3. Principle of Raman scattering

The interaction between matter and monochromatic excitation light radiation (laser) leads to scattering whose energy is expressed in frequency (unit: THz), wavelength (nm) or wavenumber (cm ^–1 ), which can be elastic, then called "Rayleigh scattering" (only the direction of the light is modified), or inelastic, with energy exchange and frequency modification, called "Raman" and "Brillouin" scattering