Core-hole excitons

Because of these problems, observations of the screening of the core-hole and the creation of excitons in core level excitation has become a favored technique for observing the metal-insulator transition the most utilized here has been core-level X-ray photoelectron spectroscopy (XPS)5. 

Absorption of the X-ray makes two particles in the solid the hole in the core level and the extra electron in the conduction band. After they are created, the hole and the electron can interact with each other, which is an exciton process. Many-body corrections to the one-electron picture, including relaxation of the valence electrons in response to the core-hole and excited-electron-core-hole interaction, alter the one-electron picture and play a role in some parts of the absorption spectrum. Mahan (179-181) has predicted enhanced absorption to occur over and above that of the one-electron theory near an edge on the basis of core-hole-electron interaction. Contributions of many-body effects are usually invoked in case unambiguous discrepancies between experiment and the one-electron model theory cannot be explained otherwise. Final-state effects may considerably alter the position and strength of features associated with the band structure. 

Despite the potential, experimental spectra of ELNES and XANES have not been fully utilized in order to monitor the local structural and chemical environment. One of the major reasons is the presence of core-hole effects which leads to a redistribution of the PDOS features [10]. In other words, the presence of this effect has been considered as a bottleneck for the full interpretation of the experimental spectra. For example, O Brien et al. compared their XANES spectra of MgO, o -Al203 and MgAl204 at cation L2,3-edge with theoretical DOS obtained by band calculations, but their unoccupied DOS did not reproduce the experimental spectra [11]. Thus, the origin of the major spectral features was concluded to be the formation of a core exciton, i. e., a bound state of the excited electron due to the presence of a core hole. 

The CB DOS must reflect the presence of a core hole as dictated by the final state rule (M). The large peak at -1.7 eV in the O K for CuO and around -1.2 eV in the Cu Lm in Figure 4 is believed to arise from these cdCT and cpCB states, respectively (47). Note that an Eip-Ei excitation energy in Table 1 corresponds to a feature at Ei (binding energy) in Fig. 4. The cdCB state in CuO is excitonic-llke as the 0 3p DOS drops into the 2 eV gap because of the core hole. CuiO is a filled band in the ground state, bo only a cCB feature appears around -1. eV. This feature in CutO has helped... 

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. 

The excitonic state in MgO (bulk) arising from the creation of a hole in the Mg-2p core level appears as an impurity level within the band gap of MgO. Previous cluster model studies provided theoretical evidence of the local character... 

An explanation for the observed effects can be based on the assumption that carboxyl groups in protonated form may be a good photoelectron acceptor. This may be associated with the known easier electrochemical reduction of organic acids at low pH [8]. Excited electron-hole pairs in ZnSe core may recombine in few possible ways. First, a direct recombination results in the appearance of excitonic emission band at A,=408 nm. The second possible pathway is the energy transfer to Mn ion followed by Mn emission at A.=590 nm. At the neutral and acidic pH an additional recombination channel may be realized via trapping of photoelectrons by carboxyl groups (prior to the energy transfer to Mn ions)... 

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