SEXAFS/NEXAFS

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

There are several ways to make a SEXAF/NEXAFS measurement surface sensitive. 

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. 

Absorption techniques (infrared spectroscopy, EXAFS, SEXAFS, NEXAFS),... 

SEXAFS, NEXAFS, surface extended and near edge X-ray absorption fine structure (Stohr et al., 1985)... 

From the above descriptions, it becomes apparent that one can include a wide variety of teclmiques under the label diffraction methods . Table Bl.21.1 lists many techniques used for surface stmctural detemiination, and specifies which can be considered diffraction methods due to their use of wave interference (table Bl.21.1 also explains many teclmique acronyms commonly used in surface science). The diffraction methods range from the classic case of XRD and the analogous case of FEED to much more subtle cases like XAFS (listed as both SEXAFS (surface extended XAFS) and NEXAFS (near-edge XAFS) in the table). 

As the table shows, a host of other teclmiques have contributed a dozen or fewer results each. It is seen that diffraction teclmiques have been very prominent in the field the major diffraction methods have been LEED, PD, SEXAFS, XSW, XRD, while others have contributed less, such as NEXAFS, RHEED, low-energy position diffraction (LEPD), high-resolution electron energy loss spectroscopy (HREELS), medium-energy electron diffraction (MEED), Auger electron diffraction (AED), SEELFS, TED and atom diffraction (AD). 

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. 

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. 

D. Norman. J. Phys. C Solid State Phys. 19,3273, 1986. Reprinted with an appendix bringing it up to date in 1990 as pp. 197-242 in Current Topics in Condensed Matter Spectroscopy Adam HUger, 1990. An extensive review of SEXAFS and NEXAFS, concentrating on physical principles. 

Figure 1 Schematic representation of the three techniques (a) x-ray photoabsorption (NEXAFS/SEXAFS), (b) photoelectron spectroscopy (photoemission) and (c) photoelectron diffraction.

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. 

Different surface-sensitive techniques respond differently to the various kinds of disorder. The measurement of LEED beams is unique in largely filtering out all defects that are unrelated to the superlattice periodicity that defines the beams. Other techniques (including diffuse LEED) generally include contributions from all defects, for instance from adsorbates located at undesired steps and crystallite boundaries. Then only a reduction of the defect concentration can remove defect contributions from the experimental data. Rotational disorder of adsorbed molecules does not matter for techniques which measure only bond lengths, and not bond orientations. Thus, NEXAFS is more sensitive to such disorder than SEXAFS. 

Stohr J (1988) SEXAFS Everything you always wanted to know about SEXAFS but were afraid to ask. In Konigsberger DC, Prins R (eds) X-ray Absorption Principles, Applications Techniques of EXAFS, SEXAFS and XANES, John Wiley Sons, New York, p 443-571 Stohr J (1992) NEXAFS Spectroscopy. Springer-Verlag, Heidelberg, Germany... 

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