  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.
