The shape amplitude representation for photo-fragmentation by a single photon, given in a
recent publication (Walter et al 2003 Phys. Rev. Lett. 90 233001), is applied in detail to
double ionization of helium and molecular hydrogen. In particular, in the case of molecular
hydrogen the symmetries of the matrix element are examined in different coordinate
systems, leading to a simplification of existing selection rules and allowing new ones to be
derived. The implications of this representation for multi-differential cross sections and
circular dichroism are given.