的优点、局限性及发展的相关技术进行了讨论。     

Determination of structural parameters from EXAFS: Application to solutions and catalysts

Summary A review is presented of applications of extended X-ray absorption fine-structure (EXAFS) spectroscopy to the determination of structural detail in inorganic solutions and supported metal catalyst particles. Similarities in these applications are noted and the possible role of disorder on the reliability of the EXAFS results is discussed. A brief summary of data analysis procedures is presented with examples. Paper presented at the Workshop on ?EXAFS Data Analysis in Disordered Systems?, held in Parma, October 5–7, 1981.     

Path degeneracy and EXAFS analysis of disordered materials

Analysis of EXAFS data measured on a material with a disordered local configuration environment around the absorbing atom can be challenging owing to the proliferation of photoelectron scattering paths that must be considered in the analysis. In the case where the absorbing atom exists in multiple inequivalent sites, the problem is compounded by having to consider each site separately. A method is proposed for automating the calculation of theory for inequivalent sites, then averaging the contributions from sufficiently similar scattering paths. With this approach, the complexity of implementing a successful fitting model on a highly disordered sample is reduced. As an example, an analysis of Ti K‐edge data on zirconolite, CaZrTi₂O₇, which has three inequivalent Ti sites, is presented.     

X-ray fine structure study and bonding in some ferrites

Extended X-ray absorption fine structure (EXAFS) of the iron K edges in copper, cobalt, zinc and manganese ferrites is studied and the EXAFS parameter α of the Lytle, Sayers and Stern's theory is shown to be a measure of fractional covalency. The linear relationship between metal to oxygen (iron to oxygen) distance and the parameter α leads us to conclude that the copper ferrite is the most covalent among the four ferrites studied in the present investigation.     

EXAFS方法在金属玻璃结构研究中的误差分析

本文对EXAFS方法在金属玻璃结构研究中所得径向距离的误差进行了分析。所做的模拟计算表明当前流行的关于这个问题的误差理论不能圆满地解释这一现象。本文分析了玻璃态和晶态的相移函数,指出在相似的体系中玻璃态和晶态相移函数与k的关系采用线性函数近似时,其斜率可能是不同的,而且无序度相差越大,两者的偏差可能也越大。在EXAFS数据处理过程中,没有考虑这种偏差的影响是造成EXAFS方法结果中径向距离误差的一个原因。本文还提出了一个修正参考样品(晶态)相移函数斜率偏差的方法,并将它应用于Cu₅₅Zr₄₅金属玻璃的EXAFS数据处理中,所得结果与用X射线异常散射方法测定的结果符合得很好。 关键词：     

The effect of statistical noise on structural parameters in EXAFS data analysis

Summary We analyse the effect of the statistical noise on the physical parameters derived from an EXAFS spectrum. Two separate classes of experimental cases are discussed, namely heavy and light backscatterers. The weight of the noise in the two cases is, in fact, quite different, depending on thek range useful for the analysis of the data. We find that for heavy backscatterers the statistical noise can usually be neglected, while for light backscatterers it can be one of the main limitations in obtaining reliable results from EXAFS. Paper presented at the Workshop on ?EXAFS Data Analysis in Disordered Systems?, held in Parma, October 5–7, 1981.     

Surface‐sensitive reflection‐mode EXAFS from layered sample systems: the influence of surface and interface roughness

The calculation of reflection‐mode grazing‐incidence X‐ray absorption spectra from single surfaces and (multi‐)layered systems is studied here. In particular, the influence of the surface and interface roughness was investigated in detail. Simulations of grazing‐incidence reflection‐mode EXAFS spectra using a simple Fresnel theory neglecting any effect of roughness are compared with the Névot–Croce model and the elaborated distorted‐wave Born approximation which both include surface and interface roughness. Data are presented for clean gold surfaces, where the strong influence of the surface roughness on the resulting spectra is demonstrated. Furthermore, in the case of layered systems, the influence of both the outer (air or vacuum side) surface roughness and the inner interface roughness on the reflection‐mode EXAFS spectra is evaluated. The practical consequences of the observed correlations are discussed, and a quantitative data analysis of a copper sample that was oxidized in ambient air for several months is shown, including the evaluation of specular reflectivity profiles at fixed energy.     

