In conditions where the interaction betweeen an atom and a short high -frequency extreme ultraviolet laser pulse is a perturbation, we show that a simple theoret ical approach, based on Coulomb-Volkov-type states, can make reliable predictions for ionization. To avoid any additional approximation, we consider here a standard case : the ionization of hydrogen atoms initially in their ground state. For any field para meter, we show that the method provides accurate energy spectra of ejected electrons, including many above threshold ionization peaks, as long as the two following conditions are simultaneously fulfilled : (i) the photon energy is greater than or equal to the ionization potential ; (ii) the ionization process is not saturated.Thus, ionization of atoms or molecules by the high -harmonics laser pulses which are generated at present may be addressed through this Coulomb-Volkov treatment.3

The interaction between an atom and a short laser pulse is studied in the case where both photon energies are smaller than the ionization potential and perturbation conditions prevail. Under these conditions, a full numerical solution of the time-dependent Schrödinger equation for a hydrogen atom initially in its ground state shows that secondary peaks show up in the above-threshold ionization (ATI) spectrum. We introduce here an easyto-implement approximation that sheds light on these features. This approach, which is based on Coulomb-Volkov-type states, is an extension of a previous theory that applies only when photon energies are greater than the ionization potential. We show that introducing a coupling to intermediate bound states into the approach permits to nicely reproduce the full numerical electron spectrum for smaller photon energies. Further, it allows us to trace back unambiguously the secondary peaks to a manifold process, i.e., excitation of transient bound states followed by multiphoton absorption, and to show that the main ATI peaks may be enhanced by intermediate state contributions. When bound states with 1 ഛ n ഛ 4 are included, the approach provides excellent predictions for photon energies as low as half the ionization potential.

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 field 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.

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