Dalgarno, A.Sadeghpour, H. R.2019-09-271992Dalgarno, A., and H. R. Sadeghpour. 1992. “Double Photoionization of Atomic Helium and Its Isoelectronic Partners at X-Ray Energies.” Physical Review A 46 (7): R3591–93. https://doi.org/10.1103/physreva.46.r3591.1050-29471094-1622http://nrs.harvard.edu/urn-3:HUL.InstRepos:41417400A simple and accurate procedure for calculating the rate of double photoionization of "two-electron" systems at very high photon energies is presented. Arguments are given to support the validity of the method used. In particular, we show that the many-body perturbation theory diagrams depend asymptotically on the choice of the gauge for the electric dipole operator. The ratio of double-to-single ionization is calculated to be 1.68% in agreement with the recent synchrotron measurements. For H- and Li+, we predict ratios of 1.51% and 0.89%, respectively.en-USJournal Article2019-09-2710.1103/PhysRevA.46.R3591