Quantum-classical hybrid approach to helium double photoionization

The photoionization process is divided into (A) the absorption of the photon by one electron and (B) the correlated motion of the electron pair leading to singly or doubly ionized helium. We relate (A) to the total cross section to be calculated analytically in the quasiclassical reflection approximation as known from molecular problems. For (B) the two-electron wave function is propagated with the semiclassical version of Feynman's path integral to separate single and double ionizing events. A probe for (B) is the ratio between double and single ionization. The results for (A) and (B) and for absolute ionization cross sections obtained by combining (A) and (B) are in good agreement with different experiments that cover together a range of photon energy from the double-ionization threshold to several hundred eV.