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Abstract

Within this thesis, the photoionization, fragmentation and autoionization of highly excited states of diatomic molecules (O_2 and N_2) induced by intense extreme-ultraviolet (XUV) radiation delivered by the Free-Electron-Laser in Hamburg (FLASH), is studied employing coincident three-dimensional recoil-ion momentum spectroscopy. First, with an XUV-pump/THz-probe scheme, the relaxation dynamics of highly excited states of O_2 created by absorption of a single 60 eV XUV photon is investigated. The relative time shift between the first emitted photoelectron and a second low-energetic photoelectron carries information about the time difference between both photoemission steps or the autoionization lifetime. We have measured that the lifetime of the autoionizing excited cation state ^4Σ_g^- with a kinetic energy release (KER) of 4 eV is 102 ± 30 fs. Second, the IR-assisted XUV multiphoton experiments of N_2 were performed in combination with a synchronized femtosecond optical laser. Aligned N_2^{n+} ions are created by sequential absorption of XUV photons and further excited or ionized by a delayed NIR laser pulse. By tracing the time-dependent KERs of double ionization products N_2^{2+} → N^+ + N^+ (N(1,1)) and triple ionization products N_2^{3+} → N^{2+} + N^+ (N(2,1)), we find an enhancement factor for N(1,1) of 2.2 and for N(2,1) of 5.7 compared to without NIR. The detailed dissociative pathways are traced by KERs and angular distribution of ionic fragments.