Raman shows that when a monochromatic visible light of frequency ν passes through a gas or liquid, the light is scattered in all possible directions and has three frequencies.
One is the incident frequency ν and other two frequencies are ν ± ∆ν. The scattered light of the incident frequency is due to well-known Rayleigh Scattering. However, the production of two additional frequencies was a new phenomenon and is known as the Raman Scattering. The scattering of light with change of frequency is known as Raman Effect. The spectrum formed due to Raman effect is called Raman spectrum and the spectral lines obtained are called Raman lines. The frequency shifts ∆ν of the Raman lines are independent of the incident frequency ν. Raman lines with the lower frequency (ν - ∆ν) is known as the Stokes line while the line of the higher frequency (ν + ∆ν) is known as the anti-Stokes line. The intensity of the Stokes line is more than that of the anti-stokes line. However, both of them are much weaker than the intensity of the incident frequency ν.
The explanation of Raman scattering in terms of the Quantum theory is very simple. When light is incident on a substance, the photons can be imagined to undergo collisions with the molecules. If the collision between photon and molecule is perfectly elastic, there will be no transfer of energy from photon to molecule or vice versa. The photon is scattered without any change of energy or frequency (Rayleigh Scattering).
If the collisions are inelastic, energy is exchanged between photon and molecules. The molecule can gain or lose amounts of energy equals to the difference in energy between two of its allowed quantum states. If the molecule gain energy from the photon, it goes from a lower energy state (E1) to higher energy state (E2), while the photon will be scattered with energy hν - ∆E (= E2 - E1). Then the scattered light will have a frequency ν - ∆E/h, which is less than that of the incident light. The resulting lines are Stokes lines located on the lower frequency side of Raman spectrum. Conversely, the molecule may be initially in an exited state, it may lose energy ∆E (= E2 - E1) to the photon and go to a lower state after collision, while the photon is scattered with energy hν + ∆E. Hence the scattered light will have a frequency ν + ∆E/h. The resulting lines are anti-Stokes lines, situated on the higher frequency side of the Raman spectrum. At ordinary temperature, there are more molecules in the lower energy state. Therefore, transitions are more likely from lower energy state to upper energy state and the energy is absorbed by the molecules from photons. The reverse process of energy being given to the photons is less likely. Therefore, Stokes lines are more intense than the
The most important aspect of the Raman effect is that the Raman spectra can be obtained even for those molecules which do not possess permanent dipole moment. Therefore, the Raman spectra provides a method for the spectroscopic investigation of those system which are not accessible by the usual infrared technique. The Raman discovery has given a method to carry out infrared spectroscopy with the help of the visible light. Every substance has its own distinct Raman spectra that can be used to identify the substance. This effect was found so useful in various fields that Raman was awarded Nobel prize in 1930. The availability of laser has increased the usefulness of the Raman effect by many folds. It can be used to study vibrational, rotational and anisotropy spectra of gases and liquids. It can also be used to study lattice spectra of the transparent crystals. It helps to understand nature of the molecular interaction, chemical bond, hydrogen bonding, isomerism etc. Its application is not limited to physics and chemistry only. It is utilized in the fields of biology, medicine, engineering and technology. In the honour of the discovery of the Raman effect, 28th February of every year is celebrated as National Science Day. It is indeed unfortunate that even after a period of more than 88 years, we are still awaiting a second Nobel prize in science to be awarded to an Indian citizen.
Dr. Mukesh Kumar
Department of Physics
S.V. College, Aligarh