Hydrogen atom in a strong laser field: Numerical simulation versus Keldysh-type theories

The dynamics of a 3D hydrogen atom in an intense laser pulse is investigated using the direct numerical integration of the nonstationary Schrödinger equation in the multiphoton regime of ionization. The results obtained are compared with the theoretical data of the strong-field approximation (SFA) method and the validity of the SFA and other Keldysh-type theories in the problem of the strong-field ionization of atoms is analyzed in a wide range of laser frequencies and intensities. The Reiss approximation (SFA) provides qualitative agreement with the numerical simulation for moderate laser intensities and reveals the channel closing phenomena during the multiphoton ionization. However, this approach is found to be inappropriate in the presence of strong fields where we numerically demonstrate the suppression of atomic ionization. The reason for the stabilization lies in the formation of a new system (dressed atom), which is not taken into account in the SFA and other Keldysh-type theories.