Ionization dynamics in interactions of atoms with ultra-short and intense laser pulses

Atom ionization by intense laser pulses, whose electric field performs less than two oscillations during the pulse, is investigated theoretically usin g both quantum and classical approaches. We show that, under these conditio ns, the ionization process exhibits a classical aspect. Further, up to lase r field amplitudes comparable to the Coulomb field of the nucleus, which is experienced by the active electron on its initial target orbital, the nucl ear held is shown to play a significant role in the dynamics of ionization. For higher laser fields, a simple approach based on Coulomb-Volkov states appears much more convenient than full numerical treatments.