Selection rules in few photon double ionization

We performed numerical simulations using a 3D model for He atom interacting with an intense ultrashort laser pulse. The results of our numerical simulations reveal a selection rule in few photon double ionization, namely in $N$-photon processes (with $N$ odd) the two ionized electrons cannot be emitted back-to-back, while this is allowed when $N$ is even. Furthermore, the difference between momentum space wavefunction projected onto plane waves and projected onto Coulomb wavefunction for the correlation between the two electrons is investigated.