Absorption-edge fine structure

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. 

X-ray techniques presently well known in catalysis include diffraction, small angle scattering, and fluorescence spectroscopy. X-ray absorption edge fine-structure spectroscopy, although known to the X-ray physicists for thirty years, is not well known in catalysis research. The promise which this field holds for catalysis has been apparent over much of the thirty year interval, but has been overshadowed by difficulties both experimental and theoretical. 

The present report is based primarily upon spectra of 95 known compounds of 4 elements in the first transition series and of several catalysts, obtained as part of an exploration of catalyst applications of absorption edge fine-structure spectroscopy. A previous progress report has been given... 

The absorption edges in Fig. 10.10 are not perfectly sharp, but have a delicate fine structure ("Kossel35 lines") that was first explored in the 1930s. Since about 1970, this fine structure is now used in EXAFS (extended X-ray absorption edge fine structure spectroscopy) and in XANES (X-ray absorption near edge spectroscopy) the oscillations are due, again, to a chemical shift, which can be used to identify the local chemical environment of the emitting element in the sample. 

In studies on the absorption-edge fine structure in organometallic complexes, Pauling s valence bond theory may be used in explaining the fine structure. 

FDH = Formate dehydrogenase CAR = Carboxyhc acid reductase FMDH = Formylmethanofuran dehydrogenase MFR = Methanofuran AOR = Aldehyde ferredoxin oxi-doreductase MPT = Molybdopterin Fdox = Oxidized ferredoxin Fdred = Reduced ferredoxin FOR = Formaldehyde ferredoxin oxidoreductase EXAFS = X-ray absorption edge fine structure kDa = Kilodaltons EPR = Electron paramagnetic resonance. 

Ashley, C.A. and Doniach, S. Theory of extended X-ray absorption edge fine structure (EXAFS) in crystalline solids." Phys. Rev. B11 1279-1288 1975. 

Figure 5.8 The polychromatic simultaneous profile technique for complete sampling of absorption edge fine structure. One oscillation photograph is shown 4.95° scan angle, ten oscillations, exposure time 2 min, SRS 1.9GeV 100 mA, energy window across each diffraction spot 67 eV centred on the dip of / at the rhenium Lm edge. From Greenhough et al (1983).

X-ray absorption spectroscopy combining x-ray absorption near edge fine structure (XANES) and extended x-ray absorption fine structure (EXAFS) was used to extensively characterize Pt on Cabosll catalysts. XANES Is the result of electron transitions to bound states of the absorbing atom and thereby maps the symmetry - selected empty manifold of electron states. It Is sensitive to the electronic configuration of the absorbing atom. When the photoelectron has sufficient kinetic energy to be ejected from the atom It can be backscattered by neighboring atoms. The quantum Interference of the Initial... 

In solid state physics, the sensitivity of the EELS spectrum to the density of unoccupied states, reflected in the near-edge fine structure, makes it possible to study bonding, local coordination and local electronic properties of materials. One recent trend in ATEM is to compare ELNES data quantitatively with the results of band structure calculations. Furthermore, the ELNES data can directly be compared to X-ray absorption near edge structures (XANES) or to data obtained with other spectroscopic techniques. However, TEM offers by far the highest spatial resolution in the study of the densities of states (DOS). 

A comparison of the absorption edge of catalyst A with and without MgO shows considerable differences (Figure 12). Though the oxidation state of V is essentially the same, the edge fine structure induced by MgO indicates that the local environment about V had been altered significantly. As with the cyclic microunit studies, the... 

Synchrotron-based N K-edge X-ray absorption near-edge fine structure (XANES)... 

Electrons Auger Electron Spectroscopy, Extended X-Ray Absorption Fine Structure, Low-Energy Electron Diffraction, Scanning Electron Microscopy, Surface Extended X-Ray Absorption Fine Structure, Ultraviolet Photoelectron Spectroscopy, X-Ray Absorption Near Edge Fine Structure, and X-Ray Photon Spectroscopy. 

In an effort to more fully elucidate the structure and reactivity of metal oxide pillared clays, we have been investigating the structure-reactivity properties of chromia-pillared derivatives (17). In the following sections, we provide an example of the structure-catalytic reactivity properties of chromia-pillared montmorillonites. Also, we report our initial efforts to structurally characterize the intercalated chromia aggregates by Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. Unlike previously reported metal oxide pillared clays, chromia-pillared clay exhibits strong K-edge absorption and fine structure suitable for determination of metal-oxygen bond distances in the pillars. 

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge fine structure (XANES) have been used with great success to characterize highly dispersed supported metals. EXAFS can provide information on the local structure of highly dispersed metal on a support, such as alumina, and XANES, measured at the same time, provides information on the valence state of the metal. 

The metal X-edge fine structure corresponds to the transition of a Is electron of the absorbing atom to the unoccupied levels of p symmetry situated just beyond the Fermi level, and to any such hybrid levels in the conduction band which may have a p admixture (23,24,116,199). The height of the absorption edge is related to the number of p electrons lacking. These transitions obey all selection rules, and in first-row transition elements the 4p orbitals are unoccupied the 4p-5p distances are about 12 eV and the distance ratio 4p-5p 5p-6p 6p-7p 4 2 1. With the broadening of the higher levels, 5p and 6p absorption often overlap. 

Surface Chemistry by X-ray Photoelectron Spectroscopy (XPS) and Near-Edge X-ray Absorption Spectroscopy Fine Structure (NEXAFS)... 

EXAFS Extended x-ray absorption fine structure [177, 178] Variation of x-ray absorption as a function of x-ray energy beyond an absorption edge the probability is affected by backscattering of the emitted electron from adjacent atoms Number and interatomic distance of surface atoms... 

Zaera F, Fischer D A, Carr R G and Gland J L 1988 Determination of chemisorption geometries for complex molecules by using near-edge X-ray absorption fine structure ethylene on Ni(IOO) J. Chem. Rhys. 89 5335-41... 

Fischer D, Marti A and Hahner G 1997 Orientation and order in microcontact-printed, seif-assembied monoiayers of aikanethiois on goid investigated with near edge x-ray absorption fine structure spectroscopy J. Vac. Sc/. Technol. A 15 (4) 2173-80... 

---