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Principles of Raman Spectroscopy

The basic principles of Raman Spectroscopy are as follows,

Raman Effect in Raman Spectroscopy

When the light of a definite frequency is passed through a gas, liquids, or solid and the scattered light is observed at right angles to the incident light, the scattered light is found to contain some additional frequency over and above that of incident frequency. The series of lines in the scattered light is Raman spectrum (lines) and the phenomenon is known as Raman Effect.

Raleigh Scattering or Raleigh Lines in in Raman Spectroscopy

In Raman spectroscopy, for the scattered light, the line which has the same frequency as that of incident light is known as Raleigh Lines and in such scattering is known as Raleigh Scattering.

Stokes Line and Anti-Stoke Lines in Raman Spectroscopy

In the scattered light, the lines which have frequencies lower (longer wavelength) than that of incident light are called Stokes Lines, and the lines which have frequencies higher  (shorter wavelength ) than that of the incident are called Anti-Stoke Lines.

Raman Shift

If Vi represents the frequency of incident light and Vs represents the frequency of scattered light, then the difference (Vi-Vs) is a constant characteristic of the substance exposed to light and is independent of the frequency of the incident light.

This difference (Vi-Vs) is called the Raman Frequency or Raman Shift.

It was also found\d that the energy change i.e. hc (Vi-Vs) corresponding to Raman Frequency always corresponds to the energy changes accompanying rotational and vibrational transitions in a molecule.

Raman Spectroscopy on the basis of Quantum Theory

According to quantum theory, Raman Spectra may be regarded as a collision between the molecules of reactants and that of photons of radiations. The collisions are of the following types,

Elastic Collision

In this collision, when the photons collide with molecules, there is neither gain nor loss of energy.

This is a perfectly elastic collision and the scattered radiations will have the same frequency as that of incident radiation. This is known as Raleigh Scattering.

Inelastic Collision

There are two possibilities,

  1. At the time of collision, photons loose their energy to molecules scattered.

Thus the scattered radiation is called stokes radiations has less energy and frequency than the incident radiation.

The corresponding lines produced are called stokes lines.

 2. At the time of the collision, molecules lose their energy to photons. Thus, the scattered radiations called Anti-Stoke radiations have more energy and frequency than the incident radiation. The corresponding lines produced are called Anti-stokes lines.

The principles of conservation of energy can be applied to the photons molecule collision.

i.e. Total energy before collision. = Total energy after the collision.

                                    hv + E = hv1 + E1

Where v = frequency of the incident photon.

             E = energy of molecule before the collision.

             v1=frequency of the incident photon.

             E1= energy of molecule after the collision.

           On rearranging, we get

            hv1=  hv + (E-E1)

                           v1=  v = (E-E1) /h

There are following three possibilities,

[a] If E = E1, v1= v . It is Raleigh Scattering.

[b] If E < E1,v1< v .Here the scattered photons have lower frequency. It is Stokes Lines.

[c] If E > E1,v1> v .Here the scattered photons have higher frequency. It is Anti-Stokes  Lines.

Rule of Mutual Exclusion

It is a very important rule about IR and Raman Spectra.

  1. The Rule of mutual exclusion states that, “If a molecule has centre o symmetry, then its Raman active vibrations are IR inactive and vice versa”.
  2. The converse of this rule is also true i.e. If for a molecule Raman and IR spectra do not have common lines then the molecules must have a center of symmetry. But id there are some common lines which coincide, then the molecule has no centre of symmetry.
  3. The rule of mutual exclusion can be best explained by studying example of CO2 molecule

            CO2 molecules show four vibrations viz,

Symmetrical Stretching Vibrations.

Rule of Mutual Exclusion
  • It shows absorption bond in Raman Spectral region a 1300cm-1 and it has zero dipole moment
  • Thus, it is Raman active and IR inactive.

Asymmetrical Stretching Vibrations

Rule of Mutual Exclusion
  • It has a resultant dipole moment and show IR absorption band at 2350 cm-1.
  • But there is no change in polarizability of the molecule.

Thus, it is IR active and Raman inactive.

Pair of Degenerate Bending Vibrations: (Symmetric)

Rule of Mutual Exclusion
  • They differ only in direction and have same frequency and energy and hence it shows only one resultant absorption band in IR region.
  • However, during bending the polarizability of the molecule remains same.
  • Thus, it is IR active and Raman inactive.

Q: Who discovered Raman Spectroscopy?

Sir Chandrasekhara Venkata Raman was a famous Indian physicist well known for his work in the field of scattering light defines Raman Spectroscopy. He was born on 7 November 1888 at Tiruchirappalli and died on 21 November 1970 at Bengaluru.

Q: What is Raman Spectroscopy?

Raman Spectroscopy is a non-destructive chemical analysis technique that provides detailed information about chemical structures and their properties. This technique is used to detect vibrational, rotational, and other states in a molecular system, capable of probing the chemical composition of materials.


Q: What are the three basic types of spectroscopy?

Following are basically three types of spectroscopy, atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES), and atomic fluorescence spectroscopy (AFS)

Q: What is spectroscopy?

Spectroscopy is the study of the interaction between matter and radiation. When the light has been absorbed by the molecule there is some change in behaviors of the molecule observed.


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