Non-Markovian behavior of ultrafast coherent ionization dynamics in a crystal exposed to a seeded free-electron-laser pulse

Abstract

We investigate the ionization dynamics of a crystal structure driven by ultrafast coherent x-ray pulses of moderate to high intensities for excitations where dipole-allowed, single-photon ionization dominates. Using a simple model of the crystal, we demonstrate that quantum coherences may already play an important role at moderate pulse intensities, leading to qualitatively novel features which cannot be described by rate equations. In particular, the ionization may exhibit a minimum as a function of the pulse duration, where the ionization drops to almost zero, although during the pulse a noticeable fraction of the electrons is promoted to unbound states. For higher intensities, the qualitative deviations between the coherent quantum-mechanical treatment and the rate description is even more pronounced. In particular, due to the presence of quantum-mechanical coherences, the full theory predicts, even for the single-photon transitions to a continuum of free-electron states, a Rabi-type behavior similar to what is known for two-level systems.