Nonrelativistic double photoeffect on K-shell electrons

We investigate the double K-shell ionization of heliumlike ions caused by the absorption of a single photon with energies being much smaller than the rest energy of an electron. In the near-threshold region, differential and total cross sections of the process are calculated for light ions, taking into account the leading orders of the 1/Z and αZ expansions. QED perturbation theory with respect to the parameter 1/Z exhibits a fast convergence in the entire nonrelativistic domain for moderate nuclear charge numbers Z>~2. Going beyond the electric dipole approximation leads to a forward/backward asymmetry in the angular distributions for the ejected electrons with respect to the incident photon beam. A comparison of theoretical predictions for the ratio of double-to-single photoionization cross sections with available experimental data for a number of neutral atoms is also presented.