Can someone qualitatively explain the difference between Raman scattering, Rayleigh scattering, and Brillouin Scattering with respect to molecules? I attempt to define each below but would appreciate either affirmation I'm correct or correction if I'm wrong. Raman scattering - Molecules contain electronic states of energy. Within each electronic state there are vibrational states of energy. If a molecule in the ground electronic / ground vibrational state absorbs a photon and reaches an excited electronic state / excited vibrational state (likely the case due to Franck-Condon principle), the molecule will likely thermally relax to the excited electronic / ground vibrational state via a non-radiative process, i.e. internal conversion, and then emit a photon of lesser energy (stokes radiation). Rayleigh scattering - Unlike Raman, its elastic. So the E of the photon absorbed equals the E of the photon emitted. Essentially, the only thing the molecule does is change the direction of propagation for the light. Brillouin Scattering - (This one is the tough one) The presence of phonons (sound) or a magnetic field (magnons) or thermal gradients interrupts the "lattice" of particles or equilibrium distribution of particles and, in turn, affects the index of refraction. The result in a change of frequency. Brillouin is the one I struggle with really. Doesn't the index of refraction merely change the direction of the light toward the normal of the surface? I'm thinking about it too simply, I know, but by what equation does index of refraction change the frequency of incident light? I've seen Rayleigh and Raman scattering described in terms of Brillouin scattering. Also, index of refraction is a more macroscopic property. What is going on at the atomic level in Brillouin scattering?