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Abstract:
In this thesis, photoelectron spectroscopy in combination with extremeultraviolet
(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.
