Probing electron dynamics in the double photoionization process of two-valence electron systems with UV and soft X-ray free-electron laser pulses

We investigate the double photoionization process in the $X$ = He, Li$^+$, C$^{4+}$, Be, B$^+$ and Ne$^{6+}$ targets triggered by the absorption of a single photon of energy in the extreme UV/soft X-ray spectral region. Our theoretical method consists in solving the time-dependent Schr\"odinger equation with a spectral method of configuration interaction type \footnote{S. Barmaki et al., \textbf{J. Phys. B} 51, 105002 (2018)}, \footnote{S. Barmaki et al., \textbf{Phys. Rev. A} 89, 063406 (2014)}. The probe of the electron dynamics in the different systems shows that the way the outer electrons will leave their parent $X$ system is systematically dictated by the amount of excess photon energy available to them relative to the ionization potential of the corresponding $X^{+}$ ion \footnote{S. Barmaki et al., \textbf{Chem. Phys.} 517, 24 (2019)}.