XUV-pump IR-probe resonant shake-up ionization of helium

Helium is an ideal prototype to investigate electronic correlation in ionization processes with high experimental [1-3] and theoretical accuracy~[3-6]. Pump-probe ionization processes through doubly excited states (DES), in particular, open a window on static correlation resolved in time~[6]. In this work we explore the role of DES in XUV-pump IR-probe ionization processes accompanied by shakeup, which is still largely unexplored. Indeed, only the recent advent of high-repetition-rate and intense XUV pulses from Free-Electron Lasers (FELs) have opened the way to study non-linear effects in coincidence measurements with high statistics~[7]. Here, we present emph{ab initio} calculations of the photoelectron signal in the single ionization of helium by intense FEL XUV radiation, in the presence of an IR dressing pulse, as a function of the photon and electron energy. Thanks to the long duration and tunability of the XUV pulse, it is possible to isolate the multiphoton shake-up signal associated to a specific intermediate DES. For this work, we have implemented a periodic extraction of the external portion of the wavefunction, which allows us to simulate the ionization of long-lived metastable states in the presence of pulses several tens of femtoseconds long within comparatively small quantization boxes.