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Above-threshold ionization of Rydberg atoms by microwave intensities of 3 10^4 W/cm^2 produces electrons with kinetic energies of up to 9 eV, corresponding to the absorption of up to 3 10^5 photons. The observations can be described in terms of a classical free electron in an oscillating field, which is the low-frequency limit of more conventional treatments of the phenomenon.
T. F. Gallagher (Mon,) studied this question.