New journal of physics 24(1), 013014 - (2022). doi:10.1088/1367-2630/ac3b2e
We study the interaction of intense extreme ultraviolet (XUV) light with the 2s2p doubly excitedstate in helium. In addition to previously understood energy-level and phase shifts, high XUVintensities may lead to other absorption-line-shape distortions. Here, we report on experimentaltransient-absorption spectroscopy results on the 2s2p line-width modification in helium in intensestochastic XUV fields. A few-level-model simulation is realized to investigate the origins of thiseffect. We find that the line-shape broadening is connected to the strong coupling of the groundstate to the 2s2p doubly excited state which is embedded in the ionization continuum. As thebroadening takes place for intensities lower than for other strong-coupling processes, e.g. observedasymmetry changes of the absorption profile, this signature can be identified already in anintermediate intensity regime. These findings are in general relevant for resonant inner-shelltransitions in nonlinear experiments with XUV and x-ray photon energies at high intensity.
Published by IOP, [London]