Raman Scattering: How is it Possible for an IR Inactive Molecule?

In summary, the conversation discusses the possibility of a homogeneous molecule being Raman active despite not having a permanent dipole moment and being IR inactive. The question arises on how a scattering process can occur without any excited states in the molecule. It is suggested that the polarizability of the molecule may play a role in Raman scattering.
  • #1
Niles
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Hi

How is it possible for a homogeneous molecule (i.e. one not having a permanent dipole moment and hence not IR active) to be Raman active? What my confusion is that since we cannot excite any states in the molecule (since it is IR inactive), then how can any scattering process even occur?


Niles.
 
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  • #2
Somewhere I remember that Raman scattering is nonlinear. Perhaps it is the quadrupole moment that matters?
 
  • #3
Niles said:
Hi

How is it possible for a homogeneous molecule (i.e. one not having a permanent dipole moment and hence not IR active) to be Raman active? What my confusion is that since we cannot excite any states in the molecule (since it is IR inactive), then how can any scattering process even occur?


Niles.
Well I think it is the polarizability that is important in Raman scattering. If the molecular polarizabilty changes by interaction of the incident e.m.waves with the internal motions of a molecule , the light will be scattered.
 

1. What is Raman scattering and how does it work?

Raman scattering is a phenomenon in which a molecule absorbs and emits light, resulting in a change in its vibrational energy. This change in energy causes a shift in the frequency of the scattered light, which can be measured and used to identify the molecule. Raman scattering works by shining a monochromatic light source, such as a laser, onto a sample and analyzing the scattered light using a spectrometer.

2. How is it possible for an IR inactive molecule to exhibit Raman scattering?

IR inactive molecules are those that do not show a change in their dipole moment when they vibrate, and therefore do not absorb or emit infrared (IR) light. However, Raman scattering relies on a change in the polarizability of the molecule, which is a measure of how easily the electron cloud can be distorted. This change in polarizability allows for Raman scattering to occur, even in IR inactive molecules.

3. Can Raman scattering be used to identify all types of molecules?

While Raman scattering can be used to identify a wide range of molecules, it is most effective for molecules that have polarizable bonds, such as those with multiple bonds or heteroatoms. It is also limited in its ability to identify molecules with low concentrations or overlapping Raman bands. Therefore, other techniques, such as IR spectroscopy, may be more suitable for certain types of molecules.

4. What are the advantages of using Raman scattering over other spectroscopic techniques?

Raman scattering has several advantages over other spectroscopic techniques. It can be used to analyze samples in a non-destructive manner, as the laser used is not strong enough to cause chemical changes. It also has a high degree of specificity, allowing for the identification of individual molecules in a complex mixture. Additionally, Raman scattering can be performed on solids, liquids, and gases, making it a versatile technique for various sample types.

5. Are there any limitations or challenges associated with Raman scattering?

One limitation of Raman scattering is its sensitivity to fluorescence, as the emitted light can interfere with the Raman signal. This can be mitigated by using a lower intensity laser or adding a fluorescence suppressant to the sample. Another challenge is the relatively weak Raman signal, which can make it difficult to detect low concentrations of molecules. However, advancements in technology, such as the use of resonance Raman spectroscopy, have helped to overcome these limitations and make Raman scattering a valuable analytical tool in scientific research.

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