神光Ⅲ原型上扩展X射线吸收精细结构谱实验研究

介绍了在大型激光装置上进行扩展X射线吸收精细结构(EXAFS)谱压缩物态参数测量的基本原理,以及为获得高质量EXAFS谱在神光Ⅲ原型装置上进行的实验研究。实验采用玻璃靶球、CH靶丸和金球作为EXAFS谱测量的背光源,通过多发次叠加、光子数累积的方法获得了信噪比良好的金属Ti在常温常压下的EXAFS谱,数据处理结果表明,实验测得的金属Ti EXAFS谱拟合结果与同步辐射实验拟合结果相吻合,表明实验设计的正确性与可靠性。对实验结果的分析表明,影响EXAFS谱质量的因素主要是光子计数、测量系统谱分辨率、噪声以及实验器件上的瑕疵。     

神光III原型上扩展X射线吸收精细结构谱实验研究

介绍了在大型激光装置上进行 扩展X射线吸收精细结构(EXAFS)谱压缩物态参数测量的基本原理,以及为获得高质量EXAFS谱在神光III原型装置上进行的实验研究。实验采用玻璃靶球、CH靶丸和金球作为EXAFS谱测量的背光源,通过多发次叠加、光子数累积的方法获得了信噪比良好的金属Ti在常温常压下的EXAFS谱,数据处理结果表明,实验测得的金属Ti EXAFS谱拟合结果与同步辐射实验拟合结果相吻合,表明实验设计的正确性与可靠性。对实验结果的分析表明,影响EXAFS谱质量的因素主要是光子计数、测量系统谱分辨率、噪声以及实验器件上的瑕疵。

     

谐波对EXAFS振幅的影响

本文讨论了晶体单色器高次衍射形成的谐波对EXAFS振幅的影响。给出了一般计算表达式和计算实例。这种影响是使EXAFS振幅减小,它与谐波和基波强度之比、样品厚度及吸收系数等因素有关。分别测量了不同厚度的铜箔样品有谐波与无谐波时的K吸收EXAFS谱,得出的单配位层EXAFS振幅的变化测量值与计算值相符。从而提供了对谐波影响进行修正的方法。 关键词：     

激光EXAFS谱仪简介

 同步辐射光源是目前世界上最好的光源,用它测量EXAFS谱自然效果极好.然而由于它设备庞大而昂贵,无法普及到一般实验室.转靶X光机比同步辐射装置要简单和便宜得多,但这种光源的性能不如同步辐射,每次实验所需时间也长得多.本文要介绍的激光EXAFS谱仪是用激光打到金属靶上所产生的X射线脉冲作为光源,并用拍照的方法记录EXAFS谱.其设备更加简单和便宜,而且测得EXAFS谱所需时间极短.当然这种方法也有它的局限性和缺点,例如一般只适用于测量较轻元素的EXAFS谱.激光EXAFS谱仪的结构如图所示.     

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。     

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。     

一个强场模型的哈密顿形式

强场中的量了系统可能激发到高激发态，构造正则方程的本征函数展开法一般不是适用。这一个１维强场模型问题用对称差商代替哈密顿算符中的偏导数构造正则方程，再用平方守恒，辛格式示数值解，结果与理论分析一致，表明提出的方法是成功的，有可能克服传统差分法求强场量了系统时间演化遇到的困难。     

包含六链方格的SU(2)改进作用量的累积展开变分研究

本文包含六链方格满足Symanzik要求的SU(2)格点规范作用量进行了累积展开变分研究,解析地计算了精确到二级累积展开的元格和六链方格的平均值,计算结果与Monte Carlo结果自洽.     

一种水声环境下易于硬件实现的盲波束形成方法研究

针对水声环境和水声信号的特点,提出了一种基于高阶累积量的神经网络盲波束形成算法。该方法把高阶累积量可消除高斯噪声干扰这一特性和神经网络可并行计算的结构优势有机地结合起来,不但有效地避免了矩阵求逆运算,而且易于用硬件实时实现盲最优波束形成。仿真实验验证了该算法的有效性和正确性。     

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：     

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.     

