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Abstract

Within this work, electron dynamics of autoionizing, doubly excited states in
helium are investigated. Therefore, strong field couplings of these states are
studied in two spectral regimes: in the infrared and in the extreme ultraviolet.
A laser system capable of tuning its output-pulses' central wavelength
within the infrared spectral regime is installed and characterized by a FROG
setup. Furthermore, high harmonics of these output pulses are generated
and discussed. First intensity dependent and time resolved experiments in
helium with this laser system are presented and supported by numerical calculations.
The transition from an unperturbed light induced state, which is
generated from the 2p4p state, to a resonant Autler-Townes splitting of the
sp_2;3+ state is observed. Furthermore, strong field couplings between the
ground and a doubly-excited state are studied in the extreme ultraviolet
only regime. With the help of simulations Rabi-like couplings and Stark
shifts for ultrashort pulses leading to a manipulated absorption line shape
are investigated. The same study is carried out for autoionizing states. This
theory part is used to explain experimentally measured line-shape changes
of the 2s2p resonance in helium interacting with FEL pulses.