Stokes lines

Stokes lines, radiation of particular wavelengths present in the line spectra associated with fluorescence and the Raman effect, named after Sir George Gabriel Stokes, a 19th-century British physicist. A molecule in its ground state is excited by a photon to a short-lived intermediate state. When the molecule emits a photon but not one with enough energy to take the molecule back down to the ground state, Stokes lines are formed of longer wavelength than that of the exciting radiation responsible for the fluorescence or Raman effect.

Anti-Stokes lines are found in fluorescence and in Raman spectra when the atoms or molecules of the material are already in an excited state (as when at high temperature). In this case the emitted photon takes the molecule back to the ground state, and thus the radiated line energy is the sum of the pre-excitation energy and the energy absorbed from the exciting radiation. Thus, anti-Stokes lines are always of shorter wavelength than that of the light that produces them. The difference between frequency or wavelength of the emitted and absorbed light is called the Stokes shift.