Ionization of a hydrogen-like ion by a strong low-frequency linearly polarized laser field

The influence of the long-range Coulomb potential on the ionization rate of a hydrogen-like ion has been investigated within the framework of the quasi-classical method. In a rather strong field, the dependence of the ionization rate on the field intensity does not have a purely power-law character in the multiphoton region. The analytical equations obtained are in agreement with the experimental data on the cross section of multiphoton ionization and give an estimate of the threshold field values in the tunneling region.