What is Raman? How does it work? Raman spectroscopy takes advantage of the inelastic scattering of monochromatic laser light by molecules. Energy from the laser is exchanged with the molecules in such a way that the scattered light photons have higher or lower energy than the incident photons. The difference in energy is due to a change in the polarization energy of the molecule and gives information about the molecular structure. Since different molecules show different energy changes, the Raman technique can be used as a qualitative analysis method.
Diagram of Raman scattering: Incident light (yellow) that loses or gains no energy is scattered back at the same wavelength is called Rayleigh scattering. If some of the energy is transferred to the ground state, the scattered light is scattered at a longer wavelength (red). Fluorescence is another effect that causes light to be re-emitted at longer wavelengths. It often masks Raman scattering.
Diagram of Raman instrumentation: Incident laser light (yellow) is scattered at the light surface. Most of the light is scattered at the same wavelength at the incident light. Light that is Raman shifted also is scattered in a random directions. A lens is used to collect the light, and a filter is used to block the wavelength of the incident light. Longer wavelengths (Raman scattering) is transmitted to the monochromator and detection system. The frequency shift of the scattered light will determine the chemical structure of the sample material.