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Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and luminescence spectra (or fluorescence) of the same electronic transition. It is named after Irish physicist George G. Stokes.
When a molecule or atom absorbs light, it enters an excited electronic state. The Stokes shift occurs because the molecule loses a small amount of the absorbed energy before re-releasing the rest of the energy as luminescence or fluorescence (the called Stokes fluorescence), depending on the time between the absorption and the reemission. This energy is often lost as thermal energy.
Stokes fluorescence
Stokes fluorescence is the reemission of longer wavelength (lower frequency) photons (energy) by a molecule that has absorbed photons of shorter wavelengths (higher frequency). Both absorption and radiation (emission) of energy are unique characteristics of a particular molecule (structure) during the fluorescence process. Light is absorbed by molecules in about 10-15 seconds which causes electrons to become excited to a higher electronic state. The electrons remain in the excited state for about 10-8 seconds then, assuming all of the excess energy is not lost by collisions with other molecules, the electron returns to the ground state. Energy is emitted during the electrons' return to their ground state. Emitted light is always a longer wavelength than the absorbed light due to limited energy loss by the molecule prior to emission.
Further reading
- Lakowicz, J.R. 1983. Principles of Fluorescence Spectroscopy, Plenum Press, New York. ISBN 0387312781.
- Guilbault, G.G. 1990. Practical Fluorescence, Second Edition, Marcel Dekker, Inc., New York. ISBN 0824783506.
See also
"Stokes shift"