Photoelectron energy spectrum in `direct' two-photon double ionization of helium

We study the photoelectron energy spectrum in direct two-photon double ionization of He, and show that in certain ranges of photon frequency it competes favourably with sequential double ionization. When the photon frequency allows two-photon double ejection in both the sequential and direct channels, the latter appears as a background dominated by the peaks of the sequential channel. However, in a different frequency regime, where the sequential channel requires three photons, the spectrum of the direct channel is separated in energy and dominates in magnitude. Our results have been obtained for a range of radiation intensity and pulse duration reasonably expected in high-order harmonic generation sources.