On the relativistic theory of tunneling

A relativistic generalization of the semiclassical theory of tunneling and multiphoton ionization of atoms and ions in the field of a high-intensity electromagnetic wave with linear, circular, and elliptic polarization is constructed. The exponential factor in the ionization probability is calculated for arbitrary values of adiabaticity parameter γ. In the case of low-frequency laser radiation, an asymptotically exact formula is derived for the ionization rate of the s atomic level, including the Coulomb, spin, and adiabatic corrections and the preexponential factor.