Few-electron dynamics in the interaction with strong fields

Full text: Simple single active electron models have been highly successful in describing the interaction of strong laser fields with atoms and have inspired a large number of experiments. Still, the validity and predictive power of these models for few-electron atoms and in particular for molecules remains to be investigated. There are only few (semi-)analytical models that include electron correlation. Alternatively, the numerical integration of the time-dependent Schroedinger equation (TDSE) is a challenging task because of the exponential growth of the problem size with the particle number, which has largely limited computations to n = 2 active electrons. In recent years, we have developed the MCTDHF (Multi-Configuration Time-Dependent Hartree-Fock) method for the solution of the TDSE for atoms and molecules in IR laser fields. The method scales like ∼ n4 with the number of active electrons, as opposed to the exponential scaling encountered in direct discretizations of the TDSE. Different from time-dependent density-functional theory, which has an even more favorable n2-scaling, MCTDHF allows for the straight forward computation of two-electron observables and for systematic convergence studies. As examples, we show calculations of the strong field ionization of linear molecules with up to 6 active electrons, electron-assisted laser ionization of an atom, high harmonic generation on a diatomic 4-electron molecule, and the XUV-IR pump-probe ionization of helium. While for our parameters, the effects of electron correlation on ionization are rather straight forward and can be incorporated into simple models, we find dramatic qualitative modifications of the high harmonic spectrum by multi-electron effects. For the pump-probe scenario, we find find significant crosstalk between the XUV pump and the IR probe pulses. (author)