Signatures of direct double ionization under xuv radiation

In anticipation of upcoming experiments on two-photon double ionization of atoms and particularly helium, under strong short-wavelength radiation sources (45 eV), we present quantitative signatures of direct two-photon double ejection, in the photoelectron spectrum (PES) and the peak power dependence, that can be employed in the interpretation of related data. We show that the PES provides the cleanest signature of the process. An inflection (knee) in the laser power dependence of double ionization is also discernible, within a window of intensities that depends on the pulse duration and cross sections.