Raman scattering otherwise Raman effect (rus. комбинационное рассеяние света otherwise эффект комбинационного рассеяния; рамановское рассеяние; эффект Рамана abbr., КР; КРС) — inelastic scattering of light (with a change in frequency/wavelength) accompanied by substance transitions between vibrational energy levels.

Description

Light scattering accompanied by energy exchange between photons and a substance is called inelastic scattering or Raman scattering. A change in photon energy results in a change in the wavelength (frequency) of scattered light. Light can also be elastically scattered by a substance without changing the photon energy and hence the light wavelength. An example of elastic scattering is Rayleigh scattering.

The mechanism of Raman scattering (RS) is illustrated in Fig. 1. In Stokes RS a photon gives some energy to a molecule in the process of interaction. As a result of this process the molecule moves from a lower energy level to a higher energy and the energy of the scattered photon decreases (the wavelength increases (Fig. 1, left)). In anti-Stokes RS, as a result of interaction with an excited-state molecule the photon energy increases and the molecule goes into a lower energy state (Fig. 1, right). For comparison, Fig. 1 shows a diagram (in the centre) corresponding to Rayleigh scattering where there is no energy exchange between the photon and the molecule. Fig. 1 b also shows a virtual energy level of the molecule in the light wave field (top dotted line).

Since in thermodynamic equilibrium the population of a level decreases with increasing energy, in spontaneous RS the frequency of anti-Stokes transitions is lower than the frequency of Stokes transitions; therefore, the intensity of the Stokes Raman lines in the spectrum is higher. In the spectrum the Stokes Raman lines are on the "red" side (the side of larger wavelengths / lower frequencies) of the Rayleigh line. Not all transitions between different vibrational energy levels are possible.

The intensity of Raman scattering (RS) is 3-6 orders of magnitude lower than for Rayleigh scattering, therefore an intense source of monochromatic radiation and a highly sensitive detector are required to observe the RS spectra. Today lasers are most often used as radiation sources. Raman spectroscopy can be used for studying structure and composition of substances and their interaction with the environment. Raman bands can be described by frequency, intensity and degree of depolarisation. Under the influence of optically anisotropic molecules on polarised light the scattered light becomes partially depolarised.

When the frequency of the exciting light approaches and coincides with the frequency of optical transition of the system, the situation of resonance Raman scattering (RRS) is observed. The spectral features of Raman scattering provide information about the structure and interaction of the electron and phonon subsystems in semiconductors.

Raman scattering in condensed matter has a number of specific features, since in solids the vibrations of molecules (atoms, ions) are strongly correlated, and in the case of crystals they should be viewed as vibrations of the crystal lattice as a whole.

Raman spectra of amorphous solids are more "blurred" than the spectra of crystalline objects, due to their disordered structure and reduced areas of spatial correlations between the oscillations of the particles (Fig. 2). There is also a noticeable broadening of lines in the Raman spectra of solid solutions and highly defective crystals.

Such broadening of the spectral lines can be caused by orientational disorder in solids related to variations in the orientation of molecules in molecular crystals, of dipoles in strongly polar crystals, and of free electron pairs in Pb2+ ions. The shift of Raman lines in the spectra of silicate glasses is a measure of polymerisation of silicate lattice.

Raman spectroscopy is a very informative method for studying nanomaterials, particularly carbon nanotubes; it allows their geometrical parameters, type of conductivity, etc. to be determined.

When a substance is heated, the intensity of anti-Stokes Raman lines increases significantly (in contrast to Stokes lines), which allows this effect to be used to measure temperature (suitable fibre-optic sensors have already been developed).

When the Raman scattering is excited by high-power sources the probability of Stokes scattering increases, and a stimulated Raman scattering (SRS) occurs. SRS amplifiers are widely used in fibre-optic communications. SRS lasers provide powerful coherent radiation in spectral regions, where there are no effective lasers of other types.

Illustrations

Fig. 1. Raman scattering spectrum bands and corresponding energy jumps.
Fig. 1. Raman scattering spectrum bands and corresponding energy jumps.
Fig. 2. Raman spectra of crystals and glass with equivalent composition [2].
Fig. 2. Raman spectra of crystals and glass with equivalent composition [2].

Authors

  • Veresov Alexander G.
  • Nanii Oleg E.

Sources

  1. Otto, M., Analytische Chemie; 2. vollständig überarbeitete Auflage/671 Seiten/ DM 88, Wiley-VCH, Weinheim, 2000.
  2. Brundle C. R. et al. Encyclopedia of materials characterizaton. — Butterworth–Heinemann, 1992. — 782 p.
  3. Leng Y. Materials characterization. Introduction to microscopic and spectroscopic methods. — John Wiley & Sons, 2008. — 351 p.
  4. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. New York: Wiley, 1997.
  5. Gerhard Abstreiter, Manuel Cardona and Aron Pinczuk, Light scattering by free carrier excitations in semiconductors // Light Scattering in Solids IV, Topics in Applied Physics, 1984, Volume 54/1984, 5-150, DOI: 10.1007/3-540-11942-6_20 // URL: http://www.springerlink.com/content/y5454l244258m672/ (reference date: 12.12.2011).
  6. Pentin Yu A, Vilkov L V Physical Methods in Chemistry. Textbook (in Russian) // Moscow: Mir, 2003. - 683 pp.

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