AntiStokes

Anti-Stokes is a term used in spectroscopy, particularly Raman spectroscopy, to describe a phenomenon where scattered light has a longer wavelength and lower energy than the incident light. This occurs when the incident photon interacts with a molecule and excites it to a higher vibrational energy state. The molecule then relaxes to a lower vibrational state, but not necessarily the ground state, emitting a photon with less energy. This energy difference corresponds to the energy of the vibrational transition.

In contrast to Stokes scattering, where the emitted photon has less energy than the incident photon, anti-Stokes scattering involves the molecule already being in an excited vibrational state before the interaction. When the incident photon interacts with this excited molecule, it gains energy from the molecule's vibrational state, resulting in the scattered photon having a higher energy and thus a shorter wavelength than the incident photon. However, the term "anti-Stokes" is typically used to describe the lower energy scattered photon, where the molecule transfers energy to the photon. The anti-Stokes signal is generally weaker than the Stokes signal because it relies on molecules already occupying higher vibrational energy levels, which is less probable at room temperature according to the Boltzmann distribution. The intensity of the anti-Stokes signal is temperature-dependent and increases with increasing temperature.