Evidence for highly correlated electron dynamics in two-photon double ionization of helium

Experiments using new sources of XUV pulses now tackle the difficult problem of few- photon direct double ionization of atoms. Despite its apparent simplicity, the fundamental process of two- photon direct double ionization of helium is far from being understood. Here, we use a time- dependent approach to study the process. Our results for the electron angular and energy distributions demonstrate that the dominant mechanism for double- electron escape involves a highly correlated electron motion. Angular correlations strongly favour back- to- back electron emission along the polarization axis, while dynamical screening leads to an equipartition of the electron energy for a broad range of field frequencies. These features are reflected in the recoil- ion- momentum distributions that are presently accessible to experiments.