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I’ve read several reasons for the Stokes effect and I can’t make out which of these are the true causes. Which of these apply to Raman or Luminescence or both? Are some of these the same thing?

1. At the initial absorption of an incident photon (when describing Raman also as a sort of absorption), a part of that photon energy is taken up by the electron for excitation (when that part of energy is the same as its S energy state) and the other part of that photon energy is taken up as vibrational/rotational energy for the whole molecule/atom. Thus the electron doesn’t get excited according to the full incident photon energy and because of this reason, it emits a lower photon energy.

2. After excitation of an electron, the electron itself undergoes rotational/vibrational relaxation which causes it to go down to the closest excited S energy state, before jumping back to the ground state and emitting a photon. Seen here at 5:40

3. After excitation of an electron, the whole atom/molecule that had an initial higher rotation/vibration energy (from where?), loses that energy to the rest of the lattice by phonon interaction which causes the excited electron to relax to a lower excited S energy state, before jumping back to the ground state and emitting a photon.

4. After excitation of an electron, the molecule/atom somehow gets into a higher vibrational/rotational energy state (why after electron excitation?) which makes the electron, on its way back, end up in a higher sublevel of the ground state, thus emitting a lower energy photon.

5. Caused by the Franck-Condon Principle

UPDATE: After reading further here, I came to a conclusion that cases 2 and 3 are caused by the Franck-Condon Principle, which is case 5. It seems that the Franck-Condon Principle can act in 3 ways. Case 2 is the FC Principle in isolated molecules/atoms. Case 3 is the FC Principle for phonon interactions. There's a third FC Principle not mentioned here that is caused in solvents. I'd like verification on if this is all correct or not.

There are now still cases 1 and 4 left for which I have the initial question.

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Raman scattering and luminescence are almost the same thing, the distinction being whether or not the system dephases during the process. This could happen, for example, by a collision. In the case of Raman scattering we take the point of view that the photon is never absorbed. In the case of luminescence, we suppose that the photon is absorbed. A subtle difference.

Case 1 describes Raman scattering, as the destruction of the photon, creation of the new photon, and creation of the phonon occur simultaneously.

  1. I don't know what an "S energy state" is, but it appears that the wording could describe both Raman and luminescence.

  2. I don't understand this one. How can a nucleus have a vibrational state? And if it does, and it goes from a higher vibrational state to a lower one, then it seems like the process is anti-Stokes scattering.

  3. Like 1., this one is vague about the intermediate state, so it could apply to either Raman or luminescence.

  4. More likely to be Raman than luminescence, because there is an implication that nothing interferes with the system while it's in the intermediate state. However, it could also apply to luminescence.

All of these statements are probably correct as far as they go. Hard to say what the author of the statements had in mind. But they are all short metaphors for the process, which we can accurately describe only with mathematics. There's no way to "understand" what is going on. The same is true for all quantum mechanical phenomena.

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  • $\begingroup$ Some info on the points that weren't clear to you: 2) S energy states is another name for the available energy states of an electron that it jumps to if it absorbs a photon of a same energy. 3) My apologies, I meant the whole molecule/atom having a vibrational state instead of just the nucleus. Also, I didn't mean anti-Stokes. What I meant is, because the whole molecule/atom loses its vibrational state by phonon interaction, the electron makes a small jump to a lower excited energy state without photon-emission, and THEN jumps back to the original ground state emitting a lower energy photon. $\endgroup$ – JohnnyGui Jan 28 '17 at 22:03

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