Raman Scattering and Raman Effect

Raman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals.

When photons are scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same kinetic energy (frequency) and wavelength as the incident photons. However, a small fraction of the scattered photons (approximately 1 in 10 million) is scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, that of the incident photons. In a gas, Raman scattering can occur with a change in energy of a molecule due to a transition (see energy level). Chemists are concerned primarily with such transitional Raman effect.

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 effect). 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 C. V. Raman and K. S. Krishnan, and independently by Grigory Landsberg and Leonid Mandelstam, in 1928. Raman received the Nobel Prize in 1930 for his work on the scattering of light. In 1998 the Raman effect was designated an ACS National Historical Chemical Landmark in recognition of its significance as a tool for analyzing the composition of liquids, gases, and solids.

Physical measurement dimensions, such as temperature or pressure and tensile forces, can affect glass fibres and locally change the characteristics of light transmission in the fibre. As a result of the dampening of the light in the quartz glass fibres through scattering, the location of an external physical effect can be determined so that the optical fibre can be employed as a linear sensor.

Optical fibres are made from doped quartz glass. Quartz glass is a form of silicon dioxide (SiO2) with amorphous solid structure. Thermal effects induce lattice oscillations within the solid. When light falls onto these thermally excited molecular oscillations, an interaction occurs between the light particles (photons) and the electrons of the molecule. Light scattering, also known as Raman scattering, occurs in the optical fibre. Unlike incident light, this scattered light undergoes a spectral shift by an amount equivalent to the resonance frequency of the lattice oscillation.

The light scattered back from the fibre optic therefore contains three different spectral shares:

  • the Rayleigh scattering with the wavelength of the laser source used,
  • the Stokes components with the higher wavelength in which photons are generated, and
  • the Anti-Stokes components with a lower wavelength than the Rayleigh scattering, in which photons are destroyed.

The intensity of the so-called Anti-Stokes band is temperature-dependent, while the so-called Stokes band is practically independent of temperature. The local temperature of the optical fibre is derived from the ratio of the Anti-Stokes and Stokes light intensities.