Core hole effect

Analysis of Core-Hole Effect in Cation L2,3-Edge of MgO, a-Al20s and Si02 Based on DV-Xa Cluster Calculations... 

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. 

Core-Hole Effect in MgO, a-AiPg and SiO state can be calculated as,... 

The core loss structure in electron energy loss spectroscopy (EELS) is known to be very similar to XANES, because the core loss spectrum is caused by physically the same process as that of x-ray absorption, corresponding to the electronic transition from core level to unoccupied excited states. Therefore, the theoretical analysis for ELNES can be carried out by almost the same procedure used for that of XANES. For the chemical state analysis of oxide ceramics, ELNES has also been proved to be very efficient with theoretical analysis by DV-Xa cluster calculation . The cluster calculation indicates that the core-hole effect due to the electronic transition is sometimes very important and the ground state calculation gives a serious errors in excited electronic state. 

X-ray absorption near edge structure(XANES) and electron energy loss near edge structure(ELNES) show similar spectra, thus almost the same theoretical analysis is valid both for these spectroscopies. The careful analysis including higher excited atomic orbitals in the basis set reproduce very well the spectral structure even in details. The core-hole effects are sometimes very important in the excited state electronic state. The theoretical analysis by the cluster model calculation provides very useful information on the local electronic property and the micro structure. 

Figure 5 Theoretical profiles (ELNES) of the Ti L edge in the TiOa-rutile and TiO compared with experimental ELNES from 100 nm particles. No core-hole effect is accounted for.

It is actually possible to use DFT calculations to compute XANES for amorphous and crystalline PCMs. To compute the XANES spectra, we use a new implementation by one of the present authors (V.L.), employing the PAW formalism [44] (as implemented in the VASP code [45, 46]) and including a self-consistent treatment of the core hole effects on the unoccupied states. 

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