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Abstract

In this thesis, photoelectron spectroscopy in combination with extreme-ultraviolet (XUV) and infrared (IR) radiation is employed to investigate non-linear effects in rare gas atoms. Electrons are measured by means of a reaction microscope which allows to determine the particles’ momentum vector in 4π solid angle. The simultaneous interplay of XUV-photon absorption and an IR-dressing field in helium atoms allows ionization via resonances which are not accessible by single photon excitation. Scanning the XUV-photon energy between 20.4 eV and 24.6 eV reveals several ionization pathways for different combinations of XUV-photon absorption and IR-photon absorption or emission. Information about these states is encoded in the corresponding photoelectron angular distributions (PADs). PADs are analyzed for different orientations of polarization between the two radiation fields which allows to change the magnetic quantum number and to alter ionization pathways. Additionally, PADs are analyzed for changing laser-intensities which reveals Ponderomotive-/Stark shifts and higher order multiphoton effects. Furthermore, two-photon double ionization (TPDI) in argon is investigated with intense FEL-XUV pulses. At a photon energy of 27.93 eV, we find sequential ionization to dominate and analyze the PAD of the two emitted photoelectrons. Despite the stepwise character of the process, we find both electrons being correlated via polarization of the intermediate ionic state. Moreover, PADs are found to be modulated by autoionizing states which have to be considered to properly describe the process.