Absorption core-like electronic states

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... 

The X-ray excitation process frequently is analyzed in terms of an excitonic electron hole pair (e.g. Cauchois and Mott 1949). The excitonic approach to X-ray absorption spectra accounts for the fact that the excited state is a hydrogen-like bound state. The X-ray exciton is different from the well-known optical excitons. In the latter cases the ejected electron polarizes a macroscopic fraction of the crystal-fine volume because the lifetime of optical excitations is in the order of lO s. The lifetime of the excited deep core level state, however, is in the order of 10 — 10 s, much too short to p-obe more than the direct vicinity of excited atom. Following Haken and Schottky (1958) the distance r between the ejected electron and core hole of an excited atom for E = 1 turns out to be r oc [h/(2m 0))] Here m denotes the effective mass of the ejected electron, to is the phonon frequency and is the dielectric constant. A numerical estimate yields r 10 A. Thus the information obtainable in an L, spectrum of the solid is very local the measurement probes essentially the 5d state of the absorbing atom as modified from the atomic 5d states by its immediate neighbors only. It is not suited to give information about extended Bloch states. On the other hand it is well suited to extract information about local correlations within the 5d conduction electrons, whose proper treatment is at the heart of the difficulty of the theory of narrow band materials and about chemical binding effects. 

Although the majority of the lipids in M. laidlawii membranes appear to be in a liquid-crystalline state, the system possesses the same physical properties that many other membranes possess. The ORD is that of a red-shifted a-helix high resolution NMR does not show obvious absorption by hydrocarbon protons, and infrared spectroscopy shows no ft structure. Like erythrocyte ghosts, treatment with pronase leaves an enzyme-resistant core containing about 20% of the protein of the intact membrane (56). This residual core retains the membrane lipid and appears membranous in the electron microscope (56). Like many others, M. laidlawii membranes are solubilized by detergents and can be reconstituted by removal of detergent. Apparently all of these properties can be consistent with a structure in which the lipids are predominantly in the bilayer conformation. The spectroscopic data are therefore insufficient to reject the concept of a phospholipid bilayer structure or to... 

Below the ionization edge, features in the absorption spectra called discrete resonances are due predominantly to transitions of the core electron to bound states that result from configurations in which the core electron has been promoted to a valence or Rydberg orbital or an appropriate linear combination. These states are degenerate with the ionization continua of other configurations and therefore can autoionize in an Auger-like process where one electron falls into the core hole and another electron is emitted. 

Pig. 6.1. The principle of x-ray absorption spectroscopy (A), in which the incident photon is absorbed by the excitation of a core electron into an unoccupied valence state. Element specificity (B), sensitivity to the local chemical environment (C), and the sensitivity to the presence of orientational order (D) result from the core electron being localized at a specific atom in an atomic-like orbital.polyimide... 

The widths and profiles of Miv-v absorption lines jnovide important information on the 4f potential barrier which shields 4f electrons fiom the external electrons in a R solid. This effect is more or less visible in the widths and profiles of absorption lines. The lines in the Mv spin-orbit group are almost symmetric and are well represented by a Lorentzian shape. The Miv lines are asymmetric and exhibit a Fano-like profile. Final states with either 3ds/2 or 3ds/2 core holes decay primarily by Auger transitions. The fluorescence decay is small (Connerade and Kamatak 1981). States built on the 3d3/2 hole state have an additional decay channel, leading to autoionization if they interact with... 

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