Direct versus delayed pathways in strong-field non-sequential double ionization

We report full-dimensionality quantum and classical calculations of the double ionization (DI) of laser-driven helium at 390 nm. Good agreement between the quantum and classical results is observed. We identify the relative importance of the two main non-sequential DI pathways: the direct-with an almost simultaneous ejection of both electrons-and the delayed. We find that the delayed pathway prevails at small intensities independently of total electron energy, but at high intensities the direct pathway predominates up to a certain upper limit in total energy, which increases with intensity. An explanation of this increase with intensity is provided.