SEXAFS

SEXAFS Surface EXAFS Same as EXAFS Same as EXAFS... 

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 we have seen, the electron is the easiest probe to make surface sensitive. For that reason, a number of hybrid teclmiques have been designed that combine the virtues of electrons and of other probes. In particular, electrons and photons (x-rays) have been used together in teclmiques like PD [10] and SEXAFS (or EXAFS, which is the high-energy limit of XAES) [2, Hj. Both of these rely on diffraction by electrons, which have been excited by photons. In the case of PD, the electrons themselves are detected after emission out of the surface, limiting the depth of sampling to that given by the electron mean free path. 

In tenns of individual techniques, table B1.2T1 lists tlie breakdown totalled over time, counting from the inception of surface stmctural detennination in the early 1970s. It is seen that LEED has contributed altogether about 67% of all stmctural detenninations included in the database. The annual share of LEED was 100% until 1978, and has generally remained over 50% since then. In 1979 other methods started to produce stmctural detenninations, especially PD, ion scattering (IS) and SEXAFS. XRD and then XSW started to contribute results in the period 1981-3. 

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). 

Addresses the need for advanoed methods in surfaoe extended x-ray absorption fine struoture (SEXAFS) for aoourate struotural determination. 

Kongingsberger D C and Prins R (ed) 988 X-Ray Absorption Principies, Appiications, Techniques of EXAFS, SEXAFS and XANES (New York Wiley)... 

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. 

The techniques can be broadly classified into two groups those which directly identify the atomic species present and then provide structural information about the identified species from diffraction or scattering effects (EXAFS, SEXAFS, and XPD) and those which are purely diffraction-based and do not direcdy identify the atoms involved, but give long-range order information on atomic positions from... 

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. 

X-Ray Absorption. Principles, Applications, Techniques of EXAFS, SEXAFS andXANES. (D. C. Koningsberger and R. Prins, eds.) Wiley, New York, 1988. 

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. 

SEXAFS can be measured from adsorbate concentrations as low as "0.05 mono-layers in favorable circumstances, although the detection limits for routine use are several times higher. By using appropriate standards, bond lengths can be determined as precisely as 0.01 A in some cases. Systematic errors often make the accu-... 

When an electron scatters from an atom, its phase is changed so that the reflected wave is not in phase with the incoming wave. This changes the interference pattern and hence the apparent distance between the two atoms. Knowledge of this phase shift is the key to getting precise bond lengths from SEXAFS. Phase shifts depend mainly on which atoms are involved, not on their detailed chemical environment, and should therefore be transferable from a known system to unknown systems. The phase shifts may be obtained ftom theoretical calculations, and there are published tabulations, but practically it is desirable to check the phase shifts using... 

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. 

One of the major advantages of SEXAFS over other surface structutal techniques is that, provided that single scattering applies (see below), one can go direcdy from the experimental spectrum, via Fourier transformation, to a value for bond length. The Fourier transform gives a real space distribudon with peaks in at dis-... 

Figure 3 The modulus of the Fourier transform of the SEXAFS spectrum for the half-monolayer coverage on Ni(IOO) The SEXAFS spectrum itself is shown in the inset with the background removed.

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