Correlation effects for two-electron systems

Correlation phenomena are of fundamental importance in considering autoionising states in two-electron ions, where the collective movement of atomic electrons dominates over self-consistency effects. Explicit evaluation of correlation diagams in the theory of adiabatic S-matrix describes doubly excited states as atomic states proper, though in some cases it is necessary to take into account their resonance nature. One of the most popular ways of theoretical extracting autoionization resonances from continuum is the configuration-interaction theory. The present report first describes some general formulae, and then compares the configuration-interaction and perturbation theory energies. The first order perturbation theory for decay amplitude is also discussed. As an approximate solution of the configuration-interaction, an equation diagonalisation method has appeared in the early 70's, and has been successively used to reveal resonance features in atomic reactions. It is natural to undertaken the task of relating the results of the diagonalisation approach to those of the 1/Z perturbation theory. (N.K.)