EXELFS

Details of oxygen K shell in NiO, illustrating NES and EXELFS oscillations and the measurement of the integrated edge intensity used for quantitative concentration determination. 

EXAFS Spectroscopy Techniques and Applications. (B. K. Teo and D. C. Joy, eds.) Plenum, New York, 1981. Contains historical items and treatments of EXELFS, the electron-scattering counterpart of EXAFS. 

EXELFS Extended Eneigy-Loss Fine Structure... 

The occurrence of fine structures has already been noted in the sections on spectral information and ionization losses (Sects. 2.5.3 and 2.5.3.2). In the following text some principal considerations are made about the physical background and possible applications of both types of feature, i. e. near-edge and extended energy-loss fine structures (ELNES/EXELFS). A wealth of more detailed information on their usage is available, especially in textbooks [2.171, 2.173] and monographs [2.210-2.212]. 

Fig. 2.39. Fi ne structures of inner-shell edges (A) Al-K edge of AI4C3 exhibiting ELNES and EXELFS, (B) O-K ELNES of different titanates and Ti-containing silicates.

EXELFS (the extended energy-loss fine structure) carries information about the bonding and co-ordination of the atoms contributing to the edge. However, the signal needs to be strong before statistically reliable information can be obtained. 

We shall see later that a special variant of EELS (EXELFS) is likely to be of great value in characterizing quasicrystalline solids. 

N and 0, in solid material. The second point is that EXELFS is especially suitable for the study of inhomogeneous samples (structurally and compositionally heterogeneous in the sense discussed in section 2.2 above) because the primary electron beam can be focussed to a diameter of ca 20. Other advantages of EXELFS have been discussed elsewhere (60, 61). The limitations of the technique include (i) the need to select an optimal thickness of sample so as to minimize multiple scattering and (ii) the susceptibility of the samples to suffer radiation damage. 

Brown (62) has recently shown that the presently available theory for the interpretation of EXELFS data is inadequate. In his study of the layered BN structure he found that whereas the distances to first nearest neighbours could be satisfactorily extracted from EXELFS data, second nearest-neighbour distances could not. The way ahead (62), here, is for the theory to be extended and tested against several known structures composed of light elements. 

ETEM is thus used as a nanolaboratory with multi-probe measurements. Design of novel reactions and nanosynthesis are possible. The structure and chemistry of dynamic catalysts are revealed by atomic imaging, ED, and chemical analysis (via PEELS/GIF), while the sample is immersed in controlled gas atmospheres at the operating temperature. The analysis of oxidation state in intermediate phases of the reaction and, in principle, EXELFS studies are possible. In many applications, the size and subsurface location of particles require the use of the dynamic STEM system (integrated with ETEM), with complementary methods for chemical and crystallographic analyses. 

A core hole is excited as in fine - structure techniques (see EXAFS, SEXAFS, ARPEFS, NPD, APD, EXELFS, SEELFS)... 

A fine - structure technique similar to EXELFS, except the incident electron is more surface - sensitive because of the lower excitation energy. 

EXELFS Extended X-ray Energy Loss Fine Structure A fine-structure technique similar to EXAFS, except that 60-300 KeV electrons rather than photons excite core-holes. Like EXAFS, this techniques is not explicitly surface sensitive. 

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. 

Extended X-Ray Energy Loss Fine Structure (EXELFS)... 

EXELFS extended energy loss fine structure... 

Figure 7.26. Electron energy-loss spectroscopy (EELS) spectra. Shown (top) is a representative EELS spectrum of a nickel oxide sample. A typical EELS spectrum shows a zero-loss peak that represents the unscattered or elastically scattered electrons, the near-edge fine structure (ELNES), and extended energy-loss fine structure (EXELFS). Also shown (bottom) are the fingerprint regions of an EELS spectrum, just beyond the core-electron edges, which provide information regarding the detailed bonding and chemical environment of the desired element.

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