The recent progress in the high-order harmonic generation technique has enabled the development of new soft X-ray sources of ultrashort duration and high-power. In this study, we investigate two-photon double ionization of helium by attosecond intense soft X-ray pulse, based on numerical solution of the time-dependent Schrödinger equation (TDSE). Specifically we consider the 13.6 nm wavelength, which corresponds to the 59th harmonic of a Ti:sapphire laser. We solve the two-electron TDSE based on the time-dependent close-coupling method. At the 13.6 nm wavelength (91.5eV), a single photon suffices to induce double ionization. With intense sources, we expect a process where a He atom absorbs two photons and ejects the two electrons, i.e., above-threshold double ionization (ATDI). Upon sequential ionization, the electron energy spectrum would normally have two sharp peaks at 67 and 37 eV. Our results shows that between the two peaks another component (anomalous component) emerges in the attosecond regime. If we assume that the first and second ionization takes place independently, the mean interval is 216 as for the 450 as pulse length. On the other hand, the correlation time of the ground state is 22 as. Hence, the second electron has no time to relax before ejection with significant probability. This manifests itself as the anomalous component. We have constructed a simple model in which the ionization interval is chosen randomly, from which we calculate the energy of ejected electrons, taking account of the core relaxation time. Thus obtained spectrum agrees surprisingly well with the TDSE result.
Department of Quantum Engineering and Systems Science, University of Tokyo