Difference between Raman scattering and Rayleigh scattering

[DanSandberg]

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?

 

[alxm]

Well, your description of Raman is an entirely correct description.. of fluorescence!

The difference here is that Raman is an excitation to a 'virtual' electronic level. It doesn't exist, and the molecule is not in the 'excited' state for any observable amount of time. (It's really just an artifact of how it's calculated, like virtual particles in QFT) The molecule then relaxes back to either a lower or higher energy level (relative its original state) by emitting another photon. Vibrational relaxation don't really enter into it, as there's no time. Of course, the excited state the molecules is left in after raman scattering might undergo vibrational relaxation.

Since the timescale here is so short (much shorter than fluorescence, not to mention phosphorescence) the re-emitted photon can be viewed as the same photon, with a change in energy corresponding to the change in energy of the electronic levels. From that point-of-view, it's the elastic scattering of a photon off the molecule.

Rayleigh can be viewed as the same process as for Raman, only with no changes in the energy levels of the molecule, hence elastic scattering.

Brillouin scattering is indeed a more macroscopic property, but it is related. It wouldn't occur to me to describe Rayleigh or Raman in terms of Brillouin scattering since I'm a chemical physicist. (indeed, a student learning physical chemistry would typically learn all about the former two, but not Brillouin). It's a solid-state physicist point of view to try to describe molecular phenomena in terms of condensed matter phenomena rather than vice versa! :) It's a bit silly. What's the refractive index of a molecule?

 

[DanSandberg]

Right - i knew that! Stupid mistake regarding the Raman, I wasn't fully awake yet.

In terms of the refractive index of a molecule, I'm not sure there is one. I suppose you could consider the wave equations for a photon and a phonon and try to extract the constructive/destructive interference but so far I'm not having much success with this. I'm going to keep at it and see if I can derive something to explain it quantum mechanically.

 

[alxm]

Ah, it was rhetorical really. There no meaningful way (to my knowledge) to define the index of refraction for a single molecule.