Photoelectron spectroscopy core relaxation

DV-Xa molecular orbital calculation is demonstrated to be very efficient for theoretical analysis of the photoelectron and x-ray spectroscopies. For photoelectron spectroscopy, Slater s transition state calculation is very effective to give an accurate peak energy, taking account of the orbital relaxation effect. The more careful analysis including the spin-polarized and the relativistic effects substantially improves the theoretical results for the core level spectrum. By consideration of the photoionization cross section, better theoretical spectrum can be obtained for the valence band structure than the ordinary DOS spectrum. The realistic model cluster reproduce very well the valence state spectrum in details. 

The main peaks in X-ray Photoelectron Spectroscopy (XPS) for molecules appear because of the photoionization of core electrons. In addition, satellite peaks on the high binding energy side of the main peak have often been observed. These peaks are generally referred to as shakeup satellite peaks. In the sudden approximation, the shakeup process which accompanies photoionization can be considered as a two-step process. First, a core electron is emitted as a photoelectron, creating an inner shell vacancy. In the next step, electron(s) in the same molecule transfer from valence orbital(s) to unoccupied orbital(s) with relaxation of orbital energies. It is important to study these satellites in order to understand the valence and excited states of molecules (1). 

The last chapter (chapter 109) is by D.R. Chopra on appearance potential spectroscopy (APS), which measures the probability for electronic excitation of a core level as a function of the incident electron energy. APS is based on a two-electron excita-tion/relaxation process and gives information on the density of unoccupied states. APS is a complementary technique to the more common Auger electron spectroscopy and X-ray photoelectron spectroscopy, and it also yields information on chemical bonding, near-neighbor configuration in the surface layer, and the chemical constituents. The author believes that when the different aspects of APS are fully exploited this technique will be accepted as a popular analytical tool for the material characterization of surfaces. 

In deep core level absorption spectroscopies the overlap of the core level wavefunction with the wavefunction of the excited state is of little or no importance. Deep core levels may be regarded as classical charge distributions, completely screened by the inner shells and the photoelectron. Most of the deep core level spectra can therefore be described neglecting many-electron processes. The large spatial overlap of the shallow 4d wayefunction with the 4f wavefunctions, however, favors relaxation processes of the excited atom involving spectacular many-electron effects as the giant resonances . The correlation of these exotic final-state structures... 

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