Extended X-Ray Absorption Fine Structure EXAFS

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. 

An EXAFS spectrum is a plot of the X-ray intensity transmitted by the sample, as a function of the energy E = fiv of the monochromatic X-rays. Each time E reaches the threshold for photoemission of a core electron with binding energy 

In a monoatomic solid, each shell gives rise to a characteristic interference. This is expressed as follows in the EXAFS function %(fc)  

The EXAFS function becomes understandable if we look at the Fourier Transform of %(k), which resembles a radial distribution function (the probability of finding an atom at a distance r from the absorbing atom)  

The function 0 (r) represents the probability of finding an atom at a distance r. The transform is weighted with either k or k , to emphasize the role of light or heavy atoms, respectively. In principle, the Fourier transform becomes more accurate when the k interval is larger, but in practice the signal to noise ratio of the spectrum sets the limit for k. 

In conclusion, XPS is among the most frequently used techniques in characterizing catalysts. It readily provides the composition of the surface region and also reveals information on both the oxidation state of metals and the electronegativity of any ligands. XPS can also provide insight into the dispersion of particles over supports, vrhich is particularly useful if the more common techniques employed for this purpose, such as electron microscopy or hydrogen chemisorption, can not discriminate between support and active phase. 

However, mathematics is essential to explain how structural data are derived from EXAFS. The EXAFS function, x(k), is extracted from the X-ray absorption spectrum in Fig. 4.10 by removing the approximately parabolic background and the step, i.e. the spectrum of the free atom. As in any scattering experiment, it is customary to express the signal as a function of the wavenumber, k, rather than of energy. The relation between k and the kinetic energy of the photoelectron is  

In a mono-atomic solid, the EXAFS function x k) is the sum of the scattering contributions of all atoms in neighboring coordination shells  

Each coordination shell contributes a sine function multiplied by an amplitude, which contains the number of neighbors in a coordination shell, Nj, as the most desirable information  

The essence of analyzing an EXAFS spectrum is to recognize all sine contributions in x(k)- The obvious mathematical tool with which to achieve this is Fourier analysis. The argument of each sine contribution in Eq. (8) depends on k (which is known), on r (to be determined), and on the phase shift p k). The latter is a characteristic property of the scattering atom in a certain environment, and is best derived from the EXAFS spectrum of a reference compound for which all distances are known. The EXAFS information becomes accessible, if we convert it into a radial distribution function, 0 (r), by means of Fourier transformation  

At the time that EXAFS was introduced in catalysis, around 1975, the technique was considered to be one of the most promising tools for investigating catalysts. These high expectations have not quite been fulfilled, mainly because data analysis in EXAFS is highly complicated and, unfortunately, not always possible without ambiguity. A number of successful applications, however, have proven that EXAFS applied with care on optimized catalysts can be a very powerful tool in catalysis [27,31,321. 

For those unfamiliar with wave vectors, hk 2n % the momentum of a wave quantum, and V(2mcEkin) = mev is the classical momentum of the electron when considered as a particle. 

So far, we have concentrated on the edge and near-edge structure of absorption spectra. Significant features are observed in this regime. For 

Quantitatively, the EXAFS x is defined as the normalized oscillatory part of the x-ray absorption ii  

In the interpretation of EXAFS data multiple scattering of the ejected photoelectron may be neglected in a good approximation. Thus it is sufficient 

EXAFS is caused by the interference of the photoelectron state with itself. Since the amplitude of the electron wave decreases with l/R and the mean free path A of electrons amounts to only a few angstroms, only neighboring atoms contribute to the interference. Thus EXAFS probes the distance between the central absorbing atom and its nearest neighbors, i.e., the short-range order (SRO) of atoms in condensed matter. 

Figaro 26. Fourier-transformed SEXAFS data from the iodine and tellurium I -edges (left)  

According to the full shell model, the Aujs duster core consists of a central Au atom surrounded by 12 further Au atoms which form the inner core of the Aujj nudeus. The 42 outer shell atoms are divided into 12 which bear phosphine ligands, 6 which link to chlorine atoms, and 24 which are uncoordinat si ace gold atoms. In addition to the temperature dependence and the absorption inten-aty results, the Mdssbauer spectra also give information on the Debye-Waller factors (f-factors or Mdssbauer fractions) which are affected by intraduster vibrations whose values can also be calculated. [83] Since inequivalent gold atoms can 

Figne 3-40. Hie temperature dependence of the MOssbauer fraction of [AusslPPhslcCIc]. Circles core atoms crosses O-bonded Au atoms stars unbounded surface atoms plus signs PPhs bonded Au atoms. 

