Time-dependent analysis of the mechanism for two-photon double escape in helium: from very long to attosecond time scales

We consider the non-sequential double ionization of helium by two XUV photons. To gain further insight into the double-escape mechanism, we examine the interaction of helium with attosecond pulses. Such an interaction allows us to unveil the timescale on which the two electrons interact. By analysing the behaviour of the electron angular and energy distributions versus the pulse duration, we show that in the limit of ultrashort pulse durations, it becomes possible to disentangle the process in which each electron absorbs one photon from the process in which one electron absorbs both photons while ejecting the other electron by a collision. These results, which confirm the double-escape mechanism that we had proposed earlier (2008 J. Phys. B: At. Mol. Opt. Phys. 41 051001), are obtained using two different time-dependent theoretical approaches. The first one is based on Jacobi matrix calculations to extract the relevant information from the ionized wave packet. In the second one, we project the final wave packet on two Coulomb functions. We also develop a simple model based on time-dependent perturbation theory.