The Raman effect was first experimentally discovered and described by the physicist Chandrasekhara Venkata Raman (* 1888, † 1970) in 1928. The Raman effect is based on inelastic light scattering at the chemical bonds of a sample. Due to vibrations in the chemical bonds this interaction causes a specific energy shift in parts of the back scattered light which results in a unique Raman spectrum.
Raman scattering is a very weak effect, typically less than one in a million excitation photons give rise to a single Raman photon. Raman scattering can be further differentiated as Stokes and Anti-Stokes scattering. Both contain information about the material and its molecular composition.
- Stokes scattering: The photon transfers energy to the molecule. The emitted photon has a lower energy than the absorbed photon.
- Anti-Stokes scattering: The molecule transfers energy to the photon. The emitted photon has a higher energy than the absorbed photon.
In addition to inelastic scattering, elastic scattering can also appear. Elastic scattering at the same energy as the incident radiation is called Rayleigh scattering. It does not contain any information about the molecules and cannot be used for chemical sample analysis in a confocal Raman microscope.