Role of saturation in the multiphoton transitions

The effects of a medium (of collisions) on the probability for two-photon absorption in a three-level system are analyzed. It is shown qualitatively that saturation can occur, under conditions more stringent than for one-photon saturation. A possible mechanism for lowering the population state of the radiation flux during multiphoton ionization is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 79–83, November, 1969.