# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1907.06874