Extended X-ray Absorption Fine-structure Spectroscopy

EXAFS is observed as a modulating change in the absorption coefficient caused by the ejected electron wave back-scattering from the surrounding atoms, resulting in interference between ejected and back-scattered waves. It is defined as  

The wavenumber is defined at a photon energy E above the absorption edge energy Eq, with respect to the mass of the electron m.  

/(k) is the sum over N back-scattering atoms i, where fi is the scattering amplitude term characteristic of the atom, cT is the Debye-Waller factor associated with the vibration of the atoms, r is the distance from the absorbing atom, X is the mean free path of the photoelectron, and is the phase shift of the spherical wave as it scatters from the back-scattering atoms. By talcing the Fourier transform of the amplitude of the fine structure (that is, X( ) real-space radial distribution function of the back-scattering atoms around the absorbing atom is produced. 

Since the fine structure observed is only associated with the particular absorption edge being studied, and the energy of the absorption edge is dependent on the element and its oxidation state, EXAFS examines the local structure around one particular element, and in some cases, an element in a given oxidation state. A fuller picture can therefore be obtained by studying more than one absorbing element in the sample. 

On analysis of the EXAFS data, the local environment around a given absorbing atom can be obtained, that is the type, number and distance of the backscattering atoms. It should be noted that it is not necessary for the surrounding atoms to be formally bonded to the absorbing atom. Typically the distance has an uncertainty 

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EXAFS Extended X-ray absorption fine structure spectroscopy. A spectroscopic technique which can determine interatomic distances very precisely. 

Harada, M., Asakura K., and Toshima, N., Structural analysis of polymer-protected platinum/rhodium bimetallic clusters using extended x-ray absorption fine structure spectroscopy. Importance of microclusters for the formation of bimetallic clusters, J. Phys. Chem., 98, 2653, 1994. 

Single-crystal X-ray determination, elemental analysis, and mass-spectroscopy are used for the characterisation of complexes and products. Cyclic voltammetry, EXAFS (Extended X-ray Absorption Fine Structure Spectroscopy), NMR, UV-vis spectroscopy, and IR can also be used to determine electronic properties of the ligands and their complexes [7],... 

M. Mosselmans, J.fw. (2000) Structural chemistry of Fe, Mn, and Ni in synthetic hematites as determined by extended X-ray absorption fine structure spectroscopy. Clays Clay Min. 48 521-527 Singh, D.B. Prasad, G. Rupainwar, D.C. Singh,V.N. (1988) As(lll) removal from aqueous solution by adsorption. Water, Air, Soil Pollut. 42 373-386... 

Bulk processes can also be probed by an appropriate photon spectroscopy. For example, EXAFS provides an excellent spatial resolution with respect to the atomic surroundings. The information from Extended X-ray Absorption Fine Structure spectroscopy is contained in the oscillations of the X-ray absorption coefficient near an absorption edge e.g., the K- or L-edge). 

Tian, G., Zhu, Y., Xu, J. 2001. Characterization of extraction complexes of Am(IB) and Nd(III) with dialkyldithiophosphinic acid by extended X-ray absorption fine structure spectroscopy. Actinides 2001, Hayama, Japan November 4—9. 

This mechanism is in agreement with the mechanism proposed by others (Belin et al., 1989 and 1995 Martin et al., 1986a, Willermet et al., 1992) using extended X-ray absorption fine structure spectroscopy (EXAFS) and infrared spectroscopy. When ZDDP is present in the lubricant formulation, the radial distribution function (RDF) indicates that crystalline iron oxide diffuses into the polyphosphate network material. 

During these temperature and gas treatments, processes like reduction, oxidation and sintering take place. In Chapter 2 it is clarified what processes are responsible for the final metal particle size and particle size distribution. This is done using a combination time resolved extended X-ray absorption fine structure spectroscopy (quick EXAFS) and mass spectrometry. 

Another contribution to variations of intrinsic activity is the different number of defects and amount of disorder in the metallic Cu phase. This disorder can manifest itself in the form of lattice strain detectable, for example, by line profile analysis of X-ray diffraction (XRD) peaks [73], 63Cu nuclear magnetic resonance lines [74], or as an increased disorder parameter (Debye-Waller factor) derived from extended X-ray absorption fine structure spectroscopy [75], Strained copper has been shown theoretically [76] and experimentally [77] to have different adsorptive properties compared to unstrained surfaces. Strain (i.e. local variation in the lattice parameter) is known to shift the center of the d-band and alter the interactions of metal surface and absorbate [78]. The origin of strain and defects in Cu/ZnO is probably related to the crystallization of kinetically trapped nonideal Cu in close interfacial contact to the oxide during catalyst activation at mild conditions. A correlation of the concentration of planar defects in the Cu particles with the catalytic activity in methanol synthesis was observed in a series of industrial Cu/Zn0/Al203 catalysts by Kasatkin et al. [57]. Planar defects like stacking faults and twin boundaries can also be observed by HRTEM and are marked with arrows in Figure 5.3.8C [58],... 

EXAFS Extended X-ray absorption fine structure spectroscopy... 

NEXAFS — Near-edge extended X-ray absorption fine structure (spectroscopy), evaluation of the X-ray absorption spectrum around and slightly below the absorption edge, for details see - EXAFS. 

Triantafillou, N. D., Purnell, S. K., Papile, C. X, Chang, X-R., and Gates, B. C., A Family of rhenium subcarbonyls on MgO Structural characterization by extended X-ray absorption fine structure spectroscopy. Langmuir 10, 4077 (1994). 

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