Raman Spectroscopy | Raman Effect | Raman Shift | Raman Spectroscopy on the basis of Quantum Theory

Introduction to Raman Spectroscopy

Raman Spectroscopy is the nondestructive instrumental technique that provides detailed information about chemical structure, phase and polymorphy, crystallinity, and molecular interactions.

Raman Effect

When the light of a definite frequency is passed through a gas, liquid, 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 the incident frequency. The series of lines in the scattered light is the Raman spectrum (lines) and the phenomenon is known as Raman Effect.

Raleigh Scattering or Raleigh Line 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.

Stoke Line and Anti-Stoke Lines in Raman Spectroscopy

In 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 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 the 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 the collision, photons lose their energy to molecules scattered.

Thus the scattered radiation is called stokes radiation 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, scattered radiations called Anti-Stoke radiations have more energy and frequency than 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 a lower frequency. It is Stokes Lines.

[c] If E > E¹,v1> v. Here the scattered photons have a 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 center 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 if there are some common lines that coincide, then the molecule has no center of symmetry.
3. The rule of mutual exclusion can be best explained by studying the example of the CO₂ molecule

            CO₂ molecules show four vibrations viz,

Symmetrical Stretching Vibrations.

• It shows an absorption band in the Raman Spectral region a 1300cm^-1 and it has zero dipole moment
• Thus, it is Raman active and IR inactive.

Asymmetrical Stretching Vibrations

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

Thus, it is IR active and Raman inactive.

Pair of Degenerate Bending Vibrations: (Symmetric)

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