XANES spectra absorption near-edge structure

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. 

Figure 2 Molybdenum K-edge X-ray absorption spectrum, ln(i /i ) versus X-ray energy (eV), for molybdenum metal foil (25- jjn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-rsy Absorption Fine Structure (S.S. Hasain, ed.) Ellis Norwood, New York, 1991.

X-ray absorption near edge structure (XANES) The X-ray absorption spectrum, as for EXAFS, may also show detailed structure below the absorption edge. This arises from excitation of core electrons to high level vacant orbitals, and can be used to estimate the oxidation state of the metal ion. 

In solid state physics, the sensitivity of the EELS spectrum to the density of unoccupied states, reflected in the near-edge fine structure, makes it possible to study bonding, local coordination and local electronic properties of materials. One recent trend in ATEM is to compare ELNES data quantitatively with the results of band structure calculations. Furthermore, the ELNES data can directly be compared to X-ray absorption near edge structures (XANES) or to data obtained with other spectroscopic techniques. However, TEM offers by far the highest spatial resolution in the study of the densities of states (DOS). 

Extended X-ray absorption fine structure (EXAFS) spectrum Part of an X-ray absorption spectrum that is used to identify the coordination of atoms, estimate bond lengths, and determine the adsorption complexes on the surfaces of adsorbents. EXAFS spectra may provide useful information on the speciation (valence state), surface complexes, and the coordination of arsenic on adsorbents (e.g. (Randall, Sherman and Ragnarsdottir, 2001 Ladeira, et al. (2001) Teixeira and Ciminelli (2005) Kober, et al. (2005)) (compare with X-ray absorption spectroscopy (XAS), X-ray absorption near edge structure (XANES) spectra, and X-ray absorption fine structure spectroscopy (XAFS)). 

X-ray absorption near edge structure (XANES) spectrum An analysis from X-ray absorption spectroscopy (XAS) and, in particular, X-ray absorption fine structure (XAFS) spectroscopy. XANES can be used to identify the valence state of arsenic in solid samples (Teixeira and Ciminelli, 2005 Kober et al., 2005). 

In the first section will be presented XAS from the physical principles to data analysis and measurements. Then section 2 will be devoted to a discussion of a few examples to illustrate the power and limitations of XAS for gaining structural information. Examples are focused on EXAFS studies on nanocrystalline materials. Detailed reviews for applications on other fields of materials science or for presenting the complementary information available by the study of the X-ray Absorption Near Edge Structure (XANES) part of the X-ray absorption spectrum can be found in a number of books [3-5], A brief overview of the recent development of the technique regarding the use of X-ray microbeams available on the third generation light sources will be finally presented in the last section. 

XANES — X-ray absorption near-edge structure an application of X-ray absorption spectroscopy where the fine structure of the absorption edge displayed in an X-ray absorption spectrum around and slightly below the absorption edge is analyzed, for details see -> surface analytical methods. 

Fig 1. The X-ray absorption spectrum of Cu,Zn-metallothionein, showing the Cu and Zn K-edges, the demarcation between the X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS), and a pictorial representation of the origin of EXAFS. 

X-ray absorption spectroscopy is a powerful method to assess the structure of mercury thiolate complexes. X-ray Absorption Near Edge Structure (XANES) spectroscopy can cleanly differentiate Hg(0), Hg(I) and Hg(II) by the energy of the emission edge. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy provides information regarding the coordination number and Hg-S bond distances. Typically, the EXAFS spectrum can establish the coordination number (/.e., number of sulfur atoms) to within 20%. More important, the bond length precision is 0.01 A. Given that each increase of one... 

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