Stokes shift

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Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and emission spectra (fluorescence and Raman being two examples) of the same electronic transition. It is named after Irish physicist George G. Stokes.

When a system (be it a molecule or atom) absorbs a photon, it gains energy and enters an excited state. One way for the system to relax is to emit a photon, thus losing its energy (another method would be the loss of heat energy). When the emitted photon has less energy than the absorbed photon, this energy difference is the Stