Raman spectrometry

The Raman effect was discovered simultaneously in 1928 by Raman and Krishnan in their study of light scattering by liquids, and by Landsberg and Mandelstam in their work on solids. Raman was awarded the Nobel Prize in 1930. This effect consists in the existence of a frequency-shifted spectrum in the light scattered by a sample subjected to monochromatic illumination. This very low-intensity spectrum is difficult to observe alongside the light scattered without any change in frequency. It is characteristic of the sample under study, and is linked to the vibrations of the atomic edifices making up the observed sample. Alongside infrared spectroscopy and inelastic neutron scattering, Raman spectroscopy is one of the branches of vibrational spectroscopy. As such, it can be used to characterize samples and apply them to qualitative or quantitative analysis.

As long as the limitations of spectrographic techniques meant that only samples carefully purified by tedious filtering and distillation to avoid the parasitic phenomena associated with fluorescence could be used, the Raman effect remained confined to fundamental studies. Yet it was during this period, in the first three decades after the discovery, that thousands of Raman spectra of all kinds of substances in liquid, solid or even gaseous state were catalogued, providing essential data for establishing molecular structures and conformations. In the 1930s, for example, a comparison of the infrared and Raman spectra of benzene showed that there were no common lines. This character, known as mutual exclusion, provided proof of the existence of a center of symmetry, incompatible with the ternary or binary symmetries suggested by most proposed formulas. This was a decisive argument in favor of the hypothesis of a plane structure with 6th-order symmetry, universally accepted today. Similarly, the absence of the characteristic carbonyl line –C=O in the Raman spectrum of C ₆ sugars had to be taken into account in the controversies that pitted proponents of a linear chain structure against those of a cyclic formula at the time. However, although the complementarity of infrared and Raman spectra and the need to study them jointly were clearly recognized, the development during the 1940s and 1950s of industrial infrared spectrometers, easy to use and well suited to analytical needs, sounded the decline of Raman techniques.

Things had come to this point around 1960, when the appearance and spectacular development of laser light sources finally provided the tool ideally suited to the monochromatic excitation that the few Raman supporters had been waiting for. The laser was the seed of a veritable revolution in Raman spectrometry techniques, which has since been continued and amplified...