Fragmentation of Atoms in Strong Fields viewed with Reaction-Microscopes

The interaction of both, highly charged ions and intense lasers fields, with matter or atoms attract increasing interest because of fundamental questions to be addressed and because of the long term perspectives of potential applications. Such applications cover a wide range from inertial fusion driven by heavy ion or intense laser beams to new techniques in material science and medical treatments. These developments rely on a profound knowledge about the interaction of radiation with single atoms and about the dynamical correlation of electrons in ultra-short and strong fields. In this context ionization of atoms in intense fields plays a key role because the coupling between radiation and matter is mediated by the electrons. Theoretically, in particular the regime of large perturbations is of interest, where the radiation field strongly modifies the atomic states, where it interacts simultaneously with several electrons and where non-linear effects prevail. This puts severe constraints on theoretical descriptions revealing substantial problems in the treatment of the correlated motion of mutually interacting particles under the action of a time dependent force. In spite of these difficulties, successful theoretical approaches have been developed so far for single ionization of atoms by charged heavy-ion impact as well as by intense laser pulses. But, the extension to more complicated situations like e.g. double or multiple ionization has been identified as a key challenge for many-body Coulomb theories. Experimentally, the recent developments of advanced many-particle coincidence techniques to study atomic fragmentation allow to identify simultaneously the momenta of all reaction products emerging from a single collision with high resolution. These so called reaction microscopes consist of recoil-ion momentum spectrometers combined with an electron analyzer with 4π efficiency and high momentum resolution. They enable experiments on single and multiple ionization reactions of atoms and molecules with unprecedented completeness and momentum resolution (for a review see [1]). Such kinematically complete experiments provide benchmark data for theories and allow for the first time the separation of different mechanisms involved in the ionization process. In this report we discuss three topics concerning the fragmentation of atoms in the strong fields generated by fast passing highly-charged ions and intense low-frequency laser pulses. The experiments have been performed at the UNILAC of GSI and at the high power laser facility of the Max-BornInstitute in Berlin.