Two-photon double ionization of He(1s2) and He(1s2s 1S) by XUV short pulses

This paper addresses the problem of two-photon double ionization (TPDI) of He(1s2) and He(1s2s1S). First, we reconsider TPDI of He(1s2) with a photon energy of 2.1 a.u.; it is well known that TPDI is mainly a sequential process, resulting in an electron spectrum dominated by two peaks. In the past, we have noticed that the peaks are shifted as the pulse duration shortens; they move toward each other. The first objective of the present work is to clarify the origin of the shift and to evaluate it quantitatively. In parallel with the resolution of the time-dependent Schroedinger equation (TDSE), we have developed a model calculation based on lowest-order perturbation theory for the laser-atom interaction and for the atomic structure representation. The model agrees with TDSE calculations, and it also explains the physical origin of the shifts. Furthermore, it provides a quantitative evaluation of these shifts. The second objective of the present work is to extend our study to TPDI of He(1s2s1S), which has received much less attention until now. We have investigated the cases of photon energies of 1.8 and 2.5 a.u. We show that TPDI is dominated by a sequential process where the first ionization leaves a remaining He+(2s) ion, which is ionized by a second photon. As in the case of He(1s2), the model agrees with TDSE calculations, and it leads to better insights into the physics underlying the double-ionization process. (authors)