Core hole spectroscopy

A popular electron-based teclmique is Auger electron spectroscopy (AES), which is described in section Bl.25.2.2. In AES, a 3-5 keV electron beam is used to knock out iimer-shell, or core, electrons from atoms in the near-surface region of the material. Core holes are unstable, and are soon filled by either fluorescence or Auger decay. In the Auger... 

How then, can one recover some quantity that scales with the local charge on the metal atoms if their valence electrons are inherently delocalized Beyond the asymmetric lineshape of the metal 2p3/2 peak, there is also a distinct satellite structure seen in the spectra for CoP and elemental Co. From reflection electron energy loss spectroscopy (REELS), we have determined that this satellite structure originates from plasmon loss events (instead of a two-core-hole final state effect as previously thought [67,68]) in which exiting photoelectrons lose some of their energy to valence electrons of atoms near the surface of the solid [58]. The intensity of these satellite peaks (relative to the main peak) is weaker in CoP than in elemental Co. This implies that the Co atoms have fewer valence electrons in CoP than in elemental Co, that is, they are definitely cationic, notwithstanding the lack of a BE shift. For the other compounds in the MP (M = Cr, Mn, Fe) series, the satellite structure is probably too weak to be observed, but solid solutions Coi -xMxl> and CoAs i yPv do show this feature (vide infra) [60,61]. 

Fig. 3.24 Electron energy scheme explaining the principle behind metastable atom electron spectroscopy. An excited atom M collides gently with an adsorbed molecule A the metastable atom is de-excited by electron transfer from the adsorbate to the core hole...

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. 

AES Auger Electron Spectroscopy Core-hole excitations are created, usually by 1-10 KeV incident electrons, and Auger electrons of characteristic energies are emitted through a two-electron process as excited atoms decay to their ground state. AES gives information on the nearsurface chemical composition. 

There are three main detection modes for EAPFS within the appearance potential spectroscopy (APS) technique./31/ First, one may monitor soft-x-ray emission due to the decay of the core hole left by the primary process. This is called SXAPS-EAPFS (Figure le). Second, it is also possible to monitor Auger electrons due to the same core-hole decay, as in AEAPS-EAPFS and AMEFS-EAPFS, cf. Figure If. Third, one may measure the remaining total intensity of... 

Additional useful information is provided as a consequence of the way Auger spectroscopy couples the core levels and the valence levels. In simple terms, the intensity of a KFF Auger transition is a measure of the valence charge available for decay into the core hole. Because a core hole is localized on a particular atom, this can give an indication of the amount of electron density in a particular valence orbital around the atom where the core hole was made. 

For chemical state analysis. X-ray emission spectroscopy(XES) for me valence state is very useful. The valence state XPS gives total DOS of the material, while XES reflects the partial DOS, because the dipole transition is predominant in the x-ray emission process. Thus a core hole in an s level, for... 

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. 

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