X-ray absorption and emission spectroscopies

The XAFS spectrum, %(E), is defined as the normalised oscillatory structure of the X-ray absorption, e.g. %(E) = ( l(E)- Po(E)/ Jo(E)), where Po(E) is the smooth varying atomic-like background absorption. Essentially, the XAFS spectrum implicates the quantum-mechanical transition from an inner, atomic-like core orbital electron to an unoccupied, bound (pre-edge transition) or unbound, free-like continuum level. The oscillatory structure therefore reflects the unoccupied part of the electronic bands/structure of the system in the presence of a core-hole. Note that this differs from the initial, ground state by physical effects induced by the fact that the 

Historically, it was controversial to recognise the local nature of the XAFS techniques (see Lytle for a recent discussion) but now the theoretical description of the techniques is reasonably well settled. The absorption of X-ray by matter is described in many text books. The treatment of the radiation as an electric field without practical spatial variation on a molecular/local scale and eliminating magnetic parts leads to the Fermi Golden Rule for the X-ray cross-section  

The intensity of the absorption process is then proportional to the square of the transition matrix element connecting the initial ((j) ) and final (( )f) states times, a delta function which ensures that it satisfies the conservation of energy theorem. The elimination of the spatial dependence of the electric field corresponds to a series expansion of its dependence up to the first term (linear dependence in r — Eq. (4.1) this yields the dipole approximation of the interaction energy between the atom electronic cloud and the X-ray radiation field. Better approximations will include quadrupole and octupole terms and so forth. However, except in a few cases, some of them detailed here, the dipole approximation gives a quantitative analysis of the XANES shape and EXAFS oscillations. 

An important approximation is to assume that the matrix element can be rewritten into a single-electron matrix element. This is based on the sudden approximation which allows the transition element in Eq. (4.1) to be rearranged in terms of an overlap term of the N-1 inactive electrons, which is roughly independent of 

As a general rule, Eqs (4.1) and (4.2) can be solved by using modern quantum-mechanical methods. Two general approaches are discerned. The first one involves ab-initio or DFT methods to 

---

X-ray absorption and emission spectroscopy is a field with a distinguished history. At the beginning, i.e., from 1913 to the early thirties, these spectroscopies were dedicated to a systematic exploration of the atomic structure in the context of the periodic system of the elements. The intense work of numerous spectroscopists, which contributed prominently to the foundations of modern atomic physics and to the development of quantum theory, was reviewed in the classical books Spektroskopie der Rontgenstrahlen by M. Siegbahn (1913) and X-Rays in Theory and Experiment by Compton and Allison (1935). 

S. P. (2000) Electronic structure of chemically-prepared LixMn204 determined by Mn X-ray absorption and emission spectroscopies. /. Phys. Chem. 

Water-molecule dimers are proposed to be the main, low-energy, component of liquid water. The molecular distances under interaction of two polarized dimers are presented in Fig.la and in Supplementary Tablel. This molecular structure is consistent with measurements of X-ray and neutron scattering [17], and with recent reports of X-ray absorption and emission spectroscopy, revealing proton delocalization [18], strong electron sharing [19] and double, rather than tetrahedral. 

X-rays provide an important suite of methods for nondestmctive quantitative spectrochemical analysis for elements of atomic number Z > 12. Spectroscopy iavolving x-ray absorption and emission (269—273) is discussed hereia. X-ray diffraction and electron spectroscopies such as Auger and electron spectroscopy for chemical analysis (esca) or x-ray photoelectron spectroscopy are discussed elsewhere (see X-raytechnology). 

The tunability of X-rays greatly facilitates techniques such as X-ray absorption fine structures (XAFS), an element sensitive tool for local structure and bonding investigation, and photoelectron spectroscopy (PES) and related phenomena such as Auger and X-ray fluorescence and emission spectroscopy. The photoelectron technique is surface sensitive and with the added advantage of tunability, it can be used widely in surface analysis. 

Atomic Absorption and Emission Spectroscopy Nuclear Magnetic Resonance Spectroscopy X-ray Methods Mass Spectrometry... 

Hafnium may be measured by atomic absorption and emission spectroscopy, x-ray fluorescence, ICP-MS methods, and neutron activation. Such instrument methods are faster than wet methods and can measure the metal at trace levels. 

The radioactive nature of the actinides, especially the transuranics, can introduce significant challenges in the characterization of their complexes. In order to prevent contamination, multiple layers of containment are often required, which can limit the types of studies that can be undertaken. However, a suite of spectroscopic tools has been used to study the chemistry and speciation of the actinides. A partial list of these techniques includes absorption, emission and vibrational spectroscopies, X-ray absorption and diffraction, and multinuclear magnetic resonance. 

These techniques fall into two categories those considered as routine (e.g. atomic absorption and emission spectroscopy, X-ray fluorescence) and a growing number of microanalytical surface techniques (e.g. laser microprobe mass analysis [LAMMA] and sensitive high-resolution ion microprobe [SHRIMP]). Each analytical technique requires specific sample preparation prior to analysis, as summarised in Table 13.1. 

X-ray spectroscopy can be classified in the same manner as every other type of spectral analysis into absorption and emission spectroscopy. However, the most popular method of x-ray spectroscopy in crude oil chemistry is the emission spectroscopy, also called x-ray fluorescence spectroscopy. The effect used by this type of spectral analysis is the same as was described for fluorescence analysis. However, x-rays are used for this analysis instead of the ultraviolet radiation used for fluorescence analysis. 

Studies of Molecular Dissociation by Means of Ultrafast Absorption and Emission Spectroscopy and Picosecond X-Ray Diffraction P, M. Rentzepis and B. Van Wonterghem... 

A Discusses structural analysis by x-ray crystallography A Explains chemical dynamics by photofragmentation translational spectroscopy A Covers kinetic analysis by ultrafast absorption and emission spectroscopy A Details syntheses of polycyclic caged amines, fuel additives, and polynitro compounds A Examines computer-aided design of monopropellants A Includes contributions by two Nobel laureates and five members of the National Academy of Sciences... 

---