Dynamics of two-photon double-ionization of helium at the attosecond scale

We study two-photon double ionization of helium. We demonstrate that attosecond pulses can be used to probe in the time-domain the electron dynamics within the atom. The interaction of helium with attosecond pulses whose duration is close to the correlation time provides information about the relaxation of the residual ion after the first electron ejection. Here, we show that such interaction of attosecond pulses with helium also modifies the double ionization probability in a non-trivial way. We examine the situations where sequential double ionization is permitted (omega >= 2 au) and where only the direct two-photon double ionization is allowed (omega < 2 au). All the calculations rely on both a model and the numerical solution of the time-dependent Schrodinger equation in its full dimensionality.