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 $\alpha Z$ 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.

• Received 15 August 2003

DOI:https://doi.org/10.1103/PhysRevA.69.032703