  We study the interaction of a two-level atom and two fields, one of them
classical. We obtain an effective Hamiltonian for this system by using a method
recently introduced that produces a small rotation to the Hamiltonian that
allows to neglect some terms in the rotated Hamiltonian. Then we solve a
variation of the Schr\"odinger equation that models decoherence as the system
evolves through intrinsic mechanisms beyond conventional quantum mechanics
rather than dissipative interaction with an environment.
