Valence-shell double photoionization of alkaline-earth-metal atoms

We apply the convergent close-coupling formalism to describe direct double photoionization (DPI) of the valence n s2 shell of alkaline-earth-metal atoms: beryllium (n=2), magnesium (n=3), and calcium (n=4). We consider the range of photon energies below the onset of resonant and Auger ionization processes where the subvalent and core electrons can be treated as spectators. By comparing alkaline-earth-metal atoms with helium, we elucidate the role of the ground state and final ionized state correlations in DPI of various quasi-two-electron atoms.