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1.
遗传算法在EXAFS谱图解析中的应用   总被引:2,自引:0,他引:2  
扩展X射线吸收精细结构(EXAFS)谱是研究物质原子近邻结构和表面结构的有力工具。EXAFS谱的解析通常采用标准样品比较法或最小二乘曲线拟合方法。但前者对标样的要求很高,而后者则参数初值难以确定,且结果有时不唯一。本文提出一种EXAFS曲线拟合的新方法一遗传算法,并对单配位层Cu样品的EXAFS谱图进行了解析,取得满意的结果。  相似文献   

2.
The approximate barium X‐ray atomic absorption in the energy region of L‐edges is reconstructed from the absorption spectrum of an aqueous solution of BaCl2. The result is corroborated by comparison with pure atomic absorption spectra of neighbour elements Xe and Cs. The application of the atomic absorption signal as a proper EXAFS background is demonstrated and discussed in the analysis of Ba hexaferrite nanoparticles with a very weak structural signal. The essential gain is found in the decrease of uncertainty intervals of structural parameters and their correlations. A simple analytical model of the absorption background for the practical EXAFS analysis is demonstrated.  相似文献   

3.
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.  相似文献   

4.
An eddy-viscous term is added to Navier-Stokes dynamics at wave numbers k greater than the inflection point kc of the energy flux F(log(k)). The eddy viscosity is fixed so that the energy spectrum satisfies E(k) = E(kc) (k/kc)(-3) for k>kc. This resulting forcing induces a rapid depletion of the energy cascade at k>kc. It is observed numerically that the model reproduces turbulence energetics at k< or =kc and statistics of two-point velocity correlations at scales r>lambda (Taylor microscale). Compared to a direct numerical simulation of R(lambda) = 130 an equivalent run with the present model results in a gain of a factor 20 in CPU time.  相似文献   

5.
Motivated by recent analytic predictions, we report numerical evidence showing that in driven incompressible magnetohydrodynamic turbulence the magnetic- and velocity-field fluctuations locally tend to align the directions of their polarizations. This dynamic alignment is stronger at smaller scales with the angular mismatch between the polarizations decreasing with the scale lambda approximately as theta(lambda) is proportional to lambda(1/4). This can naturally lead to a weakening of the nonlinear interactions and provide an explanation for the energy spectrum E(k) is proportional to k(-3/2) that is observed in numerical experiments of strongly magnetized turbulence.  相似文献   

6.
X‐ray Raman scattering (XRS) provides a bulk‐sensitive method of measuring the extended X‐ray absorption fine structure (EXAFS) of soft X‐ray absorption edges. Accurate measurements and data analysis procedures for the determination of XRS‐EXAFS of polycrystalline diamond are described. The contributions of various angular‐momentum components beyond the dipole limit to the atomic background and the EXAFS oscillations are incorporated using self‐consistent real‐space multiple‐scattering calculations. The properly extracted XRS‐EXAFS oscillations are in good agreement with calculations and earlier soft X‐ray EXAFS results. It is shown, however, that under certain conditions multiple‐scattering contributions to XRS‐EXAFS deviate from those in standard EXAFS, leading to noticeable changes in the real‐space signal at higher momentum transfers owing to non‐dipole contributions. These results pave the way for the accurate application of XRS‐EXAFS to previously inaccessible light‐element systems.  相似文献   

7.
The compound La2RuO5 was examined by the x‐ray absorption spectroscopy (XAS) methods, x‐ray absorption near edge structure (XANES) and extended x‐ray absorption fine structure (EXAFS). XANES technique was used to probe directly the average valence of Ru atoms in the compound. The energy shift of the Ru K‐edge in the XANES signal gave the average Ru valence state as 4.0 ± 0.1. EXAFS analysis provided, by yielding directly the interatomic distances and coordination numbers, the first information on the Ru atom neighborhood, on which the model for the Rietveld refinement of the unit cell of the new compound was devised. Finally, the local structure around the Ru atoms from the refinement was used in the FEFF6 code for a model EXAFS spectrum. The very good quantitative agreement with the measured spectrum proves that the refined crystal structure contains no systematic defects in the vicinity of Ru atoms. This result, together with the valence obtained from XANES, strongly confirms the proposed La2RuO5 stoichiometry. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

8.
Abstract

Although potentially extremely important for understanding the high-pressure microscopic behaviour of materials, over the years the area of high-pressure EXAFS in particular using diamond anvil cells has proved to be technically difficult. This has significantly hampered its development. The interference of X-ray dimaction from the diamonds in the diamond anvil cell with the absorption signal has proved to be a challenging problem to tackle, restricting the use of high-pressure EXAFS to energies below about 11 key Below 11 keV however the technique is also limited due to absorption of incident X-rays by the diamonds making it virtually impossible to conduct X-ray absorption experiments below about 9keV In this paper we describe in detail the methodology for scanriirig high-pressure EXAFS in diamond anvil cells and examine the origins of the associated problems and ways of dealing with them. We also demonstrate that it is possible to extend the useful range of studied absorption edges from 7keV up to at least 30keV This brings about new opportunities for high pressure EXAFS using diamond anvil cells.  相似文献   

9.
We propose a phenomenological theory of strong incompressible magnetohydrodynamic turbulence in the presence of a strong large-scale external magnetic field. We argue that in the inertial range of scales, magnetic-field and velocity-field fluctuations tend to align the directions of their polarizations. However, the perfect alignment cannot be reached; it is precluded by the presence of a constant energy flux over scales. As a consequence, the directions of shear-Alfvén fluid and magnetic-field fluctuations at each scale lambda become effectively aligned within the angle phi(lambda) proportional to lambda (1/4), which leads to scale-dependent depletion of the nonlinear interaction and to the field-perpendicular energy spectrum E(k(perpendicular)) proportional to k(perpendicular)(-3/2). Our results may be universal, i.e., independent of the external magnetic field, since small-scale fluctuations locally experience a strong field produced by large-scale eddies.  相似文献   

10.
Muffin‐tin potentials are the standard tool for calculating the potential surface of a cluster of atoms for use in the analysis of extended X‐ray absorption fine‐structure (EXAFS) data. The set of Cartesian coordinates used to define the positions of atoms in the cluster and to calculate the muffin‐tin potentials is commonly also used to enumerate the scattering paths used in the EXAFS data analysis. In this paper, it is shown that these muffin‐tin potentials are sufficiently robust to be used to examine quantitatively contributions to the EXAFS data from scattering geometries not represented in the original cluster.  相似文献   

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