Raman diffusion

The Raman diffusion , or Raman effect , is a optical Phénomène discovered by Chandrashekhara Venkata Râman. It indicates the effect by which a medium can diffuse light by modifying its frequency slightly. This shift in frequency corresponds to an energy exchange between the luminous ray and the medium.

This exchange can have several causes: vibrations of the crystal or the molecule, excitations magnetic… The measurement of this shift makes it possible to go back to certain properties of the medium. One speaks then about spectroscopy Raman. This technique is largely widespread in industry and research.

In the particular case or the diffusion is due to acoustic waves, one speaks about Diffusion Brillouin.

History

In 1922, the Indian physicist Chandrashekhara Venkata Râman published its work on " the molecular diffraction of the lumière" , first of a series of investigations with his/her collaborators which ultimement led to its discovery, on February 28th, 1928, of the optical effect which bears its name. The Raman effect was reported for the first time by C.V. Raman and K.S. Krishnan, and independently by Grigory Landsberg and Leonid Mandelstam in 1928. Raman the Nobel Prize in 1930 for its work received on the diffusion of the light.

Description

The Raman diffusion is the inelastic Scattering of a Photon by a medium. The fact that the diffusion is inelastic implies that there is an exchange of energy between the incidental photon and the molecule via the creation or the annihilation of a optical Phonon. Thus, the diffused light does not have the same wavelength as the incidental light. Two cases are distinguished:

  • Shift Stokes: the Lumière is shifted towards the Rouge (larger Wavelength, smaller energy) with the creation of a Phonon.
  • Shift anti-Stokes: the light is shifted towards the Bleu (shorter Wavelength, larger energy) with the absorption of a Phonon.
If there is no exchange of energy between the Molécule and the incidental Photon, then the diffusion is elastic and the Wavelength of the Photon diffused is not shifted. One speaks then about Diffusion Rayleigh. The shift in wavelength depends on the matter and is characteristic for him: it does not depend on the wavelength of excitation, which makes possible an analysis of the chemical composition of a sample starting from the way in which it diffuses the light (see #spectroscopy Raman).

The intensity of the line S Raman depends only on the number of molecules in the various vibrationnels modes which are associated for them. The use of the Distribution of Boltzmann makes it possible to correctly give an account of the report/ratio of intensity between the lines Stokes and anti-Stokes: the modes vibrationnels basic energy being populated more, the Stokes lines are more intense than the anti-Stokes lines.

The incidental photons can be diffused but can also modify the vibrations in the studied sample, while creating (process Stokes) or by destroying (process anti-Stokes) phonons. The lines Raman (Stokes and anti-Stokes) are characteristic of the chemical composition of the Matériau, of its crystalline structure as well as its electronic properties. The use relates to then chemistry, the enology, the Bijouterie… It is one of the rare methods which allows, to room temperature, to obtain a vibrationnelle or chemical characterization of an object. Moreover, it is nondestructive and requires a very small material portion. In certain particular cases, it is also possible to consider concentrations relative using a known reference.

Application to the Raman spectroscopy

The Raman spectroscopy, or Raman spectrometry , is a nondestructive method making it possible to characterize the molecular composition and the structure of a Matériau. This technique is complementary to infra-red Spectroscopie which also makes it possible to study the vibrationnels modes of a material.

The method consists in focusing (with a lens) a beam of Lumière Monochromatique (a beam Laser) on the sample to study and analyze the diffused light. This light is collected using an other lens and sent in a Monochromateur and its intensity is then measured with a Détecteur (standard monocanal Photomultiplicateur or CPM, multichannel type CCC).

Several geometries of diffusion are possible. One in general collects the diffused light either with 180°, or with 90°. One can also exploit the polarization of the incidental and diffused beams.

The Raman spectroscopy is a local measurement technique: by focusing the laser beam on a small portion of the medium, one can probe the properties of this medium on a volume of a few cubic microns. One speaks sometimes theRaman one.

Modes of vibration (Phonon)

An application of the spectrocopie Raman is the measurement of frequencies of vibrations of a crystal lattice or a molecule (phonons). The modes of vibrations which it is possible to measure by Raman spectroscopy are:

  • modes of vibrations whose vector of wave is quasi-no one (or the quasi-infinite wavelength). This is imposed by the conservation of the momentum in the process of diffusion. In the solids, one can thus have access only to the center of the zone of Brillouin.
  • the modes of vibration which cause a variation of the polarizability of the medium. These modes of vibration are known as active .
Moreover, among the active modes, some are detectable only in one geometry of diffusion given. An analysis of symmetries of the crystal or molecule makes it possible to predict which modes of vibrations will be detectable.

Magnetic excitations (Magnon)

The specroscopy Raman is also sensitive to the waves of spin (or let us magnons). Just as for the phonons, only the waves of spin of vector of wave quasi-no one are detectable.

See too

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