Control of spectral line shapes using ultrashort laser pulses

Spectral absorption line shapes enable a direct and state-resolved access to the bound configuration of electrons in atoms. As a fundamental example, two-electron states in helium are measured in the absorption spectrum of broadband attosecond XUV pulses at ~60 eV. The corresponding asymmetric Fano profiles are resolved with a high-resolution (~20 meV s.d.) grating-based spectrometer. Further interaction with a few-femtosecond visible laser pulse of moderate intensity (few TW/cm^2) leads to a characteristic change of the spectral profiles, tuning from asymmetric Fano to symmetric Lorentzian shapes.

With the developed time-domain understanding of this universal process, we can retrieve both amplitude and phase information of how the electronic states interact with the laser pulse. As a first application of this scheme, it is demonstrated how a bound two-electron wave packet can be experimentally observed and controlled. Furthermore, the observation of higher-lying Rydberg states opens up a new perspective to systematically study the dynamics of bound (multi-)electron states close to the ionization threshold, and how their relatively slow dynamics can be manipulated with ultrashort laser pulses.