X‐Ray Absorption Spectroscopy – the Method and Its Applications
X‐Ray Absorption Spectroscopy – the Method and Its Applications
复制标题
X 射线吸收光谱——方法及其应用
DOI：
10.1002/9783527636839.ch7
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发表时间：
2012
影响因子：
1.7
通讯作者：
H. Bertagnolli
中科院分区：
文献类型：
--
作者：
M. Bauer;H. Bertagnolli
图二：上图：用于透射（顶部）和荧光（底部）模式测量的实验设置（SY=入射同步辐射，M=单色器，F=荧光检测器，I0、I1、I2=从第一、第二和第三电离室记录的光束强度，If=荧光辐射的强度）。下图：主要激发过程的示意图X射线吸收（左），和次级过程荧光（中）和俄歇电子发射（右）。在某些情况下，通过记录与吸收成比例的过程来测量吸收是有利的。从图2（底部）可以看出，X射线光子的吸收在内壳层中产生了一个核心孔。空穴湮灭的任何过程都与光子的吸收成正比，因此可以用来测量吸收系数。壳层的空穴由来自外层的电子的辐射和非辐射跃迁填充。辐射跃迁产生X射线荧光辐射，其波长是两个壳之间的能量差的特征，因此可以容易地与吸收的辐射分离，特别是当检测器与入射光束平行取向时。非辐射跃迁对应于内部光电效应，如图2所示，并产生俄歇电子或二次电子。两个竞争过程的荧光辐射与电子发射率的比值随激发原子原子序数的增加而增加。在用于测量荧光EXAFS的典型实验中（参见图10），图2），入射强度由前检测器监测，而垂直于入射光束定位的检测器测量荧光辐射。当样品的厚度很大或感兴趣的元素的浓度很低时，例如酶的金属中心，应用荧光模式下的EXAFS测量。
Figure 2: Top: Experimental set-up for transmission (top) and fluorescence (bottom) mode measurements (SY= incoming synchrotron radiation, M= monochromator, F= Fluorescence detector, I0, I1, I2= beam intensity recorded from the first, second and third ionization chamber, If= Intensity of the fluorescence radiation). Bottom: Schematic representation of the primary excitation process X-ray absorption (left), and the secondary processes fluorescence (middle) and Auger electron emission (right).In some cases it is advantageous to measure the absorption by recording processes that are proportional to the absorption. As it can be seen from figure 2 (bottom), the absorption of a X-ray photon creates a core hole in an inner shell. Any process, by which the hole is annihilated, is proportional to the absorption of a photon, and can therefore be used as a measure of the absorption coefficient. The hole of the shell is filled by a radiative and non-radiative transition of an electron from an outer shell. The radiative transition produces a X-ray fluorescence radiation with a wavelength that is characteristic of the energy difference between the two shells and therefore can be easily separated from the absorbed radiation, especially, when the detector is perpendiculary orientated to the incident beam. The non-radiative transition corresponds to an internal photo-electric effect, as it is shown in figure 2, and generates Auger electrons or secondary electrons. The ratio of the both competitive processes fluorescence radiation to emittance of electrons increases with increasing atomic number of the excited atom. In a typical experiment for the measurement of the fluorescence EXAFS (cf. figure 2) the incident intensity is monitored by the front detector, whereas a detector, located perpendicular to the incident beam, measures the fluorescence radiation. EXAFS measurements in the fluorescence mode are applied, when the thickness of the sample is large or the concentration of the element of interest is very low, as for instance the metal centre of an enzyme.