Autoionizing Polaritons in Attosecond Atomic Ionization

Light-induced states and Autler-Townes splitting of laser-coupled states are common features in the photoionization spectra of laser-dressed atoms. The entangled light-matter character of metastable Autler-Townes multiplets, which makes them autoionizing polaritons, however, is still largely unexplored. We employ attosecond transient-absorption spectroscopy in argon to study the formation of polariton multiplets between the $3s^{-1}4p$ and several light-induced states. We measure a controllable stabilization of the polaritons against ionization, in excellent agreement with ab initio theory. Using an extension of the Jaynes-Cummings model to autoionizing states, we show that this stabilization is due to the destructive interference between the Auger decay and the radiative ionization of the polaritonic components. These results give new insights into the optical control of electronic structure in the continuum and unlock the door to applications of radiative stabilization in metastable polyelectronic systems.

Figure 1

(a) Formation of autoionizing polaritons ($\mathrm{AI}\mathrm{P}\pm$) from mixing between the bright AIS and the ALIS of the dark level. Auger (AI) and radiative (RI) decay paths for the AIPs interfere, leading to stabilization seen as narrowing of the spectral width of the lower branch in simulation (b) and experiment (c).

Figure 2

(a) Experimental (top) and theoretical (bottom) XUV photoabsorption in the vicinity of argon $3s^{-1}4p$ AIS, as a function of the IR pulse delay, for several values of IR photon energy. Color map represents the OD. Positive delay implies IR pulse arrives before XUV. Interaction of AISs with ALISs gives rise to the polariton splitting prominently visible between 0.86 and 0.98 eV IR photon energy. Experimental (b) and theoretical (c) polaritonic line shapes for the near resonant case of 0.94 eV IR energy, for several time delays. In the absence of interference between radiative and Auger channels, the two polaritons are expected to have comparable widths. Stabilization is evidenced by the delay-dependent reduction of $\mathrm{AI}\mathrm{P}-$ width and marked difference between the $\mathrm{AI}\mathrm{P}\pm$ widths in both experiment (d) and theory (e). Open symbols are for 0.93 eV case.

Figure 3

XUV absorption spectra (OD) at $\tau =0$, as a function of the IR-photon energy tuning. Experiment (a) and theory (b) show avoided crossings between the horizontal $4p$ feature and the tilted line of the ALIS from high-lying dark AISs, forming several autoionizing polaritons. (c) Theoretical predictions in a broader spectral and IR tuning range, indicating several other avoided AIS-ALIS and ALIS-ALIS crossings. (d) Theoretical OD as a function of the time delay, with all the essential states involved (top left), or removing, in turn, the $3d$ (top right), $5s$ (bottom left), and $4f$ (bottom right) essential state.