Effects in X-Ray Absorption

Inner-shell electrons of heavy atoms are subjected to the strong nuclear potential the interaction with other electrons is weak as compared to the interaction with the nucleus. Therefore, electron correlations are expected to become more important for outer shells and lighter atoms. One correlation effect is a two-electron transition induced by single-photon absorption. This process might heuristically be described by the shake model By absorption of the photon, one (inner-shell) electron is transferred to an unoccupied excited or continuum state thereby causing a change of the effective potential 

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Neel point, 27 38—46, 49 Neighboring atoms, effect in X-ray absorption spectroscopy, 35 22-23 Neodymia, conversion rates, 27 37 Neohexane... 

Kuzmin A, Parent Ph (1994) Focusing and snperfocnsing effects in X-ray absorption fine stractrrre at the iron K edge in FeFs. J Phys Conden Matt 6 4395-4404 Kuzmin A, Purans J, Parent Ph (1995) Irtfluence of the focttsing effect on XAFS in ReOs, W03 x and FeFs. Physica B208 209 45-46... 

Figure 21. Illustration of the thickness effect in X-ray absorption spectroscopy. The actual absorbance vs. energy is shown at the bottom. Due to these thickness effects, the measured signal (right) is related to the actual absorbance via a sub-linear transfer curve (Saturation). Two specific points along the curves are picked out with dotted lines and arrows, showing how the pre-edge features are raised relative to the edge. Notice also that the EXAFS amplitude in the Measured curve is reduced compared to its actual value. The Actual curve is transmission data for a Ti foil and the Measured curve is the fluorescence data for the same sample (6 pm, 45° incidence and exit angle). The Saturation curve comes from a fit between the Actual and Measured curves.

Extended energy-loss fine structures (EXELES) are analogous to the EXAES effect observed in X-ray absorption [2.224, 2.225]. These weak modulations (cf Eig. 2.39a), still observable ca. 100 eV away from the edge onset, occur because the excited elec-... 

Silica-alumina system, absorption and enhancement effects in x-ray emission spectrography, 222-224 Silicon, /as analyzing crystal, 318-327 determination by x-iay emission spectrography, 67, 219, 222-224, 260, 261, 329... 

EXAFS (Extended X-Ray Absorption Fine Structure). Characterization of the surface of metal nanoparticles had been limited to chemical methods, e.g., chemisorption of hydrogen and carbon monoxide. In 1970s, the situation was surprisingly changed due to the advances in x-ray absorption spectroscopy, especially extended x-ray absorption fine structure (EXAFS), Advances in this method have been achieved with the use of synchrotron radiation, which runs effectively at Tsukuba (Japan), Grenoble (France), etc. Now it is one of the most valuable tools to get structural information on bimetallic nanoparticles. 

In extreme cases a multiple-scattering, sharp resonant structure can result in which the electron is in a quasi-bound state (155). One example is the white line, which is among the most spectacular features in X-ray absorption and is seen in spectra of covalently bonded materials as sharp ( 2eV wide) peaks in absorption immediately above threshold (i.e., the near continuum). The cause of white lines has qualitatively been understood as being due to a high density of final states or due to exciton effects (56, 203). Their description depends upon the physical approach to the problem for example, the LiUii white lines of the transition metals are interpreted as a density-of-states effect in band-structure calculations but as a matrix-element effect in scattering language. 

Also, Suchet s method (280) for calculation of an effective charge q, which considers the effect of polarization and the sizes of the atomic radii for the cations and anions, is capable of interpreting the results of chemical shifts AE in X-ray absorption spectra in a satisfactory manner (145, 145a, 248). The effective ionic charge q is defined here as... 

The methods by which the phenomenon of interconfiguration fluctuations may be studied are (i) determination of lattice constant, (ii) magnetic susceptibility measurements, (iii) Mossbauer spectroscopy, (iv) measurement of electrical resistivity, (v) Hall effect, (vi) X-ray absorption spectroscopy and (vii) X-ray photoelectron emission spectroscopy. It is useful to note that a suite of techniques must be used to detect ICF phenomenon in a system. Nuclear magnetic resonance is sparingly used because not all the systems exhibiting ICF contain magnetically active nuclei. 

L. M. Murphy, S. S. Hasnain, R. W. Strange, I. Harvey, and W. J. Ingledew, Xafs Studies on Blue Copper Proteins The Effect of ph and Oxidation State Changes on the Copper Site, in X-ray Absorption Fine Structure , ed. S. S. Hasnain, EUis Horwood, London, 1991, p. 152. 

In the past 10 years, anomalous dispersion (AD) effects have been used more and more frequently to solve the phase problem. All elements display an AD effect in x-ray diffraction. However, the elements in the first and second row of the periodic table, for example, C, N, O, and so on, have negligible AD effects. For heavier elements, especially when the x-ray wavelength approaches an atomic absorption edge of the element, these AD effects can be very large. The scattering power of an atom exhibiting AD effects is... 

Several techniques have been developed to compensate for absorption and enhancement effects in X-ray fluorescence analyses. 

Fig. 4.1 Schematic potentials (bottom) and K-sheU spectra (top) of atoms and diatomic molecules. Resonances in K-shell spectra arise from electronic transitions from a 1 initial state to Rydberg or unfilled-MO final states. At the IP, corresponding to the threshold for transitions to continuum states, a step-like increase in X-ray absorption is expected. These effects lead to the characteristic spectra schematically shown in the upper part of the figure. In addiction to these one electron features other structures arising from multielectron transitions may be observed (Reprinted from StShr [2], with kind permission of Springer Science (2009))...

References of EXAFS proceedings [13 to 16] provide bibliography and point to the interest of EXAFS and XANES experiments in many domains of fundamental and applied research from physics to biology. See chemical effects on X-ray absorption spectra in Section 2.1.2.3.3, p. 242. 

In other applications of CT, orally administered barium sulfate or a water-soluble iodinated CM is used to opacify the GI tract. Xenon, atomic number 54, exhibits similar x-ray absorption properties to those of iodine. It rapidly diffuses across the blood brain barrier after inhalation to saturate different tissues of brain as a function of its lipid solubility. In preliminary investigations (99), xenon gas inhalation prior to brain CT has provided useful information for evaluations of local cerebral blood flow and cerebral tissue abnormalities. Xenon exhibits an anesthetic effect at high concentrations but otherwise is free of physiological effects because of its nonreactive nature. 

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