Raman Effect
Sreelakshmi P Vijayan
The Raman effect was discovered in 1928 and was named after the Indian scientist Sir C. V. Raman. Raman was awarded the Nobel Prize in Physics in 1930 for this discovery of Raman scattering. The Raman effect or Raman scattering is the inelastic scattering of photons by matter, where there is an exchange of energy and a change in the light’s direction happens. Usually, this effect involves vibrational energy being gained by a molecule as incident photons from a visible laser are shifted to lower energy. This is called normal Stokes Raman scattering.
When photons of light get scattered by a material, most of them are elastically scattered, known as Rayleigh scattering. That is, the scattered photons have the same energy ,frequency, wavelength and color as the incident photons. But they have different direction. The intensity range of Rayleigh scattering is between 0.1% to 0.01% with respect to that of a radiation source.
A very slighter fraction of the scattered photons can be inelastically scattered having energy lower than that of the incident photons. These are known as Raman scattered photons. Because of law of conservation of energy, the material either gains or loses energy in the process. Absorption of a photon excites the molecule to an imaginary state and re-emission of this photon leads to Raman or Rayleigh scattering. For Rayleigh scattering the vibrational energy of both initial and final states are the same. But for Stokes Raman scattering, the initial state has higher vibrational energy and for anti-Stokes Raman scattering the vibrational energy of the initial state is low. But in all the three cases the electronic energy of the initial and final state is the same.
Modern Raman spectroscopy
It involves the use of lasers as an exciting light source. Since lasers were not even available for more than three decades after this discovery, Raman and his co-workers used mercury lamp and photographic plates to record the spectra. It took many hours to days to produce a spectrum due to reduced sensitivity of the detectors, weak light sources, and low Raman scattering cross-sections of the materials. Then photodiode arrays and photomultiplier tubes came into use. Nowadays the most commonly used modern detectors are charge-coupled devices (CCDs).
Applications
Other alternatives of Raman spectroscopy allows Rotational energy to get studied, where gas samples are used and Electronic energy levels are examined when the source is an X-ray. Much complexed techniques involving pulsed lasers, multiple laser beams are also used. Raman effect is used as a tool for analyzing the composition of various solids, liquids and gases. Raman scattering gives data about the vibrations within a molecule. In the case of gases, data about rotational energy can be collected. For solids, high frequency phonon and magnon modes can be observed. Raman spectroscopy finds its application in the analysis of highly complex materials such as bio-organisms and human tissues. It is used in optical amplifiers. It can be used to determine the bond length of molecules having no IR spectrum. It helps in chemical imaging of simple molecules such as nucleic acid, by tagging.
The blue color of the sky is due to Raman effect, where the N2 and O2 gases in the atmosphere involves in Rayleigh scattering clubbed with inelastic Raman scattering in air.
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Check your knowledge
It is the change in wavelength of light / photons when a light beam is scattered by molecules of a medium. That is when a beam of light travels through a transparent medium/ material, a small fraction of light gets deflected.
It helps in analyzing various samples (solids, liquids & gases), identify pharmacological chemicals, discovery of counterfeit drugs, identify pigments in old canvases, identify chemical bonding etc.
Indian Government celebrate Feb 28 as National Science Day to commemorate the discovery of Raman Effect by Sir C.V Raman.
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