Question

Explain Raman effect with the help of energy level diagram.

Answer 1

Raman Effect:

The monochromatic light is scattered when it is allowed to pass through a substance. The scattered light contains some additional frequencies other than that of incident frequency. This is known as Raman effect.

The lines whose frequencies have been modified in Raman effect are called Raman lines. The lines having frequencies lower than the incident frequency are called Stoke's lines and the lines having frequencies higher than the incident frequency are called Anti-stokes lines. This series of lines in the scattering of light by the atoms and molecules is known as Raman Spectrum.

The Raman effect can be easily understood, by considering the scattering of photon of the incident light with the atoms or molecules. Let the incident light consist of photons of energy $h{v}_0$.

1. If a photon strikes an atom or a molecule in a liquid, part of the energy of the incident photon may be used to excite that atom of the liquid and the rest is scattered. The spectral line will have lower frequency and it is called stokes line.

2. If a photon strikes an atom or a molecule in a liquid, which is in an excellent state, the scattered photon gains energy. The spectral line will have higher frequency and it is called Anti-stoke's line.

3. In some cases, when a light photon strikes atoms or molecules, photons may be scattered elastically. Then the photons neither gain nor lose energy. The spectral line will have unmodified frequency.

If $v_0$ is the frequency of incident radiation and $v_s$ the frequency of scattered radiation of a given molecular sample, then Raman Shift or Raman frequency ${\Delta }_v$ is given by the relation $Dn=n_0-n_s$.

The Raman shift does not depend upon the frequency of the incident light but it is the characteristic of the substance producing Raman effect. For Stoke's lines, ${\Delta }_v$ is positive and for Anti-stoke's lines ${\Delta }_v$ is negative.

The intensity of Stoke's line is always greater than the corresponding Anti-stoke's Line. The different processes giving rise to Rayleigh. Stoke's and Anti-stokes lines are shown in Figure.

When a system interacts with a radiation of frequency $v_0$, it may make an upward transition to a virtual state. A virtual state is not one of the stationary states of the molecules. Most of the molecules of the system return back to the original state from the virtual state which corresponds to Rayleigh scattering. A small fraction may return to states of higher and lower energy giving rise to Stoke's line and Anti-stoke's line respectively.