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Abstract

Within this thesis, the dynamical response of small quantum systems after the absorption of multiple extreme-ultraviolet (XUV) photons is studied via two-color pump-probe experiments at the free-electron laser (FEL) in Hamburg (FLASH) by employing many-particle recoil-ion momentum spectroscopy. The multi-photon ionization of argon atoms is investigated at a photon energy of 27 eV and FEL intensities of 10¹³ - 10¹⁴W=cm². The sequential ionization channel is found to dominate and intermediate resonances are revealed by a delayed infrared (IR) laser pulse. Molecular hydrogen (H₂) is studied at a photon energy of 28.2 eV. Dissociation via excited states and fragmentation by sequential two-photon ionization are observed. In addition, it is shown how the ground-state dissociation of H⁺₂ can be used as a tool to determine the temporal overlap between an XUV and IR laser pulse. In the argon dimer, multiple interatomic relaxation processes are triggered by the absorption of several 27-eV-photons. Besides interatomic Coulombic decay (ICD), frustrated triple ionization and charge transfer at crossings of potential energy curves are observed. The lifetime of charge transfer is determined to be (531 +/- 136) fs using an XUV-IR pump-probe scheme. The employed reaction microscope is upgraded by an in-line XUV split-delay and focussing optics, which was designed and commissioned as part of this thesis.