Raman scattering

Raman scattering or the Raman effect /ˈrɑːmən/ is the inelastic scattering of a photon by molecules which are excited to higher energy levels. The effect was discovered in 1928 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 by a material, most of them are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons but different direction. However, a small fraction of the scattered photons (approximately 1 in 10 million) are scattered inelastically, with the scattered photons having an energy different from, and usually lower than, those of the incident photons—these are Raman scattered photons. Because of conservation of energy, the material either gains or loses energy in the process. Typically this is vibrational energy and the incident photons are of visible light, although rotational energy (if gas samples are used) and electronic energy levels (if an X-ray source is used) may also be probed. The Raman effect forms the basis for Raman spectroscopy which is used by chemists and physicists to gain information about materials. The inelastic scattering of light was predicted by Adolf Smekal in 1923 (and in German-language literature it may be referred to as the Smekal-Raman-Effekt). In 1922, Indian physicist C. V. Raman published his work on the 'Molecular Diffraction of Light', the first of a series of investigations with his collaborators that ultimately led to his discovery (on 28 February 1928) of the radiation effect that bears his name. The Raman effect was first reported by Raman and K. S. Krishnan, and independently by Grigory Landsberg and Leonid Mandelstam, on 21 February 1928. In the former Soviet Union, Raman's contribution was always disputed; thus in Russian scientific literature this effect is usually referred to as 'combination scattering' or 'combinatory scattering'. Raman received the Nobel Prize in 1930 for his work on the scattering of light. Although modern Raman spectroscopy nearly always involves the use of lasers, which were not available until more than three decades later, Raman and Krishnan used a mercury lamp and photographic plates to record spectra. In 1998 the Raman effect was designated a National Historic Chemical Landmark by the American Chemical Society in recognition of its significance as a tool for analyzing the composition of liquids, gases, and solids. For any given chemical compound, there are a total of 3N degrees of freedom, where N is the number of atoms in the compound. This number arises from the ability of each atom in a molecule to move in three different directions (x, y, and z). When dealing with molecules, it is more common to consider the movement of the molecule as a whole. Consequently, the 3N degrees of freedom are partitioned into molecular translational, rotational, and vibrational motion. Three of the degrees of freedom correspond to translational motion of the molecule as a whole (along each of the three spatial dimensions). Similarly, three degrees of freedom correspond to rotations of the molecule about the x {displaystyle x} , y {displaystyle y} , and z {displaystyle z} -axes. Linear molecules only have two rotations because rotations along the bond axis do not change the positions of the atoms in the molecule. The remaining degrees of freedom correspond to molecular vibrational modes. These modes include stretching and bending motions of the chemical bonds of the molecule. For a linear molecule, the number of vibrational modes is: