Raman Effect
In 1928 Raman and Krishnan observed phenomenon of Raman scattering of light. Raman Effect or Raman scattering is generally use to study the rotational and vibrational properties of molecules. Raman effect is in association with Rayleigh scattering in some manner. Rayleigh scattering is elastic in nature but Raman scattering is inelastic in nature. Raman effect depends upon the molecule’s polarizability.
Rayleigh and Raman Scattering
In both the scattering, light having enough energy strikes the molecules, excites its vibrational or rotational states but somehow the energy of light is not good enough to take it out of the ground electronic state. Then this light is only able to excite the molecules to virtual states, from which it decays back to lower energy states. Now further on two cases can arise.
- The molecule decays back to the initial state, which corresponds to the Rayleigh Scattering.
- The molecule decays back to a different state, which corresponds to the Raman Scattering.
Stokes and Anti-Stokes Lines
When the final energy state is higher than the original state, We call it a stokes transition and gives Stokes line and if the final state is lower than the original state then we call it an anti-Stokes transition and gives an Anti-stokes line.
Raman scattering can easily observe in terms of change in frequency,wavelength or wavenumber. When monochromatic radiation with a wavenumber ν0 is incident on systems. Most of the radiation is transmit without change but some scattering of the radiation occurs with change. When the the frequency of the scattered radiation is measured, it is found that the we observed not only the wavenumber ν0 associated with the incident radiation but also pairs of new wave numbers of the type ν′= ν0 -νM and ν′= ν0 + νM.
In molecular systems, the wave numbers νM lie in the ranges related with transitions between rotational, vibrational, and electronic levels. This scattering of radiation with a change of wavenumber is call as Raman Scattering.
It is now very much clear that Raman band is to be characterized not by its absolute wavenumber but by the magnitude of its wavenumber shift νM from the incident wavenumber. We call these wavenumber shifts as Raman wavenumber.
- For Stokes scattering, Raman shift is Δν= ν0 – νM
- For anti-Stokes scattering, Raman shift is Δν= ν0+ νM
We can distinguish Stokes and anti-Stokes Raman scattering by defining Δν to be positive for Stokes scattering and negative for anti-Stokes scattering.
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