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As a step toward the full \emph{ab-initio} description of two-photon doubleionization processes, we present a finite-pulse version of thevirtual-sequential model for polyelectronic atoms. The model relies on the\emph{ab initio} description of the single ionization scattering states of boththe neutral and ionized target system. As a proof of principle and a benchmark,the model is applied to the helium atom using the {\tt NewStock} atomicphotoionization code. The results of angularly integrated observables, whichare in excellent agreement with existing TDSE (time-dependent Schr\"odingerequation) simulations, show how the model is able to capture the role ofelectron correlation in the non-sequential regime, and the influence ofautoionizing states in the sequential regime, at a comparatively modestcomputational cost. The model also reproduces the two-particle interferencewith ultrashort pulses, which is within reach of current experimentaltechnologies. Furthermore, the model shows the modulation of the joint energydistribution in the vicinity of autoionizing states, which can be probed withextreme-ultraviolet pulses of duration much longer than the characteristiclifetime of the resonance. The formalism discussed here applies also topolyelectronic atoms and molecules, thus opening a window on non-sequential andsequential double ionization in these more complex systems.

Two-photon double ionization with finite pulses: Application of the virtual sequential model to helium