Above Threshold Ionization in Atoms Induced by Intense X-ray Radiation Fields*

This study explores Above Threshold Ionization (ATI) in atoms under intense X-ray radiation, where photon energy surpasses the ionization potential of valence electrons. We discuss conditions akin to those in current or future free-electron laser facilities. Our analytical model (derived in Ref. [1]) examines how field-free ionization potential, ponderomotive energy, and photon energy influence the transition to strong field ionization, using Keldysh and Reiss parameters. We demonstrate that both parameters are essential to capture strong-field behavior due to bound and continuum state properties. At higher X-ray intensities, ionization rates deviate from linear intensity scaling, indicating channel closure and higher-order photon absorption processes.