Helium double ionization signals under soft-x-ray coherent radiation

In this work we present theoretical ionization yields of helium for photon energies where direct two-photon double electron ejection is allowed. We explore the photon energies and the laser peak intensities of the xuv pulse where the direct double ionization of helium is expected to dominate over the sequential channel. This is achieved with the separate calculation of the partial double ionization yield (sequential and direct). We generally conclude that a careful choice of the photon energy in combination with the peak xuv intensity can result in the clear dominance of one signal (direct) over the other (sequential). Avoiding photons where resonant He+ enhancement may appear, relative double ionization signals are less sensitive to pulse duration and the ideal candidate for the observation of the direct double ionization signal appears to be photons around 45 eV (29th harmonic of a Ti:Sapphire 800 nm laser), assuming the radiation is restricted to high order harmonic generation sources. For continuously tunable sources, such as the free electron lasers, any frequency around the minima between intermediate resonances will be as convenient. In addition, we have demonstrate that beyond a certain intensity, depending on wavelength and pulse duration, the signal from the direct will be overwhelmed by the sequential.