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Discovery of the Raman Effect

Raman Spectrum ReadingIn 1928, Sir C.V. Raman and his colleague K.S. Krishnan made their discovery of what came to be called Raman scattering while searching for an optical analog to the Compton Effect.  It was nearly simultaneously discovered by G.S. Landsberg and L.I. Mandelstam, two Russian physicists, during the course of their studies on specific heats of solids. Raman and Krishnan observed the inelastic scattering of light first by focusing filtered sunlight, and later by focusing a quartz mercury arc lamp through a photographic filter to remove all lines of greater wavelength than 4356 Å, and directing it into samples of common solvents. The resultant spectra were collected on photographic plates, and showed modified lines that indicated a change of wavelength in the outgoing light. The first Raman spectra acquired of benzene are presented in Figure1. Because the efficiency of Raman scattering is so low, extremely long acquisition times were needed in this early work (2-100 hours for liquids and more than 180 hours for vapors have been reported). In total, Raman and Krishnan observed scattered secondary radiation in 60 liquids and vapors. 

Due to the inefficient nature of the scattering, several technological advances were necessary before Raman spectroscopy became a common laboratory technique. As discussed in the previous section, only 1 in 106 to 108 incident photons are returned shifted by the interaction with the molecules. The development of the laser, then, permitted much more effective measurements because of the dramatic increase in the amount of incident light. Improvements in monochromator systems, both to resolve the Raman spectrum and limit the amount of Rayleigh scattering present in spectral data, also encouraged use of Raman spectroscopy to study molecular structure and chemical systems. Miniaturized Raman systems in current use, including those in the TSI | ChemLogix product line, became possible with the advent of frequency-stabilized solid state lasers and small monolithic spectrometers.

Further, narrow notch filters are also available for the removal of the excitation light in the spectra and sophisticated software solutions exist for background elimination and in support of end-user purposes.

  1. Singh,Rajinder, “C.V. Raman and the Discovery of the Raman Effect,” Phys. Perspect.4, 399-420, 2002.

 

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