FISCI

After this necessary excursion into the broad field of electron-electron interactions as described in diagrammatic language and by certain approximations, the effects of electron correlations beyond the RRPA approach can be summarized by using the following rules (neglecting the photoelectron s self-energy diagram) ISCI and FISCI must be taken explicitly into account, and the theoretical ionization threshold has to be adapted to the experimental value. 

Similarly, FISCI can be incorporated into the wavefunction of the final ionic state ... 

The coefficient a0 belongs to the process under consideration if FISCI effects are not dramatic it will have the largest value. As well as the characteristic 2p-2h excitations attached to the coefficients f, terms of lp-lh excitations have also... 

As demonstrated in the previous section, satellites are due to electron correlation effects, and, in principle, all types which are classified in a configurational picture as initial state configuration interaction (ISCI), final ionic state configuration interactions (FISCI) and final state configuration interactions (FSO which includes interchannel interactions in the continuum) have to be taken into account. In certain cases, however, one type of correlation is more important than the others, and in the present case of 3s and 3p photoionization in argon this is FISO. This property allows a rather transparent analysis of the implications which these correlations have on the corresponding satellites. 

The interaction matrix element relevant for FISCI in the 2S manifold is given by... 

Based on early theoretical calculations, which included FISCI, the most intense satellite structures of Fig. 5.11 were attributed to the following final ionic states (see [AWK78] and references therein) ... 

The FISCI method will be considered in more detail for the 2Se manifold of 3s photoionization in argon, even though finer details of the satellite spectrum can be understood only if further electron correlations and spin-orbit effects are included (see below). In the FISCI approach, uncorrelated functions < >[ with single-particle orbitals provide the starting point. They are abbreviated to their characteristic electron orbital, i.e.,... 

Group (a) satellites (shaded ////). These contain final ionic states from the 2Se manifold, and they are due to FISCI associated with 3s ionization. One can clearly see the 3s23p4(1D)nd 2Se satellites with n = 3, 4, 5, 6 and the small 3s23p4(1S)4s 2Se satellite. 

Group (c) satellites (dotted). These arise from electron correlation effects which are beyond the FISCI mechanism. The large spatial overlap between orbitals with the same principal quantum number n =v3 still favours electron correlation effects between 3s3p6 and 3s23p43d, but ISCI and FSCI are responsible for the presence of 3s23p43d 2S+1Le final ionic states with symmetries different from 2Se. Most of these states can be identified in the electron spectrum. 

Of course, the dominance of FISCI in the argon satellite spectrum at intermediate and high photon energies only provides a rough orientation. In general, electron... 

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