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 $\gamma$. 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.