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| Home > Publications database > Nonlinear spectroscopy on an autoionizing two-electron resonance in intense, extreme ultraviolet fields at a free-electron laser |
| Dissertation / PhD Thesis | PUBDB-2024-08050 |
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2022
Heidelberg University Library
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Please use a persistent id in citations: urn:nbn:de:bsz:16-heidok-312094
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.
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