The Role of Nuclear Motion in the Photo-Double Ionization of Molecular Hydrogen

We explain the origin of recently observed variations with internuclear distance (R) of the fully differential cross sections for double ionization of aligned H$_2$ by a single photon. Using the results of converged numerical solutions of the Schr\"odinger equation, we show that these variations arise primarily from pronounced differences in the R-dependence of the parallel and perpendicular components of the ionization amplitude. We also predict that R-dependences should be readily observable in the asymmetry parameter for photo-double ionization, even in experimental measurements that are not differential in the energy sharings between ejected photo-electrons.