Eigenchannel R-matrix study of two-photon processes including above-threshold ionization in magnesium

The approach based on the eigenchannel R-matrix method and multichannel quantum-defect theory, introduced by Robicheaux and Gao to calculate two-photon processes in light alkaline-earth atoms, has been implemented in jj-coupling introducing explicitly spin-orbit effects and employing both the length and velocity forms of the electric dipole transition operator. Emphasis is placed on the identification of the intermediate-and final-state resonances appearing in the cross sections. The efficiency and potential of the method are demonstrated in magnesium, by calculating two-photon ionization cross sections, branching ratios and photoelectron angular distributions for an extended photon energy range, including above-threshold ionization. The contributions of electronic correlation, core-polarization and spin-orbit effects are investigated by a systematic comparison with experimental results and earlier theoretical calculations.