Estimation of transition rate of ionization of atoms for short range potential, based on assumptions of Keldysh approximation, shows that short-range potential does not affect the energy of the final state of ejected electron, when it leaves the atom. Coulomb potential is then treated as perturbation of final state energy leading to the ADK-theory. But Coulomb interaction was not originally included in calculating the turning point. This we corrected in [1], though only for fields with intensities below those of the atomic field. But as the ADK-theory was recently extended to the case of superstrong fields, our calculations now include extension of potential range up to 1017 W/cm2; this leads to the shift of position of the turning point τ, which then influences transition rates for atoms in the low-frequency electromagnetic field of superstrong lasers. On Figs. 1 and 2 obtained from our calculations the value of Z was switched from 1 to 10 at field intensity of 5 × 1013 W/cm2 (atomic unit system), which is done for the first time ever. The second figure also indicates an enormous activity at fields 1013 W/cm2, which is due to strong tunneling effect for this energies of laser field. After that, there is saturation at a very low level of transition rate for fields from 1015 W/cm2 to 1017 W/cm2.

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In the case of the short-range potential [1,5,8,9], the estimation of the probability of ionization of atoms is carried along taking into account the approximation (Keldysh approximation) which states that this kind of potential does not affect the energy of the final state f of the ejected electron in the laser field, because the electron is far enough from the nucleus. When the Coulomb potential is taken into account, it can be treated as a perturbation to the energy of the final state [1,10]. Yet, originally [1,10], the Coulomb potential in this kind of estimation was not included into calculating the turning point. This was done in [7], but only for the fields below the atomic field (1016 W/cm2). Now, based on the results [11,12], we are extending our calculation that included the Coulomb correction into the estimating the turning point to the fields that are much stronger (up to 1017 W/cm2). That results in the shift of the position of the turning point τ. This paper is dealing with the influence of that shift on the ionization probability for atoms in the low-frequency electromagnetic field of superstrong lasers.

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Tunneling regime, introduced by Keldysh, in the interaction of strong lasers with atoms has been now accepted as the reliable method for describing processes when low frequency lasers are involved. Yet it was always assumed that the ionized electrons are leaving the atom with zero initial momentum. Because we are interested in how non-zero momentum influences the transition probability of tunnel ionization, we obtained the exact expression for the momentum. Here the estimation of the transition probability with nonzero momentum included was conducted. Potassium atoms in the laser field whose intensity varied from 10 13 W/cm 2 to 10 14 W/cm 2 were studied. It seems that all energy of laser field is used for tunneling ionization process at the beginning of laser pulse -ionization probability is large. After that, with further action of laser pulse, ionization probability decreases, probably because part of laser pulse energy is used for increasing momentum of ejected electrons, leaving smaller amounts of light quanta available for ionization of remaining electrons. If laser pulse lasts long enough, then the amounts of light quanta available for ionization become larger, resulting in increase in ionization probability, now with greater starting energy of ejected electrons.