Alternating circular dichroism in the photoelectron spectra of the ionization of polarized atoms by ultrashort pulses

Dependence of the angle-integrated probability of the ionization of a polarized atom by a few-cycle circularly polarized (CP) pulse on the rotation direction of the pulse electric field is analyzed. Numerical simulations of the ionization of excited states of the hydrogen atom reveal that the circular dichroism effect, i.e., the difference of the probability for right- and left-hand circular polarizations of the laser pulse, exhibits oscillatory behavior vanishing for some values of the photoelectron energy. This feature is most pronounced for intermediate (on the atomic scale) CP pulse intensities and very short pulse durations. We have applied the nonstationary perturbation theory (PT) in all orders to analyze the observed effects. Results of our treatment demonstrate that PT can provide an adequate qualitative description of the ionization by ultrashort pulses whose intensities are comparable to the atomic ones only upon taking into account ionization channels involving the virtual photon emission.