When monochromatic light (for example from a laser) is focused on a sample, some of it is transmitted, some is absorbed and some is scattered. Most of the scattered light will have the same wavelength as the incident light. However a small fraction of the scattered light - approximately 1 in 107 photons - is shifted in wavelength. This is because these molecules have experienced vibrations and rotations during the interaction with the light.

The spectrum of this wavelength-shifted light is called the 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 vibrational mode of the chemical bonds in the molecule.

Since each type of molecule has its own Raman spectrum, this can be used to characterize the molecular structure and identify chemical compounds. The intensity of the Raman lines is related to the number of molecules in the sample and can be used for quantitative analysis.

Raman spectroscopy is a fast-growing technique, not only because of the analytical information it provides, but also because it is non-invasive, non-destructive and fast.

However, since such a small fraction of molecules in any sample gives a Raman signal, it is relatively insensitive. This limits its use to bulk solids and liquids and solutions down to approximately 1% w/v.

Surface Enhanced Raman Spectroscopy can increase the weak Raman signal by a million times or more, and can extend the range of applications suitable for Raman spectroscopy.


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