Ionization suppression of excited atomic states beyond the stabilization regime

A two-dimensional model atom is employed to study the ionization behavior of initially excited atomic states in highly-frequent intense laser pulses beyond the dipole approximation. An additional regime of ionization suppression is found at laser intensities where the stabilization effect is expected to break down. The appearance of this effect is due to a strong coupling of the initial wave function to the ground state of the cycle-averaged space-translated ionic potential, followed by a subsequent population transfer to the ground state during the laser pulse turn-off. Non-dipole effects are found to increase the overall ionization probabilities, but not to suppress or alter this effect. PACS numbers: 32.80.Rm, 42.50.Hz In recent years, the rapid progress in laser technology has enabled the research on highly nonperturbative phenomena of atoms in intense laser fields, such as Above Threshold Ionization (ATI) or High Harmonic Generation (HHG) [1]. With the advent of Free Electron Lasers (FEL) [2], light sources will soon be available to generate photons whose energy h̄ω may equal or even exceed the binding energies of ground state atoms. At such high frequencies, the atom may stabilize against ionization [3], such that the ionization probability must not necessarily rise with the laser intensity, but it may decrease even though the intensity is increased. The stabilization effect and dynamic ionization suppression have been extensively studied in one-electron atoms [4], and also two-electron systems have been considered [5]. Stabilization of initially excited atomic states has been of interest since they provide a means to fulfill the condition of high laser frequencies, that is, within these systems the energy of a single photon exceeds the electronic binding energy. Theoretical investigations have been carried out on the ionization of 2s and 2p states of hydrogen [6], while the existence of the stabilization effect has been experimentally verified on Rydberg states [7]. In this letter, we study the ionization dynamics of an initially excited model atom subjected to short, highly intense laser pulses whose frequency exceeds the atomic Letter to the Editor 2 -10 -5 0 5 10 -10 -5 0 5 10 0 0.02 0.04 0.06 0.08 0.1 (a)