The helium atom and helium-like ions' interaction with XFEL radiation

We study double electron ejection with one- and two-photon absorption in He and He-like ions. We use two different numerical methods aimed at solving the time-dependent Schrödinger equation for two-active electron systems interacting with a strong and short XUV laser pulse. The two approaches, which are relevant to the Configuration Interaction (CI) method, are based on an expansion of the wave function on either B-splines or Sturmian functions for the radial part and bipolar spherical harmonics for the angular part. We treat first double electron ejection with one photon in helium. Then the case of double electron ejection with two photons is studied along the isoelectronic series of He. We calculate the total probabilities for double ionization as well as the energy distribution of the electrons in the double continuum. Emphasis will be put on the role of the correlations along the He isoelectronic series. The pertinence of the distinction between direct and independent-particle (sequential) models for double ionization will be discussed in the context of short pulses.