What are the differences between Raman and IR Spectroscopy?

What are the differences between Raman and IR Spectroscopy?
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Schematic of Raman Spectrometer with microscopic attachment (Image Courtesy : http://cnx.org/)

Raman and IR spectroscopy are complementary techniques used for fingerprinting of molecules. Raman spectroscopy offers significant advantages over IR and other spectroscopy techniques.

Both Raman and IR spectra result due to changes in vibration modes of molecules. However, only those vibration modes which result in changes in the dipole moment of a module are IR active and those that result in change in polarizability are Raman active. IR and Raman activities are related to symmetry of molecules and can be expressed in terms of Rule of mutual exclusion which states that for a molecule having a centre of symmetry the Raman active vibrations are IR inactive and vice versa. However, this rule is not applicable to molecules having no centre of symmetry. In such case vibrations may be both Raman as well as IR active. The rule is valuable in establishing the structure of a molecule such N2O. The spectra of the molecule.suggests that the molecule does not have a center of symmetry. It can be concluded that the molecule has a structure N-N-O and not
N-O-N.

The dissimilarities between the two spectroscopic techniques are summarized below:

  • Raman spectra result from scattering of light by vibrating molecules whereas IR spectra result from light absorption by vibrating molecules
  • Raman activity results from change of polarizability of a molecule whereas IR activity results from changing dipole moment
  • A monochromatic light beam of high intensity laser can be used in UV, visible or IR regions in Raman measurements whereas in IR spectroscopy the range is limited to IR frequencies
  • In case of Raman scattered light is observed at right angles to the direction of the incident beam whereas in case of IR the absorption signal is measured in the same direction as the incident beam.
  • Raman technique is non-destructive. The sample can be measured directly in glass container or in case of pharmaceuticals samples can be measured in original sachets.. IR technique requires solid sample preparation using KBr or CSi powder though accessories such as HATR permit direct observation of liquids, films and gels.
  • Laser sources in Raman technique are highly intense and these facilitate focusing the coherent beam on small sample area or on exceedingly small sample volumes. This is a key advantage when only limited sample quantities are available
  • A high degree of amplification of weak Stoke signals is necessary in presence of intense Rayleigh light scattering component. This results in higher cost of the Raman spectrometer. Higher cost can be easily justified against the benefits offered by the technique.

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