A study of EXAFS associated with the Re LIII discontinuity in metal and some compounds

The authors have reanalysed the data on the EXAFS associated with the rhenium L_III absorption discontinuity in the metal and in some of its compounds, and have interpreted them in the light of the recent theory proposed by Lytle, Sayers and Stern. It has been shown that the α parameter of EXAFS is a measure of valency and that the metallic valency of rhenium is two.     

能量分辨率对EXAFS的影响

测量EXAFS谱时,系统的能量分辨率是影响其结果的重要因素。本文提出了分辨率影响结构参数的计算方法,以Ge,Ni样品为例,测量及计算了不同分辨率下的结构参数的差异。测量结果与计算结果符合得很好,讨论了分辨率如何影响EXAFS谱及结构参数,给出了修正方法。 关键词：     

Multiple bondlengths and short data ranges in sexafs and exafs

Simple, but general, model calculations are presented to show that two or more similar (but slightly different) bondlength contributions of different amplitude can lead to substantial errors in an EXAFS measurement with the apparent bondlength falling outside the ranges of the actual bondlengths. The effect is most acute for short data ranges, common in Surface EXAFS, and for low symmetry adsorption sites of the type likely to occur in many molecular adsorption systems.     

On the neglecting of higher‐order cumulants in EXAFS data analysis

The cumulant expansion is one of the most powerful and useful methods for EXAFS data analysis, in which the higher‐order cumulants allow to consider deviations from a simple Gaussian distribution. In this work, analytical expressions have been derived to show the effects of neglecting higher‐order cumulants in EXAFS analysis by the ratio method. The errors in the best‐fitting procedure owing to the omission of the higher‐order cumulants, as well as of the coordination number, can be determined.     

Determining the radial pair distribution function from X‐ray absorption spectra by use of the Landweber iteration method

The Landweber iteration approach is used to construct the radial pair distribution function (RPDF) from an X‐ray absorption (EXAFS) spectrum. The physical motivation for the presented investigation is the possibility to also reconstruct asymmetric RPDFs from the EXAFS spectra. From the methodical point of view the shell fit analysis in the case of complicated spectra would be much more eased if the RPDF for the first shell(s) are computed precisely and independently. The RPDF, as a solution of the fundamental EXAFS integral equation, is examined for theoretical examples, and a detailed noise analysis is performed. As a real example the EXAFS spectrum of curium(III) hydrate is evaluated in a stable way without supplementary conditions by the proposed iteration, i.e. by a recursive application of the EXAFS kernel.     

A comparative study of the spectra recorded at RRCAT synchrotron BL-8 dispersive EXAFS beamline with other beamlines

The aim of the present work is to make a comparative study of the EXAFS spectra recorded at the BL-8 dispersive EXAFS beamline at 2 GeV Indus-2 synchrotron source at RRCAT, Indore (India) with those recorded at other synchrotron EXAFS beamlines, viz., X-19A at NSLS, BNL (USA), EXAFS wiggler beamline 4-1 at the SSRL (USA) and beamline 11.1 at ELETTRA (Italy). For this purpose, EXAFS spectra at Cu K-edge in copper metal have been recorded at these four beamlines. Further, EXAFS spectra at Cu K-edge in a copper complex have also been recorded at BL-8 beamline and beamline 11.1 at ELETTRA (Italy). The obtained experimental μ(E) data have been background-subtracted and then normalized. The normalized data have been then converted to χ(k) data, which have been Fourier-transformed and then fitted with the theoretical model, thereby yielding different structural parameters. It has been shown that the results obtained from the EXAFS spectra recorded at the BL-8 beamline are comparable with those obtained from other synchrotron EXAFS beamlines and also with the crystallographic results reported by earlier workers. The reliability, usefulness and data quality of the BL-8 beamline have been discussed.     

