Shake-up Processes

Fig 7 a-f. Shake-up and shake-off processes in photoionization (a) Real space picture illustrating contraction of the atomic charge density in response to the creation of a core hole (b, d) Shake-up (c, f) Conjugate shake-up and (e) inelastic scattering 

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This phenomenon is closely related to the monopole-excitation 171 > or shake-up process 1521 observed for several atoms and molecules. Noble gases... 

Figure 1. Schemes of the photoemission process (a) and the resultant XPS spectrum using CO as an example. Scheme of a core-level shake-up process in XPS(b).

We have already shown that excitation energies can be diagrammatically decomposed to yield simpler quantities such as ionization potentials and electron affinities plus some remaining diagrams. MB-RSPT permits the use of this treatment for even more complex processes. In this section, we present the applicability of the theory to double ionizations observed in Auger spectra as well as excitations accompanying photoionization (shake-up processes) observed in ESCA and photoelectron spectroscopy. A detailed description of this approach is given in Refs.135,136. Here we shall present only the formal description. 

Delley and Beck (4) took a shake-up process during the X-ray absorption as a source of different 4/ configurations and the peaks B, C and D were attributed to combinations of spin-orbit splitting and configurations of Ce Afi and AfL The parameters used were those which provided a theoretical Ce 3t/ photoelectron spectrum which was in good agreement with the experimental one. However, the intervals between the obtained peaks did not agree with the experimental X-ray spectrum. 

Any discussion of these structures in terms of shake up processes of adsorbed molecules can thus be rejected. 

Before discussing even more sophisticated photoionization correlated methods, we need to mention another area in which correlation is important, namely the shake-up processes. A shake-up ionization means that electrons are excited (or de-excited) while another is ejected, as shown in Figure 14. As the energy relative to the ground state M+ species is changed because of excitation upon ejection of the electron, such states typically appear as satellites on the principal photoelectron peak. In a favorable case, their intensity can be comparable to that for a primary ionization. In principle, there are an infinite number of such shake-ups which correspond to each primary ionization, but only a few will be intense enough to be observable. 

Nearly all the peaks in the calculated Nj spectrum have a number of basis operators that contribute significantly. This indicates that the simple molecular orbital picture of the shake-up process is insufficient. " The results emphasize the need for some selection technique, such as the perturbation theory approach employed here, in the choice of the primary operator space. The addition of several extra shake-up basis operators by the perturbation selection criterion lowers the peak from 17.12 to 16.79 eV. The most important of these extra operators involves removal of a electron and de-excitation of a second Itt electron to the lw level. The importance of this operator, which acts only on the correlation part of 0>, is not obvious by pure chemical intuition. 

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