In Raman spectroscopy, a beam of light is shot into a sample or ‘analyte’. When the light contacts the analyte, a certain portion of it gets scattered. Most of the light particles undergo so-called 'Rayleigh Scattering', meaning that they do not have any change in energy level. However, about 10^-8 or 0.00000001% of the light will actually scatter with either more or less energy than it possessed initially. This is called 'Raman Scattering'. Raman spectroscopy identifies the analyte by recording that energy level difference (i.e. the ‘Raman Shift’). 

 

The light that scatters with more energy generates so-called ‘Stokes’ signals; the light that scatters with less energy generates so-called ‘Anti-Stokes’ signals. The Raman shift of the molecule is measured based on the vibration of certain bonds it contains. Each type of bond has its own characteristic raman shift. For example, C-H bonds have a different Raman shift than C=C bonds. The Raman shift of a molecule can also be based on entire groups of bonds vibrating such as benzene rings “breathing”.