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Abstract

Excitation of atoms with light that spectrally spans across the ionization threshold leads
to free electrons initially located close to the parent ions. The dynamics of these ionized
electrons driven by external light fields are studied by transient-absorption spectroscopy
on helium in this thesis. Implementing an ab initio simulation, the time-dependent
Schrödinger equation (TDSE) is solved for a model atom, giving direct access to the
strong-field-driven evolution of the electron's wave function. In a separate theoretical
approach, absorption spectra are calculated based on classical trajectories. Experimentally,
an extension of a transient-absorption beamline allowed polarization-dependent
measurements. The recorded absorption spectra of helium, depending on the intensity
of a superimposed near-infrared femtosecond laser pulse with both linear and circular
polarization, are presented and discussed. The observed absorption features can be assigned
to two intensity regimes. For moderate intensities, light-induced states adequately
explain the observed structures in the absorption spectra in agreement with the TDSE
simulations. For higher laser intensities, the photon picture breaks down and the analysis
of the dipole moment in the time domain suggests an interpretation in terms of
classical electron trajectories. The combined theoretical and experimental investigation
provides access to regimes in which the classical description of strong-field-driven electron
dynamics emerges.