The discovery of the phenomenon that is now known as extended X-ray absorption fine structure (EXAFS) was made in the 1920s, however, it wasn t until the 1970s that two developments set the foundation for the theory and practice of EXAFS measurements. The first was the demonstration of mathematical algorithms for the analysis of EXAFS data. The second was the advent of intense synchrotron radiation of X-ray wavelengths that immensely facilitated the acquisition of these data. During the past two decades, the use of EXAFS has become firmly established as a practical and powerfiil analytical capability for structure determination.  

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

EXAFS is part of the field of X-ray absorption spectroscopy (XAS), in which a number of acronyms abound. An X-ray absorption spectrum contains EXAFS data as well as the X-ray absorption near-edge structure, XANES (alternatively called the near-edge X-ray absorption fine structure, NEXAFS). The combination of XANES (NEXAFS) and EXAFS is commonly referred to as X-ray absorption fine structure, or XAFS. In applications of EXAFS to surface science, the acronym SEXAFS, for surface-EXAFS, is used. The principles and analysis of EXAFS and SEXAFS are the same. See the article following this one for a discussion of SEXAFS and NEXAFS. 

A wide selection of metal reference foils and powder films of ideal thickness for tranmission EXAFS is available from The EXAFS Materials Company, Danville, CA, USA. The transmission method is well-suited for in situ measurements of materials under industrially relevant conditions of extreme temperature and controlled atmosphere. Specially designed reactors for catalysis experiments and easy- 

In addition to transmission, EXAFS data can be recorded through the detection of 

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Figure 8.34 Experimental method for extended X-ray absorption fine structure (EXAFS)...

Figure 8.38 Curve fitting of Mo extended X-ray absorption fine structure (EXAFS) for Mo(SC6H4NH)3, taking into account (a) sulphur and (b) sulphur and nitrogen atoms as near neighbours. (Reproduced, with permission, trom Winnick, H. and Doniach, S. (Eds), Synchrotron Radiation Research, p. 436, Plenum, New York, 1980)...

Extended X-Ray Absorption Fine Structure, EXAFS 214 Surface Extended X-Ray Absorption Fine Structure and Near Et e X-Ray Absorption Fine Structure, SEXAFS/NEXAFS 227 X-Ray Photoelectron and Auger Diffraction,... 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

Alternatives to XRD include transmission electron microscopy (TEM) and diffraction, Low-Energy and Reflection High-Energy Electron Diffraction (LEED and RHEED), extended X-ray Absorption Fine Structure (EXAFS), and neutron diffraction. LEED and RHEED are limited to surfaces and do not probe the bulk of thin films. The elemental sensitivity in neutron diffraction is quite different from XRD, but neutron sources are much weaker than X-ray sources. Neutrons are, however, sensitive to magnetic moments. If adequately large specimens are available, neutron diffraction is a good alternative for low-Z materials and for materials where the magnetic structure is of interest. 

A detailed investigation of the structure of amorphous PcRu by large-angle X-ray scattering (LAXS)269 showed that in the solid state dimeric species exist with a Ru-Ru distance in the magnitude of a double bond. Current experiments using the extended X-ray absorption fine structure (EXAFS) method confirm these results.279... 

Extended X-ray absorption fine structure (EXAFS) measurements based on the photoeffect caused by collision of an inner shell electron with an X-ray photon of sufficient energy may also be used. The spectrum, starting from the absorption edge, exhibits a sinusoidal fine structure caused by interferences between the outgoing and the backscattered waves of the photoelectron which is the product of the collision. Since the intensity of the backscattering decreases rapidly over the distances to the next neighbor atoms, information about the chemical surroundings of the excited atom can be deduced. 

X-Ray Absorption Spectroscopy, X-Ray Absorption Edge Spectroscopy and Extended X-Ray Absorption Fine Structure (EXAFS) 247,280,469,980,1090,1619, 1691... 

