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Photoemission spectroscopy with high-intensity short-wavelength lasers

We theoretically study the process of photoelectron emission of the Helium atom by a high-intensity short-wavelength laser at a resonance condition of the residual singly charged ion. Photoionization followed by strong resonance coupling in the ion leads to a change in the photoelectron spectrum due to Rabi oscillations in the residual ion. Similarly to resonance fluorescence at high laser intensity, the photoelectron spectrum at high intensities evolves into a multi-peaked structure. The number of peaks in the photoelectron spectrum is related to the number of Rabi cycles following the photoionization process. Moreover, the strong laser-induced coupling to nonresonant states of the residual ion have an imprint on the photoelectron spectrum, leading to additional, isolated peaks at the lower or higher energy sides. The effect should be observable at current seeded XUV free-electron lasers and persists after volume integration in a realistic experimental geometry.