Rydberg series of autoionizing states in strong laser fields

We study the behavior of Rydberg series of resonances excited by intense laser fields. Our approach is based on a semiclassical formalism for the evolution in time of the atomic states coupled by the laser. The atomic system is described by a two-channel model in a Multichannel Quantum Defect Theory approach. We calculate the total ionization and the photoelectron spectrum after a certain interaction time. We present results both for on-resonance and off-resonance excitation of the series. We employ a more or less realistic pulse shape that corresponds to a narrow Fourier bandwidth. We show that the effects of the non-resonant members of the series on the photoelectron spectra can be important and we study it both as a function of laser intensity and as a function of the interaction time. We also show that our model correctly describes the Rabi oscillations between the ground state and the excited AI state when the field is sufficiently strong. (orig.)