Raman effect

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Scattering
Feynman diagram of scattering between two electrons by emission of a virtual photon
Types Bragg diffraction Brillouin scattering Compton scattering CREIL Dynamic Light Scattering Kikuchi line Light scattering by particles Mie theory Neutron scattering Powder diffraction Raman scattering Rayleigh scattering Rutherford scattering Small-angle scattering Tyndall effect Thomson scattering Wolf effect X-ray crystallography
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Raman scattering or the Raman effect ( /ˈrɑːmən/) is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids,^[1] and by Grigory Landsberg and Leonid Mandelstam in crystals.^[2][3]

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.^[4] 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^[5] (and in German-language literature it may be referred to as the Smekal-Raman effect^[6]). 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^[7] 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.^[8]

Stokes and anti-Stokes scattering

The different possibilities of visual light scattering: Rayleigh scattering (no exchange of energy so the incident and emitted photons have the same energy), Stokes scattering (the atom or molecule absorbs energy and the emitted photon has less energy than the absorbed photon) and anti-Stokes scattering (the atom or molecule loses energy and the emitted photon has more energy than the absorbed photon)

The Raman effect corresponds, in perturbation theory, to the absorption and subsequent emission of a photon via an intermediate quantum state of a material. The intermediate state can be either a "real", i.e., stationary, state or a virtual state. The Raman interaction leads to two possible outcomes:

• the material absorbs energy and the emitted photon has a lower energy than the absorbed photon. This outcome is labeled Stokes Raman scattering.
• the material loses energy and the emitted photon has a higher energy than the absorbed photon. This outcome is labeled anti-Stokes Raman scattering.

The energy difference between the absorbed and emitted photon corresponds to the energy difference between two resonant states of the material and is independent of the absolute energy of the photon.

The spectrum of the emitted photons is termed the Raman spectrum, and it is typically displayed according to the energy difference with the absorbed photons. The Stokes and anti-Stokes spectra form a symmetric pattern above and below the absorbed photon energy. The frequency shifts are symmetric because they correspond to the energy difference between the same upper and lower resonant states. The intensities of the pairs of features will typically differ, though. The intensity depends on the population of the initial state of the material. At thermodynamic equilibrium, the upper state will have a lower or equivalent population and the corresponding anti-Stokes spectrum will be less intense.

Distinction from fluorescence

The Raman effect differs from the process of fluorescence. For the latter, the incident light is completely absorbed and the system is transferred to an excited state from which it can go to various lower states only after a certain resonance lifetime. The result of both processes is in essence the same: A photon with the frequency different from that of the incident photon is produced and the molecule is brought to a higher or lower energy level. But the major difference is that the Raman effect can take place for any frequency of the incident light. In contrast to the fluorescence effect, the Raman effect is therefore not a resonant effect. In practice, this means that a fluorescence peak is anchored at a specific frequency, whereas a Raman peak maintains a constant separation from the excitation frequency.

Selection rules

The distortion of a molecule in an electric field, and therefore the vibrational Raman cross section, is determined by its polarizability.

A Raman transition from one state to another, and therefore a Raman shift, can be activated optically only in the presence of non-zero polarizability derivative with respect to the normal coordinate (that is, the vibration or rotation):

Raman-active vibrations/rotations can be identified by using almost any textbook that treats quantum mechanics or group theory for chemistry. Then, Raman-active modes can be found for molecules or crystals that show symmetry by using the appropriate character table for that symmetry group.

Stimulated scattering and amplification

Raman amplification can be obtained by using stimulated Raman scattering (SRS), which actually is a combination of a Raman process with stimulated emission. It is interesting for application in telecommunication fibers to amplify inside the standard material with low noise for the amplification process. However, the process requires significant power and thus imposes more stringent limits on the material. The amplification band can be up to 100 nm broad, depending on the availability of allowed photon states.

SRS is one of the processes that impede the laser coupling of inertial confinement fusion capsules.

Spectrum generation

For high-intensity CW (continuous wave) lasers, SRS can be used to produce broad bandwidth spectra. This process can also be seen as a special case of four-wave mixing, wherein the frequencies of the two incident photons are equal and the emitted spectra are found in two bands separated from the incident light by the phonon energies. The initial Raman spectrum is built up with spontaneous emission and is amplified later on. At high pumping levels in long fibers, higher-order Raman spectra can be generated by using the Raman spectrum as a new starting point, thereby building a chain of new spectra with decreasing amplitude. The disadvantage of intrinsic noise due to the initial spontaneous process can be overcome by seeding a spectrum at the beginning, or even using a feedback loop as in a resonator to stabilize the process. Since this technology easily fits into the fast evolving fiber laser field and there is demand for transversal coherent high-intensity light sources (i.e., broadband telecommunication, imaging applications), Raman amplification and spectrum generation might be widely used in the near-future.

Applications

Raman spectroscopy employs the Raman effect for materials analysis. The spectrum of the Raman-scattered light depends on the molecular constituents present and their state, allowing the spectrum to be used for material identification and analysis. Raman spectroscopy is used to analyze a wide range of materials, including gases, liquids, and solids. Highly complex materials such as biological organisms and human tissue^[9] can also be analyzed by Raman spectroscopy.

For solid materials, Raman scattering is used as a tool to detect high-frequency phonon and magnon excitations.

Raman lidar is used in atmospheric physics to measure the atmospheric extinction coefficient and the water vapour vertical distribution.

Stimulated Raman transitions are also widely used for manipulating a trapped ion's energy levels, and thus basis qubit states.

Raman amplification is used in optical amplifiers.

See also

• Scattering
• Brillouin scattering
• Nonlinear optics
• Fiber amplifier
• List of surface analysis methods
• Raman laser
• Raman spectroscopy
• Surface Enhanced Raman Spectroscopy (SERS)
• Inverse Raman effect
• Resonance Raman spectroscopy (RR)
• Coherent anti-Stokes Raman spectroscopy (CARS)
• Depolarization ratio

Notes

References

• "A new radiation", Indian J. Phys., 2 (1928) 387 - http://www.uky.edu/~holler/raman.html
• Herzberg, Spectra of Diatomic Molecules, Litton Educational Publishing, 1950, ISBN 0-442-03385-0, pp. 61ff and 66ff

External links

• Explanation from Hyperphysics in Astronomy section of gsu.edu
• Raman Spectroscopy - Tutorial at Kosi.com
• December 1930;Prof. R. W. Wood Demonstrating the New "Raman Effect" in Physics
• A short description of spontaneous Raman scattering
• Raman Effect: fingerprinting the universe