Multiconfiguration Tamm-Dancoff approximation applied to photoionization of the outer shells of Be and Mg

Abstract

A multiconfiguration version of the Tamm-Dancoff approximation (MCTD) is developed to study photoexcitation of atoms in cases where the simultaneous excitation of two atomic electrons is important. Starting from a multiconfiguration Dirac-Fock description of the ground state, the variational principle is applied to give equations for a final-state wave function consisting of both singly excited and doubly excited configurations. To illustrate the method, the MCTD equations are applied to determine the 2pns and 2pnd autoionizing resonances in the low-energy photoionization cross section of Be and the corresponding 3pns and 3pnd resonances in Mg. The predicted cross sections are found to be in good agreement with previous theoretical results as well as with the available experimental data.