In this particular article ‘Raman effect and its experimental verification’, we are going to discuss the Raman effect, experimental arrangements. Also about the classical theory of the Raman Effect.
When a monochromatic light of angular frequency ω is incident on a dust-free transparent substance (liquid, gas or solid), then in the scattered radiation, in addition to the frequency ω of incident light, a number of other frequencies, higher and lower than the incident one, are found. This phenomenon is called the Raman effect.
Stokes – Anti-Stokes Lines
In the spectrum of the scattered radiation, the new lines are called Raman lines (or bands). Raman lines (or bands) at frequencies less then the incident frequency (wavelengths greater than the incident wavelength) are called Stokes lines. And those at frequencies greater than the incident frequencies (wavelengths less than the incident wavelength) are called anti-Stokes lines. In general, the Stokes lines have greater intensity than the anti-Stokes lines. Further, the Stokes lines have greater intensity than the anti-Stokes lines. Further, the intensity of the anti-Stokes rapidly grows with rising of the temperature of the scattering substance.
General Characteristics of Raman lines
- In molecular systems, the difference between the frequencies of a Raman line and the original incident radiation is found to lie principally in the ranges associated with transitions between rotational, vibrational and electronic levels.
- The displacement in frequencies of the Raman lines from the original incident radiation frequency is independent of the frequency of the incident light and are characteristic of the scattering substance.
- If another light source with a different spectrum for incident radiation is used, other Raman lines are obtained for the same scattering substance. However, the displacements from the exciting lines are the same.
- For different scattering substance, the displacements have different magnitudes. Thus the Raman displacements are the characteristics of the scattering substance under consideration.
- The scattered radiation usually has polarization characteristics different from those of the incident radiation.
- The Raman scattering occurs over all directions. And both the intensity and polarization of the scattered radiation depend on the direction of observation.
- In the case of diatomic gases, the Raman spectrum may be regarded as an infra-red spectrum shifted into the conveniently accessible visible or ultraviolet region.
The phenomena of Raman scattering was discovered in 1928 by Indian physicist C.V Raman. Who received the Nobel Prize in physics in 1930 for his work. The first experiments demonstrating Raman Scattering were crude. In Raman’s initial experiments, a Beam of sunlight was converged successively by a telescope objective of 10 cm aperture and 230 cm focal length, and by the second lens of 5 cm focal length.
At the focus of the second lens was placed the scattering material. Which is either a liquid (carefully purified by repeated distillation in the vacuum) or its dust-free vapour. To detect the presence of a modified scattered radiation, the method of complementary light-filters was used. A blue-violet filter, when coupled with a yellow-green filter and placed in the incident light, completely extinguished the track of the light through the liquid or vapour. The reappearance of the track when the yellow filter is transferred to a place between it. And the observer’s eye is proof of the existence of a modified scattered radiation. spectrographic confirmation is also available.
Raman, initially, examined some sixty different common liquids, and every one of them showed the effect in greater or less degree. That the effect was a true scattering and not a fluorescence was indicated in the first place by its feebleness in comparison with the ordinary scattering, and secondly by its polarization. Which is in many cases quite strong and comparable with the polarization of the ordinary scattering.
Classical Theory of Raman Effect
Raman effect consists in the occurrence of (weak) displaced lines in the spectrum of the light scattered by gases (or by liquids or solids), the amount of the shift being characteristic of the substance considered.
If an atom or molecule is brought into an electric field E. The center of positive charges is moved a small distance in one direction. And that of the negative charges is moved in the opposite direction. That is an electric dipole moment P is induced in the system. The magnitude of the resulting dipole moment is proportional to that of the field; that is
P = αE…..(1)
where α is called the polarizability.