Raman Scattering

When considering Raman scattering, we can think about the physics in one of two ways: the classical wave interpretation or the quantum particle interpretation. In the classical wave interpretation, light is considered as electromagnetic radiation, which contains an oscillating electric field that interacts with a molecule through its polarizability. Polarizability is determined by the electron cloud’s ability to interact with an electric field. For example, soft molecules such as benzene tend to be strong Raman scatterers while harder molecules like water tend to be fairly weak Raman scatterers.

When light encounters molecules in the air, the predominant mode of scattering is elastic scattering, called Rayleigh scattering. This scattering is responsible for the blue color of the sky; it increases with the fourth power of the frequency and is more effective at short wavelengths. It is also possible for the incident photons to interact with the molecules in such a way that energy is either gained or lost so that the scattered photons are shifted in frequency. Such inelastic scattering is called Raman scattering.

When photons are scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons. A small fraction of the scattered photons (approximately 1 in 10 million) are scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, that of the incident photons.[4] In a gas, Raman scattering can occur with a change in energy of a molecule due to a transition to another (usually higher) energy level. Chemists are primarily concerned with the transitional Raman effect.