MODELS OF 2-LEVEL ATOMS IN QUASI-PERIODIC EXTERNAL FIELDS

We prove that the spectrum of the generalized quasienergy operator of a plane-polarized two-level atom in a strong external quasiperiodic el ectromagnetic field with nonzero constant Fourier component is pure po int, under Diophantine conditions on the frequency ratio, and excludin g a small subset of resonant values. The widespread belief that there may be only pure point spectrum in such models is briefly discussed in Section 2 and the circularly polarized case-a well-known soluble mode l-is revisited from the point of view of the quasienergy operator.