Raman Effect

In 1928 C.V. Raman discovered that when a beam of monochromatic radiation was allowed to pass through some organic substances like benzene toluene, the scattered light contained some additional frequencies other than that of the incident frequency. This is known as Raman effect. The lines with longer wavelength greater than that of the incident light are called Stokes lines and those with lower wavelength are called anti Stokes lines.
If nν_v is the frequency of scattered radiation and no the frequency of the incident radiation, then,
Dn = |ν_v - ν_o | which is called the Raman shift.

A quantum of the incident light of frequency say ν_o having energy hν_o can collide with a molecule and be scattered with unchanged frequency (elastic collisions). This is referred to as Rayleigh scattering. It is also possible for the incident quantum to induce a transition in the molecule. For simplicity, let us consider a change from the ground to the first vibrational state (v =0 to v =1) in a diatomic molecule. Let the frequency difference from 0 to 1 be represented as ν_v. The energy difference between these two states would be ν_v. The light quantum, which is scattered, now has a frequency (ν_o - ν_o) and the resulting line is the Stokes line in the Raman spectrum.

The measured value of ν_v is identical to the infrared frequency that would cause vibrational transition (mode) if it were IR active. A molecule in the vibrationally excited state v = 1 can collide with an incident light quantum of energy hν_o and frequency ν_o. The molecule can return to the ground state by giving its additional energy hν_v to the photons. This photon when scattered will a have a frequency ν_o + ν_v. The spectral line with this frequency is referred to as anti Stokes line.

Because there are other mechanisms for returning to the ground state, there are fewer molecules in the v = 1 state than v = 0 and the intensity of the anti Stokes line is much lower than that of the Stokes line. In Raman spectroscopy we study Stokes and anti Stokes lines, which are also called, Raman lines.