Abstract
The theoretical description of the ionization of an atom (ion) by external electromagnetic radiation up to now concerned two alternative situations; multiphoton ionization and tunneling ionization. For both cases the formulas describing the ionization probability when the intensity of the laser field is not too strong are well known. However, if the field is strong, then there exists a new channel; the so-called barrier-suppression (or above-barrier) ionization of the atom. How does this process occur? It is obvious that barrier-suppression ionization and sub-barrier tunneling ionization by low-frequency laser field transform smoothly into one another as the field strength F is varied near the value of the barrier-suppression field F BSI . This paper contains a review of various theoretical approaches developed during the last few years, especially analytical considerations. Some new semi-analytical expressions for ionization rates, energy and angular distributions of ejected electrons are also derived.
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