Ionization edge threshold

Near the threshold, ionization edges often show fine details, called ELNES, which contains information about the electronic structare of the probed material. In the event of ionization, electrons from a core shell are excited into unoccupied states above the Fermi level. Just near the threshold these states represent an unoccupied part of the conduction band, which is sensitive to changes of bonding and oxidation state in solids. Thus, it is possible to track bonding and... 

An attempt of identification of 5 f character has been recently reported for UO2 by resonant photoemission of the valence band region with excitation energies of 92 eV (off-resonance) and 98 eV (on-resonance). The core absorption edge used was the 5d ionization threshold, corresponding to a process ... 

Some partial photoionization cross sections, derived in this way for neon, are shown in Fig. 2.11 as a function of photon energy. The uppermost curve is the total absorption cross section. At the onset of the ionization thresholds for the ejection of Is, 2s and 2p electrons this quantity shows the corresponding absorption edges (see the discussion related to equ. (2.11)). The partition of the total cross section into partial contributions cr(i) clearly demonstrates that the dominant features are due to main photoionization processes described by the partial cross sections satellite transitions from multiple photoionization processes are also present. If these are related to a K-shell ionization process, they are called in Fig. 2.11 multiple KL where the symbol KLX indicates that one electron from the K-shell and X electrons from the L-shell have been released by the photon interaction. Similarly, multiple I/ stands for processes where X electrons from the L-shell are ejected. Furthermore, these two groups of multiple processes are classified with respect to ionization accompanied by excitation, (e, n), or double ionization, ( ,e). If one compares in Fig. 2.11 the magnitude of the partial cross sections for 2p, 2s and Is photoionization at 1253.6 eV photon energy (Mg Ka radiation) and takes into account the different... 

Although originally mainly the more pronounced near-edge features were observed and compared or interpreted (usually empirically), it is evident from a recent review (18) that today considerably greater emphasis is given to the study of the extended fine structure. This stands in relation to the improved experimental methods for detection of weak modulations and to the currently more advanced theoretical description of the EXAFS part of the spectrum. Complete understanding of the Kossel structure at the threshold part of an element s inner-shell spectrum, which contains among others valence orbital, ionization, and chemical shift information, is relatively slow due to the... 

Fig. 5. Schematic of X-ray absorption cross section as a function of the photon energy showing the threshold region (including pre-edge and edge regions), the EXAFS region, and relevant electron processes excitation of a core electron to a higher unoccupied atomic level, to the Fermi level (at absorption edge), and to the continuum (atom ionization).

The position of an edge denotes the ionization threshold of the absorbing atom. The inflection in the initial absorption rise marks the energy value of the onset of allowed energy levels for the ejected inner electron (216). For a metal this represents the transition of an inner electron into the first empty level of the Fermi distribution (242) and in case of a compound the transition of an inner electron to the first available unoccupied outer level of proper symmetry. Chemical shifts in the absorption-edge position due to chemical combination (reflecting the initial density of states) were first observed by Bergergren (27). 

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