Near-threshold double photoionization:

Double photoionization of the inert gas atom with residual ion in the state implies coherent population of and partial waves in the two-electron continuum. The final continuum states are governed by the system of coupled Schrödinger equations in hyperspherical coordinates. The general expressions for the double photoionization cross sections are derived with a special emphasis on the experimental geometries used in the current experiments. The dynamic equations are simplified within assumptions of the extended Wannier ridge model, i.e. by employing a semiclassical approximation for the motion over the hyperradius and restricting configuration space to the Wannier ridge (regions where both electrons are at equal distances from the atomic core). The resulting equations are solved numerically for the Kr atom. The initial state of the active electron pair is described by the Hartree - Fock function or by a correlated function obtained in the configuration interaction scheme. The role of various initial- and final-state correlation effects is analysed. The results are compared with the available experimental data.