Double ionization of beryllium-like ions by high-energy photons

The cross sections for double ionization of L-shell electrons by a high-energy photon are calculated in leading order of the nonrelativistic perturbation theory. Two atomic systems are considered as targets, in which the K shell is either empty or completely occupied. The first case corresponds to a helium-like ion in the doubly excited autoionizing 2s2(S1) state, while the second case describes a beryllium-like ion in the ground state. For the valence shell of the Be isoelectronic sequence, the universal high-energy scalings for the total cross section and for the double-to-single ionization ratio are found. The partial contributions due to the channels of simultaneous and sequential emissions of two electrons are also considered.