Theory of double photoionization of a two-electron atom: Circumventing the boundary conditions

We describe a method for calculating cross sections for double ionization of a two-electron atom (or ion) by one or more photons, and we present results of an application to double ionization of He by one photon. The method does not require the explicit inclusion of the asymptotic boundary conditions for two-electron escape; rather, the contribution for the single ionization channel to the inclusive rate is removed by using the projection operator that projects onto this channel. Energy and angular distributions for double ionization can be obtained by modifying this projection operator.