Why Does Raman Activity Require Anisotropic Polarizability?

In summary, Raman activity necessitates anisotropic polarizability because the scattering of light by molecules depends on the ability of their electron clouds to distort in response to an external electric field. Anisotropic polarizability implies that the polarizability varies in different directions, allowing for the energy levels of the vibrational modes to couple with the light, leading to changes in the scattering intensity. This directional dependence is crucial for the selection rules that determine which vibrational modes are Raman-active, thereby enabling the observation of specific molecular vibrations in Raman spectroscopy.
  • #1
Dario56
290
45
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?
 
Chemistry news on Phys.org
  • #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.
 

FAQ: Why Does Raman Activity Require Anisotropic Polarizability?

Why does Raman activity require anisotropic polarizability?

Raman activity requires anisotropic polarizability because Raman scattering involves the interaction of light with the vibrational modes of a molecule. Anisotropic polarizability is necessary to induce a change in the dipole moment of the molecule during vibration, which is essential for Raman scattering to occur.

How does anisotropic polarizability affect Raman scattering?

Anisotropic polarizability affects Raman scattering by influencing the intensity and selection rules of Raman-active vibrational modes. Molecules with anisotropic polarizability exhibit different levels of interaction with incident light, leading to the observation of specific Raman bands in the spectrum.

Can molecules with isotropic polarizability exhibit Raman activity?

Molecules with isotropic polarizability can exhibit Raman activity, but the intensity of the Raman signals may be significantly weaker compared to molecules with anisotropic polarizability. Isotropic polarizability limits the ability of the molecule to undergo significant changes in dipole moment during vibration, affecting the Raman scattering efficiency.

How is anisotropic polarizability determined in a molecule?

Anisotropic polarizability in a molecule can be determined through theoretical calculations, such as quantum mechanical calculations using density functional theory (DFT). Experimental techniques, such as polarized Raman spectroscopy, can also provide information on the anisotropic nature of the polarizability in a molecule.

Are there any exceptions to the requirement of anisotropic polarizability for Raman activity?

While anisotropic polarizability is a common requirement for Raman activity, there are exceptions. For example, certain symmetry-allowed vibrational modes in molecules with isotropic polarizability can still exhibit Raman activity. However, the presence of anisotropic polarizability generally enhances the Raman scattering efficiency and allows for a more detailed analysis of molecular vibrations.

Similar threads

Another Raman spectrum question
Replies
19
Views
5K
Why do molecules have rotational Raman spectra?
Replies
4
Views
7K
I EM Radiation of "Permanent" Molecular Electric Dipoles
Replies
1
Views
1K
Nuclear Magnetic Resonance (NMR) Explanation
Replies
4
Views
5K
Books to Self-Study Maths, Physics, and More
Replies
7
Views
7K
Textbooks on Condensed Matter Physics
Replies
19
Views
17K

Hot Threads

Recent Insights

Back
Top