# Is the photon truly not absorbed in Raman scattering?

In reading about Raman Scattering, I was thinking while reading it "okay, incident photo absorbed by molecule, molecule goes to higher energy vibrational state, molecule re-emits photon with either higher or lower energy than incident...how is this different than light interacting with any sort of possible transition?"

The Raman effect differs from the process of fluorescence. For the latter, the incident light is completely absorbed and the system is transferred to an excited state from which it can go to various lower states only after a certain resonance lifetime. The result of both processes is in essence the same: A photon with a frequency different from that of the incident photon is produced and the molecule is brought to a higher or lower energy level. But the major difference is that the Raman effect can take place for any frequency of incident light. In contrast to the fluorescence effect, the Raman effect is therefore not a resonant effect. In practice, this means that a fluorescence peak is anchored at a specific frequency, whereas a Raman peak maintains a constant separation from the excitation frequency.

I've read this thread as well, but it still doesn't clear up much. It also seems like there was disagreement between the different answers and the wikipedia page. For example, the chosen answer there says

In contrast to fluorescence, there is no excited state in Raman scattering

Whereas the Wikipedia article clearly uses the concept of excited vibrational states:

The energy difference between the absorbed and emitted photon corresponds to the energy difference between two resonant states of the material and is independent of the absolute energy of the photon.

It seems like many people are making a distinction between a photon being just scattered as opposed to absorbed and emitted, whereas John Rennie's answer and comments seem to be saying that the photon is always absorbed, though the order of events may differ from fluorescence.

So can someone give me a more clear picture of this? Is the photon really not absorbed in Raman scattering? Why isn't it resonant if there are different energy levels?

• Well in my opinion the different language for the processes is kind of moot. The behaviour of something like the afterglow of a phosphor can well be interpreted as resonance scattering (which shows the very exponential decay of outgoing intensity). But experimentalists will think differently. So excitation versus scattering with no intermediate excited state is not a sharply defined concept, both are just intuitive conceptualizations useful for reasoning about processes. – Sebastian Riese May 15 '15 at 19:37
• Absorption plus re-emission vs scattering may have implications for the polarization of the emitted photon vs the incident photon. So the different processes may, in fact, result in an experimentally detectable difference. – Jon Custer May 15 '15 at 19:51

In Raman scattering, the molecule absorbs the photon into a virtual state, which doesn't actually exist. Unlike an excited state, the molecule can't stay in that state for longer than a time $\Delta t$ where $\Delta t \Delta E \leq \hbar/2$ - the Heisenberg uncertainty relation.