Introduction
Among a large variety of spectroscopic techniques, available for the analysis of materials and chemicals, Raman spectroscopy is one of the most known non-destructive methods. This relies, on inelastic scattering of light by a material called Raman effect, in contrast to the most common elastic Rayleigh scattering. The first published observation was: A new type of Secondary Radiation by Raman and Krishnan (1928, published in Nature magazine) As mentioned, in inelastic scattering, the light which is passing through a transparent substance does not underlies to gain or loss of its energy, therefore it keeps the same wavelength. As opposed, in Raman scattering the photons cause the raising of electrons to a higher energy state. Almost immediately the molecule drops back down to its ground state, releasing a photon resulting the scattering. However since the molecule is dropping back to the initial state, the light photons lose or gain energy during this process, and therefore the wavelength is increased or decreased in respectively. Dependending on the energy of the photons, Stokes and anti-Stokes scattering are outstanding.
Nevertheless, in Raman spectroscopy, is ordinarily used only the Stokes half of the spectrum, due to its greater intensity. In solid materials the molecules cannot be taken individually, but only as a lattice of them. In such crystalline materials vibrations are quantised as phonons, determined by the crystal structure. In this case, only phonons with a change in polarisability are Raman active in oder to take structural information can therefore be determined from the wavenumber shifts. Crystal orientation can also be determined from the polarization of the scattered light. In Raman techniques, lasers are normally employed due to their high intensity, single wavelength and coherent beam. There are many Raman techniques, the most common yet is Fourier transform (FT) Raman due to the advantage of being faster. Some other kinds of Raman spectroscopy are: Stimulated Raman, Resonance Raman, Surface Enhanced Raman Spectroscopy (SERS), Coherent Anti-Stokes Raman Spectroscopy (CARS).
Geological applications
This method is widely used in Geology and specially in Gemolgy, Mineralogy and Petrology because it is a rapid, reliable and most of all non-destructive way to identify minerals, distinguish minerals phases with the same composition, to study fluid inclusions and generally to confirm chemical informations . All these kind of research use the micro-Raman Spectroscopy, the compination of microscope and Raman technique which gives the opportunity to small-scale study. Raman Spectroscopy is not limited for crystalline materials, but it can be used for liquid and gases as well. It should be noticed that no sample preparation or vacuum is needed, which means no added cost. Other geological section that use Raman Spectroscopy are Geoarcheology, Paleodology, and of course Planetology.
References:
Raman C. V. & Krishnan K. S., A new type of Secondary Radiation Nature, 121, 501, March 31, 1928
Colthup, Norman B., Daly, L. H., and Wiberley, S. E., Introduction to infrared and Raman spectroscopy, Boston: Academic Press, 1990.
Nakamoto, Kazuo, Infrared and Raman Spectra of Inorganic and Coordination Compounds, New York: John Wiley & Sons (1978)
Franc C. Hawthorne (Editor) Spectroscopy Methods in Mineralogy and Geology. Reviews in Mineralogy, 18, pp.698 (1988)
Raman-IR Mineral Data Bases: http://www.thermoscientific.com
http://minerals.gps.caltech.edu/files/raman/ (CalTech)
http://rruff.info/ (University of Arizona)
