Correlation Effects in Atomic Structure Using the Random-Phase Approximation

A procedure for treating correlations in atomic structure is introduced and applied to the calculation of excitation energies, oscillator strengths, and photoionization cross sections. The method is the extension of the Hartree-Fock theory known as the random-phase approximation, which has already been applied to a number of other many-body problems. In this application to atomic physics results are given for the following atoms in column II of the periodic table: beryllium, magnesium, calcium, and strontium. These atoms all have ${}_{}{}^1{}_{}{}^{}S$ ground states, and only excitations to ${}_{}{}^1{}_{}{}^{}P$ states are considered. The general conclusion of the study is that the values of the oscillator strengths and photoionization cross sections are changed significantly by the correlations, while the changes in the values of the excitation energies are quite small. Whereever comparison with experiment is possible, the inclusion of these correlations improves the agreement between theory and experiment. Their effects are, however, not as marked as in highly degenerate infinite systems or in nuclei.