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 using both quantum and classical approaches. We show that, under these conditions, the ionization process exhibits a classical aspect. Further, up to laser field amplitudes comparable to the Coulomb field of the nucleus, which is experienced by the active electron on its initial target orbital, the nuclear 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.