EXAFS NEXAFS

Structure of Surfaces and Interfaces as Studied Using Synchrotron Radiation. Faraday Discussions Chem. Soc. 89, 1990. A lively and recent account of studies in EXAFS, NEXAFS, SEXAFS, etc. 

The spherical-wave description has its own great advantages. It is best adapted to the scattering and emission by the spherical ion cores and it does not require the presence of any structural periodicity. In particular, it is well suited to the treatment of multiple scattering between different atoms within any cluster of atoms, in particular within a periodic unit cell as in LEED. It is also convenient for the treatment of fine structure arising from back-scattering by nearby atoms, as in (S)EXAFS, NEXAFS, EAPFS, etc. (i.e. in step 3 in our four-step description). 

XAS,EXAFS, NEXAFS Extracts the evidence about the chemical state environment like the oxidation state, symmetry or the local charge distribution. It also gives the local environment of atoms (coordination number, distance between the neighboring atom etc.) absorbing the incident radiation. 

Studies of passivation layers and corrosion phenomena, particularly with iron and magnesium using EXAFS, NEXAFS and related X-ray absorption spectroscopies have been reviewed elsewhere [589]. Using GIXAFS, depth profiling has become possible and spatially resolved studies of silver dissolution and lithium intercalation have been reported. 

NEXAFS Near-edge EXAFS Variant of EXAFS Variant of EXAFS... 

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,... 

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. 

The advantages of SEXAFS/NEXAFS can be negated by the inconvenience of having to travel to synchrotron radiation centers to perform the experiments. This has led to attempts to exploit EXAFS-Iike phenomena in laboratory-based techniques, especially using electron beams. Despite doubts over the theory there appears to be good experimental evidence that electron energy loss fine structure (EELFS) yields structural information in an identical manner to EXAFS. However, few EELFS experiments have been performed, and the technique appears to be more raxing than SEXAFS. 

EXAFS has recently been reviewed [307] XAS and NEXAFS monographs are also available [308,309]. 

Information concerning the orientation of the molecule can be inferred from the polarization dependence. NEXAFS is sensitive to bond angles, whereas EXAFS is sensitive to the interatomic distances. Linearly polarized X-rays are best suited for molecules possessing directional bonds. This is best exemplified for flat 7T-conjugated molecules lying flat on surfaces. When the electric field vector E is aligned along the surface normal, features due to the out-of-plane tt orbitals... 

The EXAFS technique has been especially useful for metalloproteins. It has often provided the first clues as to the identity of atoms (O, N, S) surrounding a metal atom and either covalently bonded to it or coordinated with it (Chapter 16). Interpretations are often difficult, and a common approach is to try to simulate the observed spectrum by calculation from a proposed structure.118 Tautomerism in crystalline Schiff bases (see Eq. 23-24) has been studied by nearedge X-ray absorption fine structure (NEXAFS) employing soft X-rays.119... 

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. 

A recent study of poly(di-n-hexylsilane) films by V.R. McCrary et al. (17) measured the polarization dependent Cls and Si2p NEXAFS spectra, EXAFS, and UPS as a function of temperature. Above 42°C it was found that the hexyl side chains were disordered and that the Si backbone was partially disordered (no longer an all-trans configuration). Below 42°C there was coexistence of the disordered phase with the well-ordered (all-trans backbone and side chains) phase. 

The most promising techniques for obtaining detailed surface structural information about molecular adsorbates rely on electron diffraction in one way or another. These include LEED,/27,28,29/ IV-HREELS,/30/ EAPFS,/31/ SEELFS,/32,33/ EXELFS,/34,35/ ARUPS, ARXPS, ARPEFS,/36,37,38/ PE-SEXAFS, SEXAFS, EXAFS, and NEXAFS (XANES). These and other techniques have been discussed above in part 2, and were summarized in Table I. Among these techniques, LEED has been the most productive. 

In the following the results of our analyses are presented in sequence NEXAFS for Pt and Sn EXAFS for Pt and Sn, for the reference compounds and catalysts on low and high area alumina. Our general conclusions are then compared with those derived from other diagnostics. 

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