Interference between nonsequential and sequential two-photon double ionization in short XUV pulses

We investigate the effects of electron correlation in the two-photon double ionization of helium for ultrashort XUV pulses with durations ranging from a hundred attoseconds to a few femtoseconds. We perform time-dependent \emph{ab initio} calculations of the two-electron Schr\"odinger equation. For photon energies above $65.4\,$eV, two-photon double ionization can proceed via two different channels: one where the intermediate state after one-photon absorption is the ground state of the He$^+$ ion, and another where it is an excited (shake-up) state. The ``nonsequential'' contribution from the ground state channel can then interfere with the ``sequential'' contribution from the shake-up channel. The relative strength of these contributions is controlled by the pulse duration. This observation may provide a route toward measuring the pulse duration of FEL pulses.