Ionization by intense and short electric pulses. Classical picture

Complete text of publication follows. In the last decades there has been a great revival of the classical trajectory Monte Carlo (CTMC) calculations applied to atomic collisions involving three or more particles. This approximation seems to be useful in treating atomic collisions where the quantum mechanical calculations become very complicated or intractable, which is the case usually when higher order perturbations should be applied or many particles take part in the processes. The CTMC method has been quite successful also in dealing with the ionization process in laser-atom collisions when, instead of charged particles, electromagnetic fields are used for excitation of the target. In the present work we studied the efficiency of the CTMC method. At first the electron emission spectra of a hydrogen atom when it is excited by ultra-short pulses are studied. The effect of a short pulse on one atom can be treated within the framework of a sudden momentum transfer, (or kick) i.e. in strong field approximation (SFA) where the pulse duration is much less than the classical orbital period of the electron in the initial state. We applied the Coulomb-Volkov approximation (CVA) for the determination of the energy differential electron distributions Moreover, we extended these studies for short half-cycle pulses (HCP) of finite (non-zero) duration, where the pulse can be longer than the classical orbital period of the electron in the initial state and, therefore, the sudden approximation - when the effect of a short pulse on one atom can be treated within the framework of a sudden momentum transfer - is no longer valid. We analyzed the efficiency also of the various quantum approaches like the singly-distorted Coulomb-Volkov (SDCV) and the doubly-distorted Coulomb-Volkov (DDCV) approximations. Quantum and classical results are compared to the numerical solution of the time-dependent Schrodinger equation (TDSE). We conclude that the region of validity of the different quantum and classical approximations studied in this paper depends on the kick strength Δp and the pulse duration. The CTMC-T method is able to reproduce the quantum ionization probabilities for large Δp values, even for long pulse durations. For small Δp values quantum approaches are preferable compared to the quasiclassical one, as we depart from the sudden limit. It is in the intermediate limit, i.e., Δp ∼ 1, where known quantum quantum and classical approximations fail. Precisely in this region, DDCV becomes a better alternative than the SDCV, although a deficiency arises at small electron ejection energy for high values of Δp. Summarizing, our DDCV presents a reliable approximation where the CTMC and CTMC-T fails. Acknowledgements. This work was carried out with financial support of CONICET, UBACyT X147, and ANPCyT PICT 2006-00772 of Argentina, the Argentine- Hungarian collaboration HU/08/02, the CNCSISUEFISCSU proj. No. PNII-IDEI 539/2007, the Hungarian National Office for Research and Technology and the Hungarian Scientific Research Found OTKA (K72172). KT were also partially supported by the European COST Action CM0702.