Raman spectroscopy is the study of the interaction between light and matter in which that is inelastically scattered. In Raman spectroscopy experiments, photons of a single wavelength (in the visible range this would be light of a single color) are focused onto a sample. Generally, laser is used as a high-intensity monochromatic light source. The photons interact with the molecules and consequently are reflected, absorbed, or scattered. Raman spectroscopy studies the scattered photons.
Photons interacting with molecules most commonly scatter elastically. This type of scattering is called Rayleigh scattering, in which the scattered photons have the same wavelength as the incident light. However, approximately 1 out of a million photons are inelastically scattered. With Raman scattering, the incident photon interacts with matter and its wavelength either shifts to lower or higher wavelengths. Shifted photons to the higher wavelengths are the most common, and this kind of scattering is called Stokes scattering. What happens is that the photon interacts with the electron cloud of the functional groups bonds and excites an electron into a virtual state. The electron then relaxes into an excited vibrational or rotational state. This causes the photon to lose some of its energy and is detected as Stokes Raman scattering. This loss of energy is directly related to the functional group, the structure of the molecule to which it is attached, the types of atoms in that molecule and its environment. The Raman spectra of each molecule are unique. Therefore, it can be used as a fingerprint for identification of molecular compounds onto a surface, in a liquid or air.