Photoionization in strong laser fields: from atoms to complex molecules

In this thesis, the results of the strong-field photoionization of various gas targets, particularly of methane are presented. The photoionization was performed by focusing an ultrashort two-color laser pulse into the supersonic gas jet of a Reaction Microscope. The capability of the Reaction Microscope of simultaneous detection and identification of all charged ionization products enables the channel- and fragment-specific analysis of the ionization process. The tunable relative phase between both components of the two-color field is an additional control parameter for the experiment. The discrepancy in the yield of lowenergy photoelectrons for methane ionization channels with a coincident molecular ion on the one hand and a coincident atomic ion on the other hand resembles similar results from experiments in molecular hydrogen, which could be explained by autoionization and simultaneous vibrational relaxation of a molecular Rydberg state. Despite the lack of reference data for Rydberg states of methane with excitations higher than n = 6, it was possible to estimate their energies and for one ionization channel, it could be demonstrated that electrons released by autoionization of Rydberg states between n = 9 and n = 12 are the origin of the enhanced low-energy electron yield