Raman scattering

The effect is exploited by chemists and physicists to gain information about materials for a variety of purposes by performing various forms of Raman spectroscopy.

The inelastic scattering of light was predicted by Adolf Smekal in 1923[3] and in older German-language literature it has been referred to as the Smekal-Raman-Effekt.

[4] 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 16 February 1928) of the radiation effect that bears his name.

Because lasers were not available until more than three decades after the discovery of the effect, Raman and Krishnan used a mercury lamp and photographic plates to record spectra.

[11] The following focuses on the theory of normal (non-resonant, spontaneous, vibrational) Raman scattering of light by discrete molecules.

X-ray Raman spectroscopy is conceptually similar but involves excitation of electronic, rather than vibrational, energy levels.

[13] The basics of infrared absorption regarding molecular vibrations apply to Raman scattering although the selection rules are different.

[14] Molecular vibrational energy is known to be quantized and can be modeled using the quantum harmonic oscillator (QHO) approximation or a Dunham expansion when anharmonicity is important.

A molecule can be excited to a higher vibrational mode through the direct absorption of a photon of the appropriate energy, which falls in the terahertz or infrared range.

Absorption of a photon excites the molecule to the imaginary state and re-emission leads to Raman or Rayleigh scattering.

A classical physics based model is able to account for Raman scattering and predicts an increase in the intensity which scales with the fourth-power of the light frequency.

It shows the intensity of the scattered light as a function of its frequency difference Δν to the incident photons, more commonly called a Raman shift.

The locations of corresponding Stokes and anti-Stokes peaks form a symmetric pattern around the RayleighΔν=0 line.

The frequency shifts are symmetric because they correspond to the energy difference between the same upper and lower resonant states.

Their ratio depends on the temperature, and can therefore be exploited to measure it: In contrast to IR spectroscopy, where there is a requirement for a change in dipole moment for vibrational excitation to take place, Raman scattering requires a change in polarizability.

In general, a normal mode is Raman active if it transforms with the same symmetry of the quadratic forms

The rule of mutual exclusion, which states that vibrational modes cannot be both IR and Raman active, applies to certain molecules.

This generally is only relevant to molecules in the gas phase where the Raman linewidths are small enough for rotational transitions to be resolved.

[17] The symmetry of a vibrational mode is deduced from the depolarization ratio ρ, which is the ratio of the Raman scattering with polarization orthogonal to the incident laser and the Raman scattering with the same polarization as the incident laser:

is the intensity of Raman scattering when the analyzer is rotated 90 degrees with respect to the incident light's polarization axis, and

the intensity of Raman scattering when the analyzer is aligned with the polarization of the incident laser.

[19][18] The Raman-scattering process as described above takes place spontaneously; i.e., in random time intervals, one of the many incoming photons is scattered by the material.

For high-intensity continuous wave (CW) lasers, stimulated Raman scattering can be used to produce a broad bandwidth supercontinuum.

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.

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[26] 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.

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

Raman spectroscopy can be used to determine the force constant and bond length for molecules that do not have an infrared absorption spectrum.

Raman spectroscopy has been used to chemically image small molecules, such as nucleic acids, in biological systems by a vibrational tag.