Raman Scattering - Stokes and Anti-Stokes Scattering

Stokes and Anti-Stokes Scattering

The Raman effect corresponds, in perturbation theory, to the absorption and subsequent emission of a photon via an intermediate quantum state of a material. The intermediate state can be either a "real", i.e., stationary state or a virtual state. The Raman interaction leads to two possible outcomes:

• the material absorbs energy and the emitted photon has a lower energy than the absorbed photon. This outcome is labeled Stokes Raman scattering.
• the material loses energy and the emitted photon has a higher energy than the absorbed photon. This outcome is labeled anti-Stokes Raman scattering.

The energy difference between the absorbed and emitted photon corresponds to the energy difference between two resonant states of the material and is independent of the absolute energy of the photon.

The spectrum of the emitted photons is termed the Raman spectrum, and it is typically displayed according to the energy difference with the absorbed photons. The Stokes and anti-Stokes spectra form a symmetric pattern above and below the absorbed photon energy. The frequency shifts are symmetric because they correspond to the energy difference between the same upper and lower resonant states. The intensities of the pairs of features will typically differ, though. The intensity depends on the population of the initial state of the material. At thermodynamic equilibrium, the upper state will have a lower or equivalent population and the corresponding anti-Stokes spectrum will be less intense.