Photoionization of quasi-two-electron atoms dominated by the doubly excited autoionization states

A theoretical procedure based on the configuration-interaction approach of Fano [Phys. Rev. 124, 1866 (1961)] is presented for the photoionization between states dominated by strong configuration interaction. By correctly identifying the main contributing interaction between the dominating bound configurations, the effectiveness of this procedure is demonstrated by its application to photoionization from the Mg 3s3p¹P excited state to the ¹S continuum dominated by the 3p² ¹S doubly excited autoionization state above the first ionization threshold. In addition to the good agreement with the experimental data, the individual contributions from different physical processes are also examined in detail.