Two electron response to an intense x-ray free electron laser pulse

New x-ray free electron lasers (FELs) promise an ultra-fast ultra-intense regime in which new physical phenomena, such as double core hole formation in at atom, should become directly observable. Ahead of x-ray FEL experiments, an initial key task is to theoretically explore such fundamental laser-atom interactions and processes. To study the response of a two-electron positive ion to an intense x-ray FEL pulse, our theoretical approach is a direct numerical integration, incorporating non-dipole Hamiltonian terms, of the full six-dimensional time-dependent Schroedinger equation. We present probabilities of double K-shell ionization in the two-electron positive ions Ne8+ and Ar16+ exposed to x-ray FEL pulses with frequencies in the range 50 au to 300 au and intensities in the range 1017 to 1022 W/cm2.