EXAFS, detectability

Zhong Z, Chapman D, Bunker B, Bunker G, Fischetti R, Segre C (1999) A bent Laue analyzer for fluorescence EXAFS detection. J Synchrotron Rad 6 212-214... 

Ideally, spectroscopic data would be available to constrain the possible choices of surface species (in both CA and GC modeling) however, no x-ray absorption (XAS) data were collected that describe U(VI) speciation at the surface of the schist materials because the U concentrations were below EXAFS detection limits. XAS and FTIR spectroscopic studies have indicated that U(VI) forms strong, edge-sharing, bidentate bonds with the surfaces of ferrihydrite and hematite and that the attached uranyl cation may then form ternary surface complexes with carbonate anions (Ihirgar et al.. 1999, 2()()0). At least nine monomeric U(VI) stir-... 

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. 

Blum L, Abruna FI D, White J, Gordon J G, Borges G L, Samant M G and Melroy 1986 Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS J. Chem. Phys. 85 6732-8... 

In addition to transmission, EXAFS data can be recorded through the detection of... 

The last three detection schemes apply only under very special circumstances. Transmission EXAFS is strictly a probe of bulk structure, i.e., more than about a thousand monolayers. The electron- and ion-yield detection methods, which are used in reflection rather than transmission schemes, provide surface sensitivity, 1-1,000 A, and are inherendy insensitive to bulk structure. X-ray fluorescence EXAFS has the widest range of sensitivity—from monolayer to bulk levels. The combination of electron or ion yield and transmission EXAFS measurements can provide structural information about the X-ray absorbing element at the surface and in the bulk, respectively, of a sample. 

Self-absorption occurs when the path-length is too large [35] and the X-rays emitted have a significant probability of being absorbed by the remainder of the sample before being detected. This has the consequence of reducing the amplitude of the EXAFS oscillations and producing erroneous results. As the sample becomes more dilute this probability decreases. All the atoms in the sample determine the amount of self-absorption hence the need for thin samples. 

Figure 6.20 shows an example in which QEXAFS has been used in combination with XRD to study the temperature programmed reduction of copper oxide in a Cu/ZnO/Al203 catalyst for the synthesis of methanol [43,44]. Reduction to copper metal takes place in a narrow temperature window of 430-440 K, and is clearly revealed by both the EXAFS pattern and the appearance of the (111) reflection of metallic copper in the XRD spectra. Note that the QEXAFS detects the metallic copper at a slightly lower temperature than the XRD does, indicating that the first copper metal particles that form are too small to be detected by XRD, which requires a certain extent of long range order [43,44],... 

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