Electron correlation (EC) in atoms is at the same time very important and
challenging investigation topic due to its many-body interactions, nevertheless it may
be the main process to describe the absolute electron impact cross section for the
ionization of atoms and molecules. The outer-shell double photoionization of a multielectron
target is usually a less important process in comparison to ionization of a
single electron by a photon and it is determined completely by electron-electron
interaction. The objective in this chapter has been upon a review of the recently found
relationship between SO amplitudes and the number of target electrons as well as their
density (electrons per volume). Through the comparison between the asymptotic values
of the experimental ratios of the double-to-single photoionization cross-sections from
the literature, a scaling law was achieved. This scaling allows us to predict the SO
amplitudes for several atomic elements up to xenon within a factor two. The electron
SO amplitudes following outer-shell photoionization have been plotted as a function of
the target atomic number, Z, and static polarizability, α . Our results are in
qualitative agreement with the experimental data.
