Raman Spectroscopy
Raman Spectroscopy
Raman spectroscopy is a powerful tool for determining chemical species and detects certain interactions of light with matter. In particular, this technique exploits the existence of Stokes and Anti-Stokes scattering to examine molecular structure.
When a beam of light is passed through a transparent substance, a fraction of radiant energy in scattered at all angles from the direction of the beam by the molecules or aggregates of molecules present. The fraction of the beam that is scattered is undetectable to the eye when the particles are of molecular dimensions. This type of scattering is called Rayleigh scattering.
If monochromatic radiation, or radiation of a very narrow frequency bond is used the scattered energy will consist almost entirely of radiation of the incident frequency (Rayleigh scattering) and in addition certain discrete frequencies above and below that of the incident beam will be scattered. This scattering of certain frequencies above and below that of incident beam is called Raman scattering. Such a study was carried out by sir C.V. Raman in 1928. This study also confirmed the predictions made by Smekel in 1923 that if a substance in the gaseous, liquid or solid state is irradiated with monochromatic light, the scattered light should contain radiation with different frequencies than the frequency of incident light. The lines whose wavelengthes have been modified in Raman effect are know as Raman lines. The lines whose wavelength is greater as compared to that of the wave length of incident light are called stokes' lines and the lines having wavelength shorter than the wavelengths of incident light are called Anti-stokes lines.
Stokes radiations is generally more intense than Anti-stokes. A sensitive apparatus is needed for its study. The Raman shift, Δ𝜈 is given by the relation-
Δ𝜈 = 𝜈i - 𝜈s
Where 𝜈i is the frequency of incident radiation and 𝜈s is the radiation scattered by the given molecules species.
Raman shift is independent of the frequency of incident light but it is considered as a characteristic of the substance that causes Raman effect Δ𝜈 is positive for stokes lines and Δ𝜈 is negative for antistokes lines.
Characteristic Properties of Raman Lines
Raman lines exhibit the following characteristics-
1. The intensity of stokes lines is greater in comparision to that of corresponding Anristokes lines.
2. Raman shift (Δ𝜈) is generally observed in far and near infrared region of the spectrum.
3. The Raman lines are symmetrically displaced about the parent lines. With the increase in temperature there occurs a decrease in separation of individual Raman lines from the parent lines.
4. The frequency difference between the modified and parent line represents the frequency of absorption band of the material under investigarion.
