Raman Effect refers to the change in the wavelength of light when a light beam gets deflected by molecules. This phenomenon is named after Indian physicist Sir Chandrasekhara Venkata Raman, who first published his observations of the effect in 1928.
Whenever a beam of light traverses a transparent, dust-free sample of a compound, a small portion of the light emerges in directions other than that of the incident beam. This scattered light has a largely unaltered wavelength, but a small part has wavelengths different from the incident light. This phenomenon takes place because of the Raman Effect.
We can understand Raman scattering more efficiently by considering the incident light to be comprised of photons that strike the molecules of the sample. The collisions between the two are primarily elastic, and the photons are subsequently scattered with unchanged frequency and energy.
However, sometimes the molecule occasionally takes up energy from or gives up energy to the photons. As a result, the latter are scattered with reduced or increased energy and correspondingly lower or higher frequency. These changes in the frequency serve as measures of the amounts of energy involved in the transition between the initial and final states of the scattering molecule.
The Raman effect is a fairly weak phenomenon. For a liquid substance, the intensity of the affected light is possibly only 1/100,000 that of the incident beam of light. Nevertheless, every molecule produces a characteristic pattern of Raman lines, with its intensity proportional to the number of scattering molecules in the path of the light beam.
By virtue of the above property, Raman spectra find useful applications in both qualitative and quantitative analyses. Scientists have discovered that the energies corresponding to the Raman frequency shifts are actually the energies associated with transitions between various vibrational and rotational states of the scattering molecule.
Unless we are dealing with simple gaseous molecules, it is pretty tough to observe pure rotational shifts. Similarly, you won’t find discrete rotational Raman lines in liquids because of hindered rotational motions of the molecules. The Raman Effect primarily deals with vibrational transitions, which provide larger observable shifts for solids, liquids, and gases alike.
At ordinary temperatures, gases possess relatively low molecular concentration and thus produce pretty feeble Raman effects. Therefore, scientists prefer to study solids and liquids in this regard. Both physicists and chemists use the Raman Effect to obtain information about materials for different purposes by performing various forms of Raman spectroscopy.