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Abstract

In this work fundamental light–matter interaction is studied in excited-state two-electron systems under the influence of an intense laser field in two respects: First, motivated by the results of a numerical simulation on the role of initial-state electron correlation for the ionization process, strong-field ionization out of selectively prepared doubly excited states (DESs) in helium is studied in a two-colour extreme ultraviolet (XUV)–infrared (IR) experiment using a reaction microscope (REMI). Detected recoil-ion and photoelectron momentum distributions help to identify a variety of different IR-induced ionization pathways for both single and double ionization out of different DESs as the initial state for strong-field interaction. Turning the focus from the atomic to the molecular two-electron system, in the second study, a novel all-optical approach enables visualisation of the dynamics of a vibrational wave packet in an electronically excited state of neutral H_2 through molecular self-probing by the ground state encoded in the reconstructed time-dependent dipole response of the excited system from XUV spectroscopy data. In a pump–control scheme, an additional interaction with a 5-fs near-infrared (NIR) pulse of adjustable intensity modifies the vibrational wave-packet revival. The adoption of an impulsive control mechanism together with state-resolved extraction of the accumulated strong-field induced phases leading to the observed revival shift brings access to state-dependent polarizability of different vibronic states in the excited wave packet. In future, both experimental approaches can be applied to multi-electron systems to study and control correlation in specifically prepared excited quantum systems.