Above-threshold double ionization of helium with attosecond intense soft x-ray pulses

We theoretically study a process in which helium is doubly ionized by absorbing two soft x-ray (91.45 eV) photons from attosecond, intense high-order harmonic sources. We directly solve the time-dependent Schrodinger equation and obtain electron energy distribution in the double continuum. Our results show that between the two peaks in electron energy spectra, expected for usual sequential ionization, an additional component (anomalous component) is present. The total two photon above-threshold double ionization yield including the anomalous component is explained by sequential processes: the two electrons are ejected one by one, absorbing a single photon each. This observation rejects the intuition that nonsequential double ionization would be responsible for the anomalous component. With the help of simple semiclassical stochastic models, we discuss two possible origins of the anomalous component, namely, postionization energy exchange and second ionization during core relaxation, of which the latter is more plausible: the ionization interval is so short that the second electron is ejected while the two electrons are still exchanging energy.