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Few-photon induced breakup of helium is studied using a newly developed abinitio numerical framework for solving the six-dimensional time-dependentSchroedinger equation. We present details of the method and calculate(generalized) cross sections for the process of two-photon nonsequential(direct) double ionization at photon energies ranging from 39.4 to 54.4 eV, aprocess that has been very much debated in recent years and is not yet fullyunderstood. In particular, we have studied the convergence property of thetotal cross section in the vicinity of the upper threshold (54.4 eV), versusthe pulse duration of the applied laser field. We find that the cross sectionexhibits an increasing trend near the threshold, as has also been observed byothers, and show that this rise cannot solely be attributed to an unintendedinclusion of the sequential two-photon double ionization process, caused by thebandwidth of the applied field.

A numerical study of two-photon ionization of helium using the Pyprop framework