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I have a question considering the difference between Raman Scattering and fluorescence.

I know that Raman Scataring is a non-resonant process, while fluorescence is resonant, that Raman scattering retains coherence while fluorescence doesnt and that in Raman scattering the photon is "emited" immediatelly while there is some specific time dependent decay in fluorescence. These things are clear.

What is not clear to me is why the Raman Scattering is supposed to be 2-photon process, while fluorescence is 1-photon process (I actually read this in some book that had some kind of Feynman diagrams in it - but no further information). Can someone please explain this? The more detailed explanation, the better :).

Thank you very much.

EDIT !!!

I found the book I was talking about. There is this picture. And it basically says that in case of fluorescence the system goes through a transition state (excited state) at one time - photons interact with bra and ket vector at the same time. While in Raman one photon interacts with bra vector, it is emited and only then the second photon interacts with ket vector (and is once again emited). This is their reasoning for this 1-photon/2-photon stuff.

Do I understand it correctly? And is it even explained well?

enter image description here

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marked as duplicate by Rococo, sammy gerbil, Chris, Emilio Pisanty, valerio Feb 28 '18 at 12:42

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I think this is a matter of how one defines one's terms, what one means by photon, and that's a slippery business. I'm assuming that fluorescence here means resonant elastic scattering. (Otherwise it's hard to see how it can be called a one-photon process.)

In scattering, we usually associate a photon mode with a plane wave having a well-defined frequency and wave vector. By this definition, fluorescence is a two-photon process. One photon destroyed, another created. (That's my personally preferred picture.) Another point of view notes that the frequency of the photon hasn't changed. The only thing that has changed is the direction. The incident photon has been re-directed. When I look at the the way the theory is usually presented in QED, I see two photons.

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