X‐Ray Absorption Spectroscopy of Amorphous Solids, Liquids, and Catalytic and Biochemical Systems—Capabilities and Limitations

Abstract

Although the effect on which X-ray absorption spectroscopy (EXAFS spectroscopy) is based has been known for about 70 years, up until about 20 years ago it had not found any application. The reasons for this were on the one hand the shortage of efficient X-ray sources, and on the other hand the lack of a suitable theoretical description. With the advent of synchrotron radiation the situation has changed completely. Today, EXAFS spectroscopy is applied to a variety of very different problems and used more and more frequently to answer chemical questions. The extraordinary behavior of EXAFS spectroscopy that allows the determination of local structures in almost any mixture, independent of the sample's physical state as well as element-specifically with high sensitivity, carries with it the danger that the amount of the obtainable structural information will be overestimated. A realistic estimate of the capability of EXAFS spectroscopy has in the meantime been achieved for crystalline materials, but not for amorphous materials. In this case the viewpoints range from complete rejection to overinterpretation of the EXAFS spectrum. In this article an attempt is made to show how EXAFS spectroscopy can be applied in the investigation of noncrystalline materials. First, the measurement techniques, data analysis procedures, and the simulation of EXAFS spectra are explained. Starting with the simplest systems. the amorphous metals and nonmetals, and using representative but not necessarily current examples, an explanation is given of the structural information provided by EXAFS spectroscopy, of how the local environment about the two components can be determined in binary systems, and of how even the crystallization process and structural changes can be investigated in chemical reactions. Thereafter, EXAFS studies on solutions, molecular liquids, and melts are presented. The final section is concerned with typical applications of EXAFS spectroscopy, the investigation of homogeneous and heterogeneous catalysts as well as the elucidation of active sites in enzymes. Theoretical FoundationsA monochromatic X-ray beam of energy E is attenuated by the passage through a material of thickness d according to Equation (a).['] ZJE) and Z(E) are the intensity of the incident and transmitted beams, respectively. p ( E ) is the linear absorption coefficient. Its dependence on E is depicted in Figure 1 a. p ( E ) decreases with increasing E of the incident X-rays until a threshold is reached at which the energy is sufficient to remove an electron from an inner shell. At this threshold, p ( E ) increases abruptly. Beyond this point, p ( E ) resumes its steady decrease. More precise measurements of the absorption coefficient indicate that the curve shown in Figure l a atoms. When other atoms are in the vicinity of the absorbing atom, the absorption coefficient shows small oscillations up to about 1000 eV above the absorption edge. This is shown schematically in Figure 1 b. Fig. 1. Graph of the absorption coefficient p ( E ) for an isolated ...

X-ray absorption spectroscopy on the calcium cofactor to the manganese cluster in photosynthetic oxygen evolution

“…Distances can be reliably determined with an accuracy of 2%, whereas the N can incur errors up to 30%. 8 Absolute fit quality was judged through a combination of criteria:…”

“…The first peak can be simulated by 9 ± 2 oxygen at 2.57 ± 0.02 Å, where the error in coordination number is estimated at ±30%. 8 The known coordination numbers of Sr 2 + range from 6 to 9, and Sr-O distances are distributed from 2.56 to 2.71 Å. 84,85,92 Fourier peak II ('R'= 3.0 Å) was consistently present in the four sets of intact Sr-PS II and absent in all four inactive Sr-PS II data sets.…”

“…The N app values in Table 3.1 are subject to ± 25% systematic error as an inherent limitation of the EXAFS technique. 7,8,62,131 Determination of φ…”

“…(We assume that θ is accurately determined, with negligible error.) First, the N derived from EXAFS curve fitting can incur an error of ± 25% as described earlier 7,8,62 due to an inherent limitation of the EXAFS technique. This error range was also validated in a recent EXAFS study of Mn distorted cubane complexes (Chapter 3), where the N Mn-Mn derived from curve fitting was compared to known crystallographic values.…”

“…As will be described in more detail later, XAS has many advantages for probing Sr (Ca), Mn and S in biology. [6][7][8] Among these is the element-specificity and ability to detect dilute concentrations. One experimental setup is shared by the four major investigations presented here, and this is presented in Figure 1.1.…”