Trends in the theory of atomic photoionization

For VUV photon energies, detailed theoretical understanding of the photoionization process has been achieved for the case of a single electron ionized from one of the outer subshells of a closed-shell atom. This understanding is based qualitatively upon a few properties of realistic atomic central potentials and quantitatively upon a few distinct types of electronic interactions known generically as particle-hole interactions. Current theoretical research is mainly directed at understanding a broader class of atomic photoionization phenomena involving, e.g., open-shell atoms, electronic interactions not of the particle-hole class, real two-electron processes, and external electric and magnetic fields.