Photoelectron spectroscopy of 3s3p doubly excited helium dressed with strong near-infrared laser fields

TL;DR

This study uses time-resolved photoelectron spectroscopy with synchronized XUV and NIR pulses to investigate and control doubly excited helium states influenced by strong laser fields, supported by ab initio calculations.

Contribution

It demonstrates NIR-induced coupling of autoionizing states in helium and provides a quantitative analysis of two-electron resonances using a multichannel Fano model.

Findings

Delay-dependent spectral shifts observed in helium autoionizing states.

Emergence of additional spectral structures near NIR sidebands.

Theoretical calculations confirm NIR-induced coupling effects.

Abstract

We report time-resolved photoelectron spectroscopy of the $3s3p$ doubly excited states of helium dressed by an intense near-infrared (NIR) laser field. Using synchronized XUV free-electron-laser and 800-nm NIR laser pulses, we observe a pronounced delay-dependent shift of resonance-related spectral minima together with the emergence of additional structures around the NIR sideband energy. \textit{Ab initio} theoretical calculations support these observations and identify the features as signatures of NIR-induced coupling of the bright ($3s3p {}^{1}P^{o}$) autoionizing state to nearby dark ($^{1}D^{e}$ and $^{1}S^{e}$) resonances below the $N = 3$ threshold. A multichannel Fano resonance analysis of the measured spectra yields delay-dependent line-shape parameters and resonance energies, establishing a quantitative route to characterize and control correlated two-electron resonances in strong laser fields.