Nonrelativistic double photoeffect on lithium-like ions at high energies

The total cross section for double ionization of lithium-like ions by a high-energy photon is calculated in leading order of the nonrelativistic perturbation theory. The partial contributions due to simultaneous and sequential emissions of two electrons are taken into account. The cross section under consideration is shown to be related to those for double photoeffect on the ground and excited 21,3S states of helium-like ions. The double-to-single ionization ratio is equal to R=0.288/Z2 for lithium-like ions with moderate nuclear charge numbers Z. However, even for the lightest three-electron targets such as Li and Be+, analytical predictions are found to be in good agreement with the numerical calculations performed within the framework of different rather involved approaches.