Non-Perturbative Above-Threshold Ionization Driven by X-Ray Free-Electron Lasers

We explore a new frontier in non-perturbative strong-field physics made possible by the latest advances in X-Ray Free-Electron Laser (XFEL) technology, which extends multiphoton above-threshold ionization (ATI) from the well-explored visible/infrared into the new attosecond X-ray regime. In helium, distinct angular and spectral signatures emerge, isolated from perturbative harmonic contributions. We develop a theoretical framework that enables ultrafast probing and control of non-perturbative dynamics, paving the way for future short-wavelength strong-field experiments.