- © 2014 Mineralogical Society of America
BRIEF HISTORICAL PERSPECTIVE AND SIMPLE THEORY
When monochromatic radiation νo, is incident on a system (gas, solid, liquid, glass, whether colored or transparent) most of the radiation is transmitted through the system without change, but some scattering of this radiation can also occur (approximately 1 in 107 photons). The scattered radiation corresponds to ν′ = νo ± νm. In molecular systems, the energy of the scattered light (in wavenumbers, νm) is found to lie principally in the range associated with transitions between vibrational, rotational and electronic energy levels. Furthermore, the scattered radiation is generally polarized differently from that of the incident radiation with both scattered intensity and polarization dependent upon the direction of observation.
During the 1920’s different physics groups worked on this subject around the world: 1) an Indian group composed of Raman and Krishnan (1928), who made the first observations of the phenomenon in liquids in 1928 (Raman won the Nobel Prize in Physics in 1930 for this work); 2) Landsberg and Mandelstam (1928) in the USSR reported the observation of light scattering with change of frequency in quartz and finally 3) Cabannes and Rocard (1928) in France confirmed the Raman and Krishnan (1928) observations while Rocard (1928) published the first theoretical explanation.
The principle of Raman spectroscopy is the illumination of a material with monochromatic light (laser) in the visible spectral range followed by the interaction of the incident photons with the molecular vibrations or crystal phonons which induces a slight shift in the wavelength of the scattered photons. Scattering can occur with a change in vibrational, rotational or electronic energy of a molecule. If the scattering is elastic and the incident photons have the same energy as the scattered photons, the process is called Rayleigh scattering and this is the dominant scattering interaction. If …