Atoms in the field of strong multimode synchronized radiation

The behaviour of a two-level atomic system in a high-frequency field of periodically varying intensity is considered. An example of such a field is synchronized multimode laser radiation. Exeressions for the quasi-energy levels of ar atom in a field of periodic amplitude which are valid for an arbitrary number of synchronized radiation modes are derived in the adiabatic (with respect to phase) approximation. The spectrum of resonance frequencies Ωsub(k) (k=1, 2, 3...) corresponding to the intersection of the quasi-levels of the system is determined. The frequencies Ωsub(k) define the positions of the parametric resonances. It is shown for a biharmonic field that if the interMode frequency interval Ω is equal to Ωsub(k) then absorption (amplification) of certain modes and their combination tones become extremal