Strong-field extreme-ultraviolet dressing of atomic double excitation

TL;DR

This paper demonstrates the experimental observation and theoretical analysis of strong-field effects on autoionizing two-electron states in helium using intense XUV pulses, revealing energy and phase shifts that modify Fano resonance profiles.

Contribution

It provides the first experimental evidence of strong-field dressing of autoionizing states in helium with intense XUV pulses and links these effects to phase and energy shifts in the Fano resonance.

Findings

Observation of spectral modifications of Fano line shapes with increasing pulse energy

Quantum-mechanical calculations linking energy and phase shifts to Fano asymmetry

Transient energy shifts of a few meV induced by strong XUV fields

Abstract

We report on the experimental observation of strong-field dressing of an autoionizing two-electron state in helium with intense extreme-ultraviolet laser pulses from a free-electron laser. The asymmetric Fano line shape of this transition is spectrally resolved, and we observe modifications of the resonance asymmetry structure for increasing free-electron-laser pulse energy on the order of few tens of $\mu$J. A quantum-mechanical calculation of the time-dependent dipole response of this autoionizing state, driven by classical extreme-ultraviolet (XUV) electric fields, reveals a direct link between strong-field-induced energy and phase shifts of the doubly excited state and the Fano line-shape asymmetry. The experimental results obtained at the Free-Electron Laser in Hamburg (FLASH) thus correspond to transient energy shifts on the order of few meV, induced by strong XUV fields. These results open up a new way of performing non-perturbative XUV nonlinear optics for the light-matter interaction of resonant electronic transitions in atoms at short wavelengths.