EXAFS distance shell 1/absorbing atom

The copper EXAFS of the ruthenium-copper clusters might be expected to differ substantially from the copper EXAFS of a copper on silica catalyst, since the copper atoms have very different environments. This expectation is indeed borne out by experiment, as shown in Figure 2 by the plots of the function K x(K) vs. K at 100 K for the extended fine structure beyond the copper K edge for the ruthenium-copper catalyst and a copper on silica reference catalyst ( ). The difference is also evident from the Fourier transforms and first coordination shell inverse transforms in the middle and right-hand sections of Figure 2. The inverse transforms were taken over the range of distances 1.7 to 3.1A to isolate the contribution to EXAFS arising from the first coordination shell of metal atoms about a copper absorber atom. This shell consists of copper atoms alone in the copper catalyst and of both copper and ruthenium atoms in the ruthenium-copper catalyst. 

The data collected are subjected to Fourier transformation yielding a peak at the frequency of each sine wave component in the EXAFS. The sine wave frequencies are proportional to the absorber-scatterer (a-s) distance /7IS. Each peak in the display represents a particular shell of atoms. To answer the question of how many of what kind of atom, one must do curve fitting. This requires a reliance on chemical intuition, experience, and adherence to reasonable chemical bond distances expected for the molecule under study. In practice, two methods are used to determine what the back-scattered EXAFS data for a given system should look like. The first, an empirical method, compares the unknown system to known models the second, a theoretical method, calculates the expected behavior of the a-s pair. The empirical method depends on having information on a suitable model, whereas the theoretical method is dependent on having good wave function descriptions of both absorber and scatterer. 

The EXAFS function is obtained from the X-ray absorption spectrum by subtracting the absorption due to the free atom. A Fourier transform of the EXAFS data gives a radial distribution function which shows the distribution of the neighbouring atoms as a function of internuclear distance from the absorbing atom. Shells of neighbours, known as coordination shells, surround the absorbing atom. Finally, the radial distribution function is fitted to a series of trial structural models until a structure which best fits the... 

With a Fourier transformation of (k) in the distance space, one obtains a separation of the contribution of the various coordination shells. This Fourier transform yields the structural parameters Rj, Nj and ah and thus the near range order of the specimen with respect to the absorbing atoms. The EXAFS analysis for the different absorber atoms within the material yields their specific near range order. Thus, one may get the structure seen form several kinds of absorbing atoms. EXAFS does not require highly crystalline materials. It is a suitable method to study disordered, or even amorphous, structures. The a values provide quantitative information about the thermal and structural disorder. 

EXAFS is an X-ray absorption technique which gives detailed local structure information, such as the type, number and distance of neighbouring atoms [36]. As in XPS, the basic process is the photoelectric effect a photon is absorbed by an atom or ion and an electron is emitted from an inner shell. 

The result is peaks in r space that correspond to interatomic distances between the central absorber atom and the surrounding scatterers in the individual coordination shells. As is typical in all Fourier transformations, the resolution of this transform depends on the range EXAFS signal used. In other words, it is important to have a good EXAFS signal without noise up to a x range of 14 or 15. The r value is shifted... 

In the EXAFS technique the incident X-rays are absorbed by the atom which is being studied. In doing so they interact directly with the electrons of the atom and the excited electrons then probe the immediate environment of the atom. As a result X-rays are emitted and this appears as fine strucmre in an absorption. From this it is possible to deduce coordination numbers, geometry and mean distances of the atom under study to its nearest neighbours, i.e. to give information about the primary solvation of an ion. Recently it has proved useful in probing the secondary solvation shell. 

Nj and Rj are the most important structural data that can be determined in an EXAFS analysis. Another parameter that characterizes the local structine aroimd the absorbing atom is the mean square displacement aj that siunmarizes the deviations of individual interatomic distances from the mean distance Rj of this neighboring shell. These deviations can be caused by vibrations or by structural disorder. The simple correction term exp [ 2k c ] is valid only in the case that the distribution of interatomic distances can be described by a Gaussian function, i.e., when a vibration or a pair distribution function is pmely harmonic. For the correct description of non-Gaussian pair distribution functions or of anhar-monic vibrations, different special models have been developed which lead to more complicated formulae [15-18]. This term, exp [-2k cj], is similar to the Debye-Waller factor correction used in X-ray diffraction however, the term as used here relates to deviations from a mean interatomic distance, whereas the Debye-Waller factor of X-ray diffraction describes deviations from a mean atomic position. 

EXAFS spectra provide information regarding the local environment surrounding the atom whose absorption edge is being studied. The specific local information includes the atom type of neighbouring atomic shells, the distance of those shells from the central (absorbing) atom and the static or dynamic disorder. The latter is... 

By Fourier transforming the EXAFS oscillations, a radial structure function is obtained (2U). The peaks in the Fourier transform correspond to the different coordination shells and the position of these peaks gives the absorber-scatterer distances, but shifted to lower values due to the effect of the phase shift. The height of the peaks is related to the coordination number and to thermal (Debye-Waller smearing), as well as static disorder, and for systems, which contain only one kind of atoms at a given distance, the Fourier transform method may give reliable information on the local environment. However, for more accurate determinations of the coordination number N and the bond distance R, a more sophisticated curve-fitting analysis is required. 

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