  We report on multiple photon monochromatic quantum oscillations (Rabi
oscillations) observed by pulsed EPR (Electron Paramagnetic Resonance) of
Mn$^{2+}$ (S=5/2) impurities in MgO. We find that when the microwave magnetic
field is similar or large than the anisotropy splitting, the Rabi oscillations
have a spectrum made of many frequencies not predicted by the S=1/2 Rabi model.
We show that these new frequencies come from multiple photon coherent
manipulation of the multi-level spin impurity. We develop a model based on the
crystal field theory and the rotating frame approximation, describing the
observed phenomenon with a very good agreement.
