Low-Frequency Strong-Field Ionization and Excitation of Hydrogen Atom

The dynamics of hydrogen atom in the presence of a strong radiation field of the titanium-sapphire laser is studied for the Keldysh parameters gamma >= 1 and gamma <= 1. It is demonstrated that the ionization is sup plemented with the effective population of the excited states with the principal quantum numbers n = 5-10 in the entire range of variation in the Keldysh parameter. The population of the excited Rydberg states can be interpreted as a consequence of the multiphoton resonance involving the initial 1s state and a group of excited states in the vicinity of the continuum boundary with the simultaneous repopulation of these states by Lambda-type Raman transitions under the action of the laser field. The resulting coherent Rydberg packet appears to be stable with respect to ionization, so that the ionization of the atomic system in the presence of strong electro magnetic field is suppressed. Physical reasons for the stabilization are discussed. An interpretation of the effective population of the Rydberg states in the recent experiments on the ionization of atomic helium by the titanium-sapphire laser is proposed. DOI: 10.1134/S1054660X1010004X