Raman scattering
Raman scattering or the Raman effect is the inelastic scattering of a photon by molecules which are excited to higher vibrational or rotational energy levels.
It was discovered by C. V. Raman and his student K. S. Krishnan in liquids,
and independently by Grigory Landsberg and Leonid Mandelstam in crystals.
The effect had been predicted theoretically by Adolf Smekal in 1923.When photons are scattered from an atom or molecule, most of them are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons. A small fraction of the scattered photons (approximately 1 in 10 million) are scattered inelastically by an excitation, with the scattered photons having a frequency and energy different from, and usually lower than, those of the incident photons.