On the theory of tunneling and above-barrier ionization of atoms and ions in a strong laser field

Energy and angular distributions of photoelectrons emerging from the tunneling ionization of an atom or an ion under the action of high-power laser radiation (for all the values of the Keldysh parameter gamma) are calculated. The cases of linearly, circularly, and elliptically polarized electromagnetic waves are considered. The probability of above-barrier ionization of hydrogen atoms in a low-frequency (gamma much less than 1) laser field is calculated. Formulas for the momentum spectrum of electrons in the case when an atomic level is ionized with an ac (linearly polarized) electric field of general form are presented. The method of "imaginary time" is applied to analyze ionization of atoms in a nonmonochromatic field and the Coulomb correction in the range where gamma less than or similar to 1. Analytical approximations for the asymptotic coefficient C-kappa of the atomic wave function at infinity (for s electrons) are discussed.