Accuracy evaluation in temperature‐dependent EXAFS measurements of CdTe

The evaluation of uncertainty in temperature‐dependent EXAFS measurements is discussed, considering the specific case of a recent experiment performed on CdTe. EXAFS at both Cd and Te K‐edges was measured at different times and at different beamlines in a temperature range from 5 to 300 K. Attention is focused on the nearest‐neighbours parameters: bond thermal expansion, parallel and perpendicular mean‐square relative displacements and the third cumulant. Different causes of uncertainty, a comparison of experimental results with theoretical models, the difference between EXAFS and crystallographic thermal expansions and the meaning of the third cumulant are discussed.     

Pressure effects in Debye-Waller factors and in EXAFS

Anharmonic correlated Einstein model (ACEM) and statistical moment method (SMM) have been developed to derive analytical expressions for pressure dependence of the lattice bond length, effective spring constant, correlated Einstein frequency and temperature, Debye-Waller factors (DWF) or second cumulant, first and third cumulants in Extended X-ray Absorption Fine Structure (EXAFS) at a given temperature. Numerical results for pressure-dependent DWF of Kr and Cu agree well with experiment and other theoretical values. Simulated EXAFS of Cu and its Fourier transform magnitude using our calculated pressure-induced change in the 1st shell are found to be in a reasonable agreement with those using X-ray diffraction (XRD) experimental results.     

Structural investigations on an amorphous Se90Te10 alloy produced by mechanical alloying using EXAFS, cumulant expansion and RMC simulations

We investigated an amorphous Se(90)Te(10) alloy produced by mechanical alloying using two different approaches. First, we used extended x-ray absorption fine structure (EXAFS) spectroscopy and the cumulant expansion method using the Einstein model for the temperature dependence of the cumulants to obtain the cumulants C(*)(1), C(*)(2), and C(*)(3). From these, we found information about the structure of the alloy as well as the thermal and structural disorder, anharmonicity of the effective interatomic pair potentials, thermal expansion of the Se-Se and Se-Te bonds and asymmetry of the partial distribution functions g(Se-Se)(r) and g(Se-Te)(r). The cumulants C(*)(1), C(*)(2), and C(*)(3) also allowed us to reconstruct the g(EXAFS)(ij)(r,T) functions from EXAFS. Then, we made reverse Monte Carlo (RMC) simulations using the total structure factor S(K) obtained from synchrotron x-ray diffraction and the EXAFS oscillations χ(k) on the Se K edge as input data to obtain the g(RMC)(ij)(r) functions. Both methods furnished very similar g(ij)(r) functions, and the structural data obtained from them were also very similar. The results obtained from both methods showed the presence of Se-Te pairs indicating that there is alloying at the atomic level. In addition, we could not find any evidence of the presence of Te clusters in the alloy.     

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.     

EXAFS spectroscopy,from one perspective

Summary Extended X-ray absorption fine-structure (EXAFS) spectroscopy has been demonstrated to be a most useful probe of short-range interatomic correlations in a wide variety of materials. The theory of the EXAFS is presented in its simplest form so as to provide the basis for a discussion about the kind and quality of the structural information which can be extracted from EXAFS data. The adequacy of several of the approximations commonly incorporated in this theory is examined with emphasis on those areas of the theory which bear most directly on the analysis of EXAFS data. Paper presented at the Workshop on ?EXAFS Data Analysis in Disordered Systems?, held in Parma, October 5–7, 1981.     

Muffin‐tin potentials in EXAFS analysis

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.     

一种声发射信号方位角自适应监测算法*

四阶累积量可用于声发射信号处理,结合盲波束形成算法可对声发射信号的方位角进行估计。由于四阶累积量的计算量较大,采用迭代计算的思想,提出一种基于四阶累积量的自适应声发射信号方位角监测算法。与改进前的算法相比,自适应算法的计算复杂度和所需存贮空间均显著降低,可用于声发射信号的方位估计和监测。计算机仿真及实测数据分析均验证了算法的有效性。