Ultrarelativistic limit for the two-electron photoionization cross section

The ratio of double- to single-electron photoionization cross section is calculated in the ultrarelativistic photon frequency region, namely where the photon energy is much larger than the electron rest mass. This ratio can be presented as a sum of the shake-off term and the quasifree term. They are expressed via essentially different integrals of the initial-state two-electron wave function. The shake-off term is calculated in the dipole approximation. The quasifree contribution is nondipole and is determined by the probability of finding both ionizing electrons at the same point in the initial state. The calculations are performed for the negative hydrogen ion, the helium atom, and the heliumlike ions in the ground and excited states using high-precision nonvariational wave functions. The nuclear charge dependence of this correction for heliumlike ions in the ground and the four lowest ¹S excited states is calculated. The possibility to detect the quasifree contribution experimentally is discussed.