Adaptation of the Modified Adiabatic Approximation to Strong Field Ionization

Savichev's modified adiabatic approximation [Sov. Phys. JETP 73, 803 (1991)] is used to obtain a general form of a quantum-mechanical amplitude of ionization by a low-frequency laser field. The method possesses only one requirement that the frequency of the laser field must be low. Connections of the obtained result with the quasi-classical approximation and other previous investigations are discussed.