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Abstract

In this work, the influence of intense extreme-ultraviolet (XUV) fields on helium is
experimentally investigated. Therefore, XUV pulses from a free-electron laser (FEL)
are combined with transient absorption spectroscopy (TAS) and explored with numerical
quantum-mechanical simulations. A novel TAS beamline enables measurements
on the prototypical atomic three-body system, helium, at the free-electron laser in
Hamburg (FLASH). In particular, the energetically lowest two-electron resonance,
2s2p, with its asymmetric Fano absorption line shape is of interest. This bound state is embedded in the single-ionization continuum and thus represents an atomic
interferometer. Its main property, the sensitivity to phase, is used in this work to
detect manipulations induced by strong XUV pulses. In the experiments, a distortion
of the absorption line is observed in the presence of highly intense XUV pulses. Firstly,
the line shape’s symmetry change is investigated with a numerical few-level model
simulation and found to be connected to the transient dressing of the excited state.
Employing realistically modelled stochastic pulses, the investigation is extended to the
line shape’s dependence on the pulse duration. Finally, the line broadening is explained
by the model simulation and allows for disentangling the contributing mechanisms,
two-photon absorption and the increased reversion to the ground state.