Raman spectroscopy is an experimental technique where incident light is scattered by molecular vibrations in molecules or solids (phonons).

Thus if the scattered light is recorded as a function of energy of the scattered light (with respect to the incident light) one may obtain a spectra of the molecular vibrations. Below a schematic view of the process is shown.

In the Raman process the scattered light have an energy equal to the incident excitation minus (Stokes) or plus (Anti Stokes) the energy of the molecular vibration (depicted in the upper right of Fig. above).