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 Post subject: Raman Spectroscopy:technique and its gemological application
PostPosted: Thu Jan 21, 2021 1:56 pm 
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Raman Spectroscopy: technique and its gemological application
Alberto Scarani, Mikko Åström,
One of the essential requirements of gemological analysis is that the gemstones should not be damaged or permanently altered. In other words the analysis must be non-destructive. This perfectly understandable limitation prevents gemologists and gemological laboratories to use many techniques widely available for analyzing other kind of samples by sacrificing small parts of them. Raman spectroscopy is considered to be the best-known identification technique in material analysis as it is extremely reliable and, at the same time, fast and absolutely non-invasive for gemstones.

The Raman effect

When a substance is irradiated with light, a small portion of this radiation is scattered. In scattering a photon, which can be considered as a single moving piece of light, randomly changes its direction of propagation. This effect is called as elastic scattering and is also known as Rayleigh scattering effect.

A very small fraction (approximately 1 in 10 million photons) of the light scatters in a different way. In this so called inelastic scattering or Raman scattering something else than just direction of propagation changes too. In this case, there is an exchange of energy between the photon of the incident radiation and the molecule, which will reach an excited state. Part of this energy is used by the matter to start a vibrational motion. Subsequently, the matter moves back to the ground state by releasing a photon whose energy is missing the quota needed to ignite the vibrational motion. The result is that the light has changed its color.

Raman scattering is closely related to the vibration modes of a substance; since these vibrations depend on both mass of the atoms, and the bonding forces & symmetries between them, it follows that the Raman spectrum of a substance represents a fingerprint which allows its unambiguous identification. For the discovery of this scattering, which took place in 1928, Sir C.V. Raman was awarded the Nobel Prize in Physics in 1930 (Figure 1).

An extraordinary aspect of this scattering, commonly referred to as Raman shift, is that it is not only unique and distinct for any given material but remains constant regardless the wavelength of used excitation source. This element is far from secondary if we think that in gemology, as in many other fields, we have to deal with a multitude of different materials which often react differently depending on the energy of the emission used to identify them. Some samples will in fact be easier to detect by using a source in the visible wavelength range, while for others that may exhibit luminescence reactions at these wavelengths, it may be necessary to use infrared units for excitation.

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