  By means of quantum stochastic calculus we construct a model for an atom with
two degenerate levels and stimulated by a laser and we compute its fluorescence
spectrum; let us stress that, once the model for the unitary atom-field
dynamics has been given, then the spectrum is computed without further
approximations. If only the absorption/emission term is included in the
interaction, we reobtain the Mollow spectrum in the case of a monochromatic
laser and the Kimble-Mandel spectrum in the case of a "phase diffusion model"
for a non monochromatic laser. However, our model can describe also another
type of light scattering, a "direct scattering" due to the response of the atom
as a whole, which we expect to be small, but which interferes with the
scattering due to the absorption/emission channel. When both the scattering
channels are introduced we obtain a modification of the Mollow-Kimble-Mandel
spectrum, which shares the main features with the usual case, but which
presents some asymmetries even in the case of no detuning.
