STOKES LINES

radiation of particular wavelengths present in the line spectra associated with fluorescence and the Raman effect (q.v.), named after Sir George Gabriel Stokes, a 19th-century British physicist. Stokes lines are 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 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.