Fe-S and Fe-Fe distances in [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters are all very similar 2.3 and 2.7 A, respectively. In the [6Fe-6S] prismane model cluster, however, there is an additional Fe-Fe distance at 3.7 A (Fig. 2). If a [6Fe-6S] cluster were present in the Fepr protein, then this longer Fe-Fe distance should he visible with extended X-ray absorption fine structure (EXAFS). As a consequence, EXAFS studies were carried out at the CCLRC Synchrotron Radiation facility in Daresbury, UK. The two Fepr proteins (those of D. vulgaris and D. desulfuricans), as well as a synthetic [6Fe-6S] cluster, were subjected to an EXAFS study. Low-temperature EXAFS... 

Time-resolved X-ray absorption is a very different class of experiments [5-7]. Chemical reactions are triggered by an ultrafast laser pulse, but the laser-induced change in geometry is observed by absorption rather than diffraction. This technique permits one to monitor local rather than global changes in the system. What one measures in practice is the extended X-ray absorption fine structure (EXAFS), and the X-ray extended nearedge strucmre (XANES). 

Extended X-ray absorption fine structure (EXAFS) studies have been very useful for obtaining structural information on bimetallic cluster catalysts. The application to bimetallic systems is a particularly good one for illustrating the various factors which have an influence on EXAFS. Moreover, the applicability of EXAFS to this area has been very timely, in view of the enormous interest in bimetallic systems in both catalytic science and technology. 

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption fine structure (EXAFS) was used to extensively characterize Pt on Cabosll catalysts. XANES Is the result of electron transitions to bound states of the absorbing atom and thereby maps the symmetry - selected empty manifold of electron states. It Is sensitive to the electronic configuration of the absorbing atom. When the photoelectron has sufficient kinetic energy to be ejected from the atom It can be backscattered by neighboring atoms. The quantum Interference of the Initial... 

These conclusions from the infrared reflectance spectra recorded with Pt and Pt-Ru bulk alloys were confirmed in electrocatalysis studies on small bimetallic particles dispersed on high surface area carbon powders.Concerning the structure of bimetallic Pt-Ru particles, in situ Extended X-Ray Absorption Fine Structure (EXAFS>XANES experiments showed that the particle is a true alloy. For practical application, it is very important to determine the optimum composition of the R-Ru alloys. Even if there are still some discrepancies, several recent studies have concluded that an optimum composition about 15 to 20 at.% in ruthenium gives the best results for the oxidation of methanol. This composition is different from that for the oxidation of dissolved CO (about 50 at.% Ru), confirming a different spatial distribution of the adsorbed species. 

In the present study, we synthesized in zeolite cavities Co-Mo binary sulfide clusters by using Co and Mo carbonyls and characterized the clusters by extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and high resolution electron microscopy (HREM). The mechanism of catalytic synergy generation in HDS is discussed. 

Intermediates were also observed in the synthesis of a neutral cluster, Ir4(CO)i2, from Ir(CO)2(acac) in the cages of zeohte NaY these were characterized by IR and extended X-ray absorption fine structure (EXAFS) spectroscopies, the latter being a technique ideally suited to investigation of small, highly dispersed species present in small amoimts in sohds. The spectra indicated dimeric intermediates, possibly Ir2(CO)8 [ 16], when the reaction was carried out in the near absence of water in the zeohte in contrast, the reaction in the dehydrated zeolite was faster, and no evidence of intermediates was observed [16]. 

Fig. 3 Ir4 cluster supported at the six-ring of zeolite NaX as represented by density functional theory samples were characterized by Extended X-ray absorption fine structure (EXAFS) spectroscopy and other techniques [32]...

Extended X-ray absorption fine structure (EXAFS) and photoemission measurements carried out on the clusters ranging from isolated atoms to aggregates large enough to acquire bulk metal properties, is also could be a measure for the electronic properties [40]. Copper shows a... 

Extended X-ray Absorption Fine Structure (EXAFS) the measurements were mostly made at the Gilda Italian Beamline (equipped with a bending magnet) at the European Synchrotron Radiation Facility in Grenoble (France). 

Local Structure of the Eu2+ Impurity. From the experimental perspective, the doping of lanthanide ions into solid state materials can be probed by different instrumental technics such as nuclear magnetic resonance (NMR),44 extended X-ray absorption fine structure (EXAFS),45,46 or electron paramagnetic resonance (EPR),47 which instead of giving a direct clue of the local geometry offers only data that can be corroborated to it. From the theoretical point of view,... 

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