Difference between Raman scattering and Rayleigh scattering

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Discussion Overview

The discussion focuses on the differences between Raman scattering, Rayleigh scattering, and Brillouin scattering, particularly in relation to molecular interactions and energy states. Participants explore the mechanisms behind each type of scattering and the implications for understanding molecular behavior in various contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes Raman scattering as involving transitions between electronic states of energy, where a photon is absorbed and then emitted with lesser energy, suggesting a thermal relaxation process.
  • Another participant challenges this description, asserting that Raman scattering involves excitation to a 'virtual' electronic level, which does not exist for an observable time, and that the re-emitted photon can be viewed as the same photon with a change in energy.
  • Rayleigh scattering is characterized by elastic scattering, where the energy of the absorbed photon equals the energy of the emitted photon, with no changes in the energy levels of the molecule.
  • Brillouin scattering is noted to be related to macroscopic properties, such as changes in the index of refraction due to phonons or thermal gradients, but participants express uncertainty about its relationship to molecular phenomena.
  • One participant questions how the index of refraction can change the frequency of incident light and seeks to understand the atomic-level interactions involved in Brillouin scattering.
  • Another participant expresses skepticism about defining the index of refraction for a single molecule, suggesting that it may not be meaningful.

Areas of Agreement / Disagreement

Participants generally express differing views on the nature of Raman scattering and its distinction from fluorescence, with some affirming the initial description while others challenge it. There is also disagreement regarding the relevance of Brillouin scattering to molecular phenomena, with no consensus on the definitions or implications of the index of refraction at the molecular level.

Contextual Notes

Participants highlight limitations in understanding the index of refraction for single molecules and the complexities involved in relating macroscopic properties to molecular behavior. There are unresolved questions about the atomic-level mechanisms in Brillouin scattering and the definitions of energy states in Raman scattering.

DanSandberg
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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?
 
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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?
 


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.
 


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

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