Raman Effect : Definition, Features and Applications

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What are Stokes Lines? Write Down the Special Features of Raman Effect

In 1928 Indian scientist Sir C V Raman observed that when a beam of monochromatic light was passed through organic liquds such as benzene and toluene, the scattered light contained other frequencies in addition to that of the incident light. This phenomenon is known as Raman effect. In addition to liquids, gases and transparent solids exhibit this effect.

Raman observed the scattered light by spectrometer placed at right angle to the incident light and found that in addition to the unmodified original spectral line (main line), a number of new lines were present on both sides of the main line. These lines are known as Raman lines and the spectrum produced is called Raman spectrum.

Features of Raman Spectra:

i) In the Raman spectrum, some weak spectral of lower and higher frequencies are observed on both sides of the main line [Fig.].

Spectral lines of lower frequency (or longer wavelength) and higher frequency (or shorter wavelength) are called Stokes lines and anti-Stokes lines respectively. While only Stokes lines are observed in the fluorescence spectrum, anti-Stokes lines are observed only in the Raman spectrum.

The frequency of the incident radiation and scattered radi-ation is same in the region of original spectral lines of Raman spectrum. This scattering of light by molecules, without change in frequency is known as Rayleigh scatter-ing and the spectral line is called Rayleigh line. Anti Stokes line, Rayleigh line and Stokes line altogether are called Raman lines.

ii) Stokes lines and anti-Stokes lines are situated on both sides of the Rayleigh line at equal frequency interval. The frequencies of the lines are directly related to that of the incident light.

iii) The frequency difference (∆f) of the Stokes and the anti Stokes lines from the Rayleigh line does not depend on the frequency of the main line. But it depends on the nature of the scatterer. The frequency difference (∆f) is called Raman shift. If f₀ be the frequency of the Rayleigh line, the frequencies of Stokes line and that of anti-Stokes line are given by,
f’ = f₀ – ∆f (Stokes line)
f” = f₀ + ∆f (anti-Stokes line)

iv) The intensity of a Raman line when expressed as a fraction of the Rayleigh line is usually a few hundredths in liquids and a few thousandths in gases. No accurate data are available as regards the absolute intensities of Raman lines in liquids and gases. The Stokes lines are always more intense than the corresponding anti-Stokes lines.

v) Raman lines are generally polarised.

Explanation of Raman Effect on the basis of quan-tum Theory: According to quantum theory, any radiation is considered as the flow of photon particles, each of energy hf. When such a light photon falls on the molecules of a solid, liquid or gas, the photon undergoes three types of collisions with the molecule.

i) The molecule may merely deviate the photon without absorbing its energy which will result in the appearance of unmodified line in the scattered beam [Fig.(a)].

ii) The molecule may absorb part of energy of the incident photon, giving rise to the modified Stokes line whose frequencies will evidently be less than that of the incident radiation(Fig.).

iii) It may also happen that, the molecule itself being in an excited state, imparts some of its intrinsic energy to the incident photon and this will produce the anti-strokes line of frequency greater than that of the incident radiation[Fig.].

In the first case, elastic collision takes place between the photon and the molecule. So the frequency of the scattered photon becomes equal to its initial frequency. It proves the existence of Rayleigh line in the Raman spectra. In the second and third cases inelastic collision takes place between them. As a result two cases may arise. The frequency of the scattered photon may decrease (origin of stokes line) or increase(origin of anti-Stokes line).

Let intrinsic energy of the molecule before collision = E_p
intrinsic energy of it after collision = E_q
mass of molecule = m
velocity of the molecule before collision = v
velocity after collision = v’
energy of the incident photon = hf
energy of the scattered photon = hf’
From the principle of conservation of energy we have,
E_p + \(\frac{1}{2}\)mv² + hf = E_q + \(\frac{1}{2}\)mv’² + hf’ …. (1)
As the collision does not appreciably change the temperature of the surrounding, we may assume that the kinetic energy of the molecule remains practically unaltered in the process.
Hence from the equation (1) we have,
E_p + hf = E_q + hf’ or, h(f’ – f) = E_p – E_q
or f’ = f + \(\frac{E_p-E_q}{h}\) …. (2)

Now,

1. if E_p = E_q, then f’ = f i.e., frequency of the scattered light remains identical with the incident light. So it denotes Rayleigh line.
2. if E_p < E_q, then f’ < f i.e., frequency of the scattered light increases. So this represents anti-stocks line.

Applications of Raman Effect :

This effect has many applications.

1. It has been put to use in the study of the structure of molecules and crystals.

2. This effect has also been applied in the study of certain aspects of nuclear physics, such as the spin statistics as well as the isotopic constitution of the nucleus.