X-ray absorption fine structure

the unique and unusual molecular structure in SCF solutions profoundly affects the reactivity in these systems. Thus, the elucidation of this structure is one of the first requirements for developing a predictive capability for reaction rates and pathways. A powerful spectroscopic technique that has only recently been used for in situ characterization of the molecular structure in supercritical fluid solutions is X-ray absorption fine structure (XAFS). XAFS provides detailed structural information about the number of nearest-neighbor atoms, bond distances, and bond strengths (from the Debye-Waller factor). The application of XAFS to a wide range of SCF solutions provides another powerful tool to explore the detailed structure of SCFs. 

A summary of the characteristics of XAFS spectroscopy with respect to SCF solutions are shown below. 

To date there have been only a handful of studies of fluids but there are a wide range of potential areas where XAFS may provide key structural information necessary to elucidate reaction rates and pathways. Only a few of the potential uses of XAFS for determination of SCF structure and reactions have thus far been explored. A few possible applications of the technique include (1) supercritical water hydration, (2) supercritical water ion pairing, (3) redox chemistry of inorganic species under supercritical conditions, (4) in situ characterization of organometallic structures, (5) binding of solutes or solvents to organometallic species, and (6) weak chemical interactions of the solute or solvent with other solutes. 

The first application of XAFS to SCFs was to explore the extent of ion hydration (Sr ) in SCH2O [39]. In later studies, solutions containing different 

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

EXAFS Extended x-ray absorption fine structure [177, 178] Variation of x-ray absorption as a function of x-ray energy beyond an absorption edge the probability is affected by backscattering of the emitted electron from adjacent atoms Number and interatomic distance of surface atoms... 

For bulk structural detemiination (see chapter B 1.9). the main teclmique used has been x-ray diffraction (XRD). Several other teclmiques are also available for more specialized applications, including electron diffraction (ED) for thin film structures and gas-phase molecules neutron diffraction (ND) and nuclear magnetic resonance (NMR) for magnetic studies (see chapter B1.12 and chapter B1.13) x-ray absorption fine structure (XAFS) for local structures in small or unstable samples and other spectroscopies to examine local structures in molecules. Electron microscopy also plays an important role, primarily tlirough unaging (see chapter B1.17). 

Lee P A, Citrin P H, Eisenberger P and Kincaid B M 1981 Extended x-ray absorption fine structure—its strengths and limitations as a structural tool Rev. Mod. Rhys. 53 769-806... 

Iwasawa Y (ed) 1996 X-Ray Absorption Fine Structure for Cataiysis and Surfaces (Singapore World Scientific)... 

Stdhr J, Kollin E B, Fischer D A, Flastings J B, Zaera F and Sette F 1985 Surface extended x-ray-absorption fine structure of low-Z adsorbates studied with fluorescence detection Rhys. Rev. Lett. 55 1468-71... 

Zaera F, Fischer D A, Carr R G and Gland J L 1988 Determination of chemisorption geometries for complex molecules by using near-edge X-ray absorption fine structure ethylene on Ni(IOO) J. Chem. Rhys. 89 5335-41... 

Ingalls R, Crozier E D, Whitmore J E, Seary A J and Tranquada J M 1980 Extended x-ray absorption fine structure of sodium bromide and germanium at high pressure J. Appl. Phys. 51 3158... 

Fischer D, Marti A and Hahner G 1997 Orientation and order in microcontact-printed, seif-assembied monoiayers of aikanethiois on goid investigated with near edge x-ray absorption fine structure spectroscopy J. Vac. Sc/. Technol. A 15 (4) 2173-80... 

Wu S, Lipkowski J, Tyiiszczak T and Hitchcock A P 1997 Eariy stages of copper eiectrocrystaiiization eiectrochemicai and in situ x-ray absorption fine structure studies of coadsorption of copper and chioride at the Au(111) eiectrode surface J.Phys. Chem. B 101 10 310-22... 

Rehr J J, Mustre de Leon J, Zabinski S I and Albers R C 1991 Theoretical x-ray absorption fine structure standards J. Am. Chem. Soc. 113 5135-40... 

Rehr J J, Albers R C and Zabinski S I 1992 High order multiple scattering calculation of x-ray absorption fine structure Phys. Rev. Lett. 69 3397-400... 

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)...

Surface Extended X-Ray Absorption Fine Structure and Near Edge X-Ray Absorption Fine Structure (SEXAFS/NEXAFS)... 

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. 

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 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. 

XAFS V, Proceeding of the Fifth International Conference on X-ray Absorption Fine Structure. (J. M. de Leon, E. A. Stern, D. E. Sayers, Y. Ma, and J. J. Rehr, eds.) North-Holland, Amsterdam, 1989. Also in Physica B. 158, 1989. Report of the International Workshop on Standards and Criteria in X-ray Absorption Spectroscopy (pp. 701-722) is essential reading. 

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