In the context of http://en.wikipedia.org/wiki/Rayleigh_scattering" [Broken] scattering, the effect can be viewed as photon absorption giving excitation to a non-existent ('virtual') energy level, followed by immediate re-emission of a photon and return to the same (Rayleigh) or another (Raman) state.
Contrast this to http://en.wikipedia.org/wiki/Fluorescence" [Broken], which occurs between real energy states.
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