Different regimes of strong-field dynamics of atoms in intense low-frequency laser pulses

In this work, the dynamics of a 3D hydrogen atom in an intense ultrashort low-frequency laser pulse is investigated by direct numerical integration of the non-stationary Schrödinger equation in a wide range of laser pulse parameters. Significantly different regimes of ionization are found to exist. In the case when the Keldysh parameter is much less than unity, but the laser intensity exceeds the barrier suppression threshold, the atomic behavior is found not to correspond to the tunneling ionization and is shown to reveal the features of the Kramers--Henneberger regime, resulting in the stabilization phenomenon. The applicability range of the re-scattering scenario is discussed.