Double photoionization of two-electron atoms based on the explicit separation of dominant ionization mechanisms

Double ionization by a single photon is often discussed in terms of two mechanisms, namely, shakeoff and knockout, dominant at high and low photon energies, respectively. We have developed a model to formulate the explicit but separate contribution of both mechanisms at all energies [Phys. Rev. Lett. 89, 073002 (2002)]. The separation is based on a quasiclassical formulation of knockout which is free from any shakeoff part since the latter is purely quantum mechanical. The relevance of each mechanism from threshold up to several keV photon energy is quantified and discussed in detail for the photoionization from the ground state of two-electron atoms. Photoionization ratios, integral and singly differential cross sections calculated for helium and other members of its isoelectronic sequence are compared to benchmark experimental data and recent theoretical results. A connection to Samson's half-collision model [Phys. Rev. Lett. 65, 2861 (1990)] is also given.