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Abstract

In the course of this work two-photon double ionization (TPDI) of helium and D₂ molecules was studied at the free-electron-laser in Hamburg (FLASH). Using a reaction microscope the three-dimensional momentum vectors of the ions generated by the double ionization were obtained. For helium the nonsequential channel of TPDI was studied at a photon energy of 52 eV. From the measured momentum distribution of the He²⁺-ions information about the correlated electron dynamics could be inferred, providing the first experimental evidence for the so called virtual sequential ionization. The comparison of the observed single differential cross sections with the results of two time-dependent close-coupling calculations showed significant differences. TPDI of D₂ molecules was studied at a photon energy of 38 eV. It was demonstrated that both the sequential and the nonsequential channel contribute significantally to the double ionization yield. A split-mirror setup was used to perform a XUV-pump-probe experiment tracing the ultrafast nuclear wave-packet motion in the 1sσ _g-state of the D⁺ ₂-ion. It was shown that even with a mean pulse-length in the order of 30 to 40 fs structures with a width < 10 fs can be resolved due to the particular time structure of the free-electron-laser radiation.