Fluorescence EXAFS

Figure 1 Schematic view of a typical EXAFS experiment at a synchrotron radiation facility. Note that it is possible to record transmission and fluorescence EXAFS simultaneously with reference EXAFS.

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. 

Fluorescence EXAFS studies of a (In,Mn)As thin layer (10 nm) grown on a GaSb buffer layer and of (In,Mn)As quantum dots (QDs) on GaAs were also performed. The results show that in the thin (In,Mn)As layer, the In-site substitutional Mn and the NiAs-type MnAs coexist, whereas the majority of Mn atoms are substituted into the In-sites of InAs in (In,Mn)As QDs. It is argued that the difference of the strain deformation between the thin layer (with strain) and thick layer and QDs (strain relaxed) is responsible for the differences in the local structure of the Mn atoms (Ofuchi et al. 2001b). 

Panhli, F., Manceau, A., Sarret, G., Spadini, L., Bert, V., Kirpichtchikova, T., Matthew, M., and Ahamdach, N. (2005). Evolution of Zn speciation induced by phytostabilization in a contaminated sediment, using scanning electron microscopy, x-ray fluorescence, EXAFS spectroscopy, and principal component analysis. Geochim. Cosmochim. Acta 69, 2265-2284. 

Fig. 10.12. Cell for temperature dependent fluorescence EXAFS measurements of liquids wilh soli X-rays. The cell eonsi.sts of holder (1). window flange (2), double-sided flange (.3) containing Ihe liquid sample, blind flange t4). thermocouple (5), cartridge heater (6), copper gaskets (8), screw bolls (9) and polymer coated beryllium window (10). During the experiment, die cell is mounted on the cold linger (7) of a sample maiiipu-lalor I lOHl.

Parkhurst DA, Brown GE Jr, Parks GA, Waychunas GA (1984) Structural study of zinc complexes in aqueous chloride solutions by fluorescence EXAFS spectroscopy. Abstracts with Program Geol Soc Am AnnMtg 16 618... 

Lagarde P, Delaunay R, Flank AM, Jupille J (1993) Site of sulfur impurities in silicate glasses and REFLEXAFS studies around the Si -edge. Jap J Appl Phys 32 619-621 Lee PA, Citrin PH, Eisenberger PM (1981) Extended X-ray absoption fine structure-its strength and limitations as a structural tool. Rev Mod Phys 53 769-806 Lee JM, Yoo H-H, Joo M (1999) Numerical determination of a true absorption spectrum from grazing-incidence fluorescence EXAFS data. J Synchrotron Rad 6 244-246 Lieser KH, Flakowski M, Hoffman P (1994) Determination of trace elements in small water samples by total reflection X-ray fluorescence (TXRF) and by neutron activation analysis (NAA). Fresenius J Anal Chem 350 135-138... 

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... 

Other setups are described, allowing simultaneous EXAFS/UV-vis measurements [20] and combined transmission/fluorescence EXAFS/XRD (X-ray diffraction) measurements [22]. The specially designed reaction cuvette for simultaneous EXAFS/UV-vis measurements is shown in Figure 3.4. 

A schematic representation of the setup used for transmission/fluorescence EXAFS/XRD measurements of heterogeneous catalysts in the gas and hquid phases is shown in Figure 3.5, wherein online product analysis is performed by either mass spectrometry or FTIR spectroscopy. 

Figure 3.5 Experimental setup for combined transmission/fluorescence EXAFS/XRD measurements. (Reproduced from Ref [22].)...

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... 

Acronyms abound in phofoelecfron and relafed specfroscopies buf we shall use only XPS, UPS and, in Sections 8.2 and 8.3, AES (Auger elecfron specfroscopy), XRF (X-ray fluorescence) and EXAFS (exfended X-ray absorption fine sfmcfure). In addition, ESCA is worth mentioning, briefly. If sfands for elecfron specfroscopy for chemical analysis in which elecfron specfroscopy refers fo fhe various branches of specfroscopy which involve fhe ejection of an elecfron from an atom or molecule. Flowever, because ESCA was an acronym infroduced by workers in fhe field of XPS if is mosf often used to refer to XPS rather than to electron spectroscopy in general. 

Shaw, C.F. Ill, Schaeffer, N.A., Elder, R.C., Eidsness, M.K., Trooster, J.M. and Calls, G.H.M. (1984) Bovine serum albumin-gold thiomalate complex gold-197 Moessbauer, EXAFS and XANES, electrophoresis, sulfur-35 radiotracer, and fluorescent probe competition studies. Journal of the American Chemical Society, 106, 3511-3521. 

Figure 1.1 The electiomagnetic spectrum, showing the different microscopic excitation sources and the spectroscopies related to the different spectral regions. XRF, X-Ray Fluorescence AEFS, Absorption Edge Fine Structure EXAFS, Extended X-ray Absorption Fine Structure NMR, Nuclear Magnetic Resonance EPR, Electron Paramagnetic Resonance. The shaded region indicates the optical range.

Inner electrons are usually excited by X-rays. Atoms give characteristic X-ray absorption and emission spectra, due to a variety of ionization and possible inter-shell transitions. Two relevant refined X-ray absorption techniques, that use synchrotron radiation, are the so-called Absorption Edge Fine Structure (AEFS) and Extended X-ray Absorption Fine Structure (EXAFS). These techniques are very useful in the investigation of local structures in solids. On the other hand, X-Ray Fluorescence (XRF) is an important analytical technique. 

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