Shake-up of a two-electron system is investigated in the strong infrared laser field limit, both theoretically and experimentally. During tunnel ionization the electron shakes-up a second electron to an excited bound state. Theoretically, a complete analytical theory of shake-up in intense laser fields is developed. Shake-up is measured experimentally by using the molecular clock provided by the internuclear motion. The number of measured events is found to be in excellent agreement with theory.
Besides shake-up there are a wealth of multi-electron phenomena in strong laser fields. We introduce the multi-configuration time-dependent Hartree-Fock (MCTDHF) method as a new approach towards the numerical solution of the time-dependent Schrödinger equation arising in ultrafast laser dynamics. MCTDHF approximates the exact wave function by several Slater determinants. By doing so the method produces a lower dimensional, non-linear system of coupled differential equations compared to the original Schrödinger equation.
To assess the reliability and efficiency of MCTDHF we test the method on two examples. We find rapid convergence for several quantities towards the exact results. The method converges, where time-dependent Hartree-Fock fails qualitatively.
By using one dimensional MCTDHF calculations we then investigate ionization of multi-electron systems. The laser induced multi-electron dynamics depend on the ratio of laser frequency to plasmon frequency discriminating two different regimes. In the over-resonant limit tunnel ionization is destroyed. Ionization takes place by a classical over the barrier mechanism. In the under-resonant limit tunnel ionization remains dominant, but is weakened by a polarization induced growth of the tunneling barrier.