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What is Raman spectroscopy?

When monochromatic light is focused on a sample, some part of it is transmitted, some is absorbed and some is scattered.  Most of the scattered light will have the same wavelength of the incident light.  However a small fraction of the scattered light - approximately 1 in 10^7 photons - are shifted in wavelength.  This is because the molecules of sample molecules have, in the meantime, experienced vibrations and rotations during the interaction with the light.

The spectrum of this wavelength-shifted light is called Raman spectrum.  Raman spectra consist of sharp bands that are characteristic of the specific molecule in the sample.  Each line of the spectrum corresponds to a specific vibration of the atoms of the molecule.

This fundamental characteristic of Raman spectroscopy is  the specificity of the molecular recognition and the quantitative analysis possible by weighing the intensity of a Raman lines.

Useful links

A tutorial-style introduction to the basics of the Raman effect.
Wikipedia Definition

Reference:

Modern Raman Spectroscopy: A Practical Approach edited by Ewen Smith, and Geoffrey Dent, John Wiley and Sons, Ltd
Analytical Applications of Raman Spectroscopy edited by Michael J. Pelletier, Blackwell Science