Why Does Raman Activity Require Anisotropic Polarizability?

  • #1
Dario56
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It's mentioned that the normal mode of molecule needs to involve the change in molecular polarizability to be Raman active.

Explanation is provided in Physical Chemistry textbook by Atkins on the example of the rotational Raman spectra. Only the frequency of the electric field ##(f_i)## occurs in the induced dipole formula if the polarizability is isotropic. If it's anisotropic, two additional frequenices occur ##(f_i + 2f_R)## and ##(f_i - 2f_R)## corresponding to Raman shift (Stokes and anti-Stokes lines), where ##f_R## is rotational frequency of the molecule. This explanation is clear, but it's quite math based without much intution.

Can you give more intuitive explanation of the relation between anisotropic polarizability and Raman activity?
 
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  • #2
I had typed out a long explanation but then realized it was far more math-based than your OP! The most intuitive explanation is probably that the Raman effect is a two-photon process. Since the EM field is represented by a vector, two-photon processes must be represented by a rank-2 tensor. We call that tensor the polarizability tensor. I don't know if that helps at all.
 

What is Raman activity and how is it related to polarizability?

Raman activity refers to the ability of a molecule to scatter light inelastically, which results in a shift in the wavelength of the scattered light. This phenomenon is closely related to the polarizability of a molecule, which is a measure of how much the electron cloud around the molecule can be distorted by an external electric field. In Raman spectroscopy, the change in polarizability during vibration is what leads to the inelastic scattering of light, producing a Raman signal.

Why does Raman activity require anisotropic polarizability?

Anisotropic polarizability means that the polarizability of a molecule varies with direction. For a molecule to be Raman active, its polarizability must change with respect to its vibrational modes during the interaction with light. If the polarizability is isotropic (same in all directions), no change in polarizability would occur during vibration, and thus, no Raman scattering can be observed. Anisotropic polarizability allows for the directional change needed to detect different vibrational modes.

What is the role of symmetry in Raman spectroscopy?

In Raman spectroscopy, the symmetry of a molecule plays a crucial role in determining which vibrations are Raman active. Symmetry elements of the molecule can either allow or forbid changes in polarizability with certain vibrational modes. Group theory and symmetry operations help predict the Raman activity of these modes. Molecules with higher symmetry might have fewer Raman active modes due to more symmetry-related restrictions on changes in polarizability.

How does the laser wavelength influence Raman activity?

The wavelength of the laser used in Raman spectroscopy affects the efficiency of the Raman scattering process. The polarizability change and, consequently, the Raman activity can vary depending on the energy of the incident light. Different electronic environments in the molecule can interact differently with various laser wavelengths, affecting the intensity and even the presence of certain Raman bands. Selecting an appropriate laser wavelength is crucial for optimizing the detection of specific vibrational modes.

Can all molecular vibrations produce a Raman signal?

Not all molecular vibrations produce a Raman signal. For a vibration to be Raman active, it must involve a change in the polarizability of the molecule. Vibrations that do not result in a significant change in polarizability are typically not Raman active. Additionally, the symmetry of the molecule can forbid certain vibrations from being Raman active based on selection rules derived from group theory. Therefore, both the nature of the vibrational mode and the symmetry of the molecule determine Raman activity.

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