I am currently studying the textbook Infrared and Raman Spectroscopy, 2nd edition, by Peter Larkin. In a section entitled The Raman Scattering Process, the author says the following:

Light scattering phenomena may be classically described in terms of EM radiation produced by oscillating dipoles induced in the molecule by the EM fields of the incident radiation. The light-scattered photons include mostly the dominant Rayleigh along with the very minor amount of Raman scattered light. The induced dipole moment occurs as a result of the molecular polarizability $\alpha$ where the polarizability is the deformability of the electron cloud about the molecule by an external electric field. Fig. 2.9 shows the response of a nonpolar diatomic placed in an oscillating electric field.

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Here we represent the static electric field by the plates of a charged capacitor. The negatively charged plate attracts the nuclei, while the positively charged plate attracts the least tightly bound outer electrons resulting in an induced dipole moment. This induced dipole moment is an off-resonance interaction mediated by an oscillating electric field.

Can someone please better explain what the "electron and proton center" in figure 2.9 is supposed to be, and how it relates to Raman scattering? Is the "electron and proton center" of the diatomic molecule only at that single point that is pointed to by the arrow in figure 2.9? If so, then why? Or is each of the three points with the identical symbols an "electron and proton center"? Again, if so, then why?

I would greatly appreciate it if someone would please take the time to clarify this.


What does it even mean for a diatomic molecule to be "homogeneous"? I understand the concept of "homogeneous" mixtures, but not a "homogeneous" molecule.

  • $\begingroup$ In the water molecule, the electron center is at the center of the Oxygen atom, on its nucleus. The proton center is in the center of the triangle ABC where A is the nucleus of the Oxygen atom, and B and C are the nuclei of the 2 H atoms. The proton center is not far, but not exactly on the Oxygen nucleus. $\endgroup$ – Maurice May 14 '20 at 20:10
  • $\begingroup$ @Maurice Are you saying that the textbook author is incorrect? Is water even a "homogeneous diatomic molecule"? What does it even mean for a diatomic molecule to be "homogeneous"? I understand the concept of "homogeneous" mixtures, but not a "homogeneous" molecule. $\endgroup$ – The Pointer May 14 '20 at 20:13
  • $\begingroup$ @porphyrin Ahh, ok, I see. So, in that case, all three points with the same symbol are "electron and proton centres"? $\endgroup$ – The Pointer May 14 '20 at 20:52
  • $\begingroup$ @porphyrin Thanks for taking the time to explain this to me. Feel free to post an answer, so that I can accept it and close this question. $\endgroup$ – The Pointer May 14 '20 at 21:02

I guess for homogeneous read homo-nuclear. He is trying to explain how the electric field of the radiation induces a dipole by shoving the electrons around, hence reference to polarisability: this relates to the standard description. This induced oscillating field then radiates, hence scattering. The electron and proton centre is when the field is zero, i.e. whatever that distribution is in the molecule in no external field


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