1. N. B. Delone and V. P. Krainov, Springer Ser. At., Opt., Plasma Phys. 13 (2000).
2. L. P. Rapoport, B. A. Zon, and N. L. Manakov, Theory of Multiphoton Processes in Atoms (Atomizdat, Moscow, 1978) [in Russian].
3. L. V. Keldysh, Sov. Phys. JETP 20, 1307 (1964).
4. M. V. Fedorov, J. Exp. Theor. Phys. 122, 449 (2016).
5. V. S. Popov, Phys. Usp. 47, 855 (2004).