Raman technique is a molecular, non-destructive, vibrational technique.
In 1928, Mr. Raman discovers that by bombarding molecules (organic at this time) with a radiation in the visible these reemitted photons of the same frequency as the incident radiation but also very slightly different photons of frequencies (absolutely not present in the incident radiation). In 1930 the Raman being mostly used. After the Second World War, the development of the infra-red detector relegated the Raman to the background until the strong development of lasers.
Considering monochromatic primary radiation, laser between 200 and 1000 nm for the most common, the re-emitted radiation contains the wavelength of the laser but also and on both sides of this wavelength of the reemitted characteristic lines by the material subjected to radiation.
Moreover a fine analysis of the distribution of these lines below the wavelength of the laser and superior to the latter shows that the two distributions are identical and that their point of symmetry on the X axis (x = wavelength , Y = intensities) is the wavelength of the laser. At lower wavelengths than the laser, these lines are called Antistoke lines and above the laser wavelength, they are called Stoke lines. Since the spectrum is symmetrical, the domain of wavelengths longer than that of the laser is conserved in Raman spectroscopy.
The analysis of the spectrum on either side of the re-emitted intensity of the laser also shows that these lines (the Raman effect in general) and 1,000,000 times smaller than the intensity of the wavelength of the laser. laser re-emitted by the sample. The Raman yield is very low. It requires focused lasers and powers (a few hundred milliwatts) that can damage (burn) samples especially if they are dark.
An analysis of the possible vibrations of the excited molecule (elongation, torsion, rotation …) shows that at each difference in wavelength between the original wavelength of the laser and a line corresponds a mode of vibration. Indeed a difference in wavelength corresponds to an energy. A mode of vibration corresponds to an energy necessary to implement to enter resonance. These ‘delta’ energies correspond to those observed in infra-red. However some vibrations are active in IR and not in Raman and vice versa.