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.