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5. M. V. Fedorov, Electron in a Strong Light Field (Nauka, Moscow, 1980) [in Russian]; F. H. M. Faisal, Theory of Multi-photon Processes (Plenum, New York, 1987); M. H. Mittleman, Introduction to the Theory of Laser Atom Interections, 2nd ed. (Plenum, New York, 1993).