When optical control pulses interact with the autoionizing states, the underlying physics is described in terms of the entangled state of photons and Auger resonances, termed as ‘autoionizing polaritons’. We developed a new approach to understand the formation and decay of such autoionizing polaritons. Specifically, we employed attosecond transient absorption to demonstrate the stabilization of autoionizing states by light fields, thus verifying a theoretical prediction made near forty years ago. By fine-tuning the laser parameters, we achieved control over the interferences between radiative ionization and autoionization of polaritons. In the figure below: (a) Formation of autoionizing polaritons (AIP) by the IR induced mixing between the bright autoionizing state (AIS) and the autoionizing light-induced state (ALIS) of the dark level. (b) Autoionizing (AI) and radiative (RI) ionization paths interfere, leading to stabilization seen as narrow spectral width of the AIP- branch in the experimental spectrum. (c) XUV absorption around argon 4p AIS as a function of the IR pulse delay, for 0.94eV IR photon energy. Interaction of AIS with ALIS gives rise to the polariton splitting in experiment and theory. (d) Stabilization and lifetime control is evident in the delay dependent reduction of AIP- line width.

Atomic and molecular structure is highly dynamic in the presence of an intense ultrafast laser pulse. Fundamentally new excitation, relaxation, and ionization pathways materialize within a few femtoseconds due to transient electronic modifications, which cannot be investigated using conventional lasers. We performed attosecond-resolved experiments to investigate the electronic structure of an atom under the action of a strong IR field. Using a two-color scheme, we tuned the XUV photons below the ionization threshold of helium and obtained precision measurements of the ion-yield and electron angular distributions in the presence of IR pulse. Our measurements highlighted the role of quantum interferences between photo-excitation paths. As the intensity ramps on femtosecond time scales, we observe switching between ionization channels mediated by different atomic resonances. The quantum phase difference between interfering paths is extracted for each ionization channel and compared with theTDSE simulations.