Local order in light emitting porous silicon studied by XEOL and TEY

The local structure of porous silicon has been studied exciting its optical luminescence by X-rays (XEOL). The photoluminescence yield and the total electron yield (TEY), recorded simultaneously as a function of the X-ray energy at the Si K edge, give rise to the extended X-ray absorption fine structures (EXAFS). Analysis of EXAFS data confirms that the optical luminescence of porous Si originates from the nanocrystalline cores and shows that XEOL–EXAFS and TEY–EXAFS are sensitive to different Si local environment. It can be assumed that XEOL–EXAFS is related only to the light emitting sites while TEY–EXAFS is sampling both luminescent and non-luminescent Si sites.     

Lift‐off protocols for thin films for use in EXAFS experiments

Lift‐off protocols for thin films for improved extended X‐ray absorption fine structure (EXAFS) measurements are presented. Using wet chemical etching of the substrate or the interlayer between the thin film and the substrate, stand‐alone high‐quality micrometer‐thin films are obtained. Protocols for the single‐crystalline semiconductors GeSi, InGaAs, InGaP, InP and GaAs, the amorphous semiconductors GaAs, GeSi and InP and the dielectric materials SiO₂ and Si₃N₄ are presented. The removal of the substrate and the ability to stack the thin films yield benefits for EXAFS experiments in transmission as well as in fluorescence mode. Several cases are presented where this improved sample preparation procedure results in higher‐quality EXAFS data compared with conventional sample preparation methods. This lift‐off procedure can also be advantageous for other experimental techniques (e.g. small‐angle X‐ray scattering) that benefit from removing undesired contributions from the substrate.     

EXAFS results on metallic amorphous alloys and on metallurgy

Summary A review of some general trends of EXAFS in alloys and metallic glasses is given. The intrinsic qualities of the EXAFS method in metallurgy, structure of amorphous materials, clustering and spin glasses are discussed. Paper presented at the Workshop on ?EXAFS Data Analysis in Disordered Systems?, held in Parma, October 5–7, 1981.     

A study of the EXAFS associated with the K absorption discontinuity of copper in CuGa and CuGe alloys

The extended X-ray absorption fine structure (EXAFS) associated with the K absorption discontinuity of copper has been studied in the pure metal and in some copper-gallium and copper-germanium alloys, using a bent crystal X-ray spectrograph. The results obtained are discussed in the light of the recent theory of EXAFS proposed by Lytle, Sayers and Stern. It has been shown for the first time that there exists in alloys a correlation between the EXAFS parameter α₁ and e/a ratio. This correlation has been used to obtain the value of the lattice parameter “a” for Cu-27 at.% Ge alloy, which is not available in literature.     

On EXAFS Debye‐Waller factor and recent advances

The effects of structural and vibrational disorder on the EXAFS signals are parameterized in terms of the Debye Waller (DW) factor. Here the vibrational contribution is addressed, which for most systems can be singled out by studying the temperature dependence of the EXAFS DW factor, which corresponds to a good accuracy to the parallel mean square relative displacement (MSRD) around the inter‐atomic equilibrium distance. By comparing the first‐shell EXAFS thermal expansion with the crystallographic thermal expansion one can evaluate the perpendicular MSRD. The results of recent measurements on copper and on several tetrahedral semiconductors are here critically compared and some properties of the MSRDs are discussed, such as the dependence of correlation, force constants and vibrational anisotropy on crystal structure and bond ionicity as well as the relative merits of the correlated Debye and Einstein models. The anharmonic contribution to the parallel MSRD of CdTe has been evaluated and a quasi‐harmonic analysis has been attempted, leading to an estimation of the bond Grüneisen parameter.