Core electron excitation binding energies

Fig. 3.27 (a) The probability Qt to create a core hole in a level with binding energy E with a primary electron of energy Ep maximizes for Ep/Ej 2-3. (b) Auger decay is the preferred mode of de-excitation in light elements, while X-ray fluorescence becomes more important for heavier elements. 

In electron correlation treatments, it is a common procedure to divide the orbital space into various subspaces orbitals with large binding energy (core), occupied orbitals with low-binding energy (valence), and unoccupied orbitals (virtual). One of the reasons for this subdivision is the possibility to freeze the core (i.e., to restrict excitations to the valence and virtual spaces). Consequently, all determinants in a configuration interaction (Cl) expansion share a set of frozen-core orbitals. For this approximation to be valid, one has to assume that excitation energies are not affected by correlation contributions of the inner shells. It is then sufficient to describe the interaction between core and valence electrons by some kind of mean-field expression. 

If electromagnetic radiation with a sufficiently short wavelength hits a specimen, then electrons may be excited and leave its surface. By the use of X-rays, also electrons of core levels may be emitted. X-ray Photoelectron Spectroscopy (XPS) examines the kinetic energy of these photo-emitted electrons in an electrostatic energy analyzer. A simple energy balance permits the evaluation of the binding energy Eg of the level from which the electron is emitted. 

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