Surface extended x-ray absorption fine structure, SEXAFS

Surface extended X-ray absorption fine structure (SEXAFS). In order to understand the SEXAFS approach it may first be useful to give a brief recap of X-ray absorption. In addition to the scattering process described above, an X-ray photon may instead be absorbed. X-ray absorption results in the excitation of a core electron into one of the continuum of empty levels in the lowest unoccupied band of the sample this is shown for copper in Figure 2.72. To a first approximation, the attenuation of the beam is given by a form of the Beer-Lambert expression ... 

Surface-enhanced resonance Raman scattering (SERRS), 21 327-328 advantage of, 21 329 Surface Evolver software, 12 11 Surface excess, 24 135, 136 Surface extended X-ray absorption fine structure (SEXAFS), 19 179 24 72 Surface filtration, 11 322-323 Surface finish(es). See also Electroplating in electrochemical machining, 9 591 fatigue performance and, 13 486-487 Surface finishing agents, 12 33 Surface force apparatus, 1 517 Surface force-pore flow (SFPF) model,... 

All analytical methods that use some part of the electromagnetic spectrum have evolved into many highly specialized ways of extracting information. The interaction of X-rays with matter represents an excellent example of this diversity. In addition to straightforward X-ray absorption, diffraction, and fluorescence, there is a whole host of other techniques that are either directly X-ray-related or come about as a secondary result of X-ray interaction with matter, such as X-ray photoemission spectroscopy (XPS), surface-extended X-ray absorption fine structure (SEXAFS) spectroscopy, Auger electron spectroscopy (AES), and time-resolved X-ray diffraction techniques, to name only a few [1,2]. 

Two rather different techniques that exploit the same underlying phenomenon of coherent interference of elastically scattered low energy electrons are photoelectron diffraction [5] and surface extended X-ray absorption fine structure (SEXAFS) [6,7]. Figure 1.1. shows schematically a comparison of the electron interference paths in LEED and in these two techniques. In both photoelectron diffraction and SEXAFS the source of electrons is not an electron beam from outside the surface, as in LEED, but photoelectrons emitted from a core level of an atom within the adsorbate. In photoelectron diffraction one detects the photoelectrons directly, outside the surface, as a function of direction or photoelectron energy (or both). The detected angle-resolved photoemission signal comprises a coherent sum of the directly emitted component of the outgoing photoelectron wavefield and other components of the same wavefield elastically scattered by atoms (especially in the substrate) close... 

Figure 1.1. Schematic diagram showing the electron elastic scattering pathways contributing to the techniques of low energy electron diffraction (LEED), backscattering photoelectron diffraction (including the scanned-energy mode - PhD) and surface extended X-ray absorption fine structure (SEXAFS). Black disks represent substrate atoms, grey-shaded disks represent adsorbate atoms.

Some comments should be made on areas which are not included. No discussion is given on specific instrumentation. There is virtually no discussion of structural characterization of surfaces, the emphasis being on elemental and chemical composition analysis. For this reason, several techniques which are primarily structural tools, are not discussed at all (e.g., Low Energy Electron Diffraction, LEED (1), Surface Extended X-Ray Absorption Fine Structure SEXAFS (2), and neutron scattering (3)), and the structural analysis capabilities of XPS (4), SIMS (5), and Ion Scattering (6) are not covered. 

The surface extended X-ray absorption fine structure (SEXAFS) method can use either an electron or an ion detection signal (Koningsberger and Prins, 1988). The classification of analytical techniques may be considered in terms of incident and emitted radiation, resolution, and sensitivity, according to Table 4.7, which lists eight of the many possible techniques (Briggs and Seah, 1990 Buckley, 1981 Watts, 1990). Many of the surface analysis techniques were introduced into many laboratories over the years of 1968 to 1970. This resulted from the maturing of clean vacuum systems which could achieve pressures, down to 10"8 Pa. At these low pressures, it is possible to obtain and maintain atomically clean surfaces. 

Surface extended x-ray-absorption fine structure (SEXAFS)... 

The availability of high intensity, synchronous X-ray sources at national synchrotron centers has allowed the development of a number of new tools that can study passive or corrosion Blms as they are forming in aqueous environments. Surface X-Ray Diffraction (SXRD) provides a direct determination of the crystal structure of the atoms of the surface. Surface Extended X-Ray Absorption Fine Structure (SEXAFS) can provide information on the geometric structure of atoms within a few angstroms around the absorbing atom. 

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