| Summary: | The femtosecond-timescale formation of a Fano resonance in doubly excited helium has been recently observed, enabling a time-domain view into two-electron correlation dynamics. Measuring the absorption spectral line shape, the buildup of the resonance is revealed by imposing a temporal gate on the dipole response using strong-field ionization. Here, we apply this approach to the Rydberg series of the doubly excited states in helium. This reveals the characteristic times of emergence of isolated two-electron resonances from the continuous single-ionization background absorption, as well as their time-dependent line-shape asymmetry and the time it takes to separate individual spectral absorption lines within the series. Furthermore, we time resolve the dynamics of the excited wave packet by reconstructing its dipole response in the time domain and thereby characterize the ionization gate to close as fast as only 1.2 fs. These results represent an approach to resolve on the femtosecond and attosecond timescale the strong-field-ionization dynamics of excited coherent wave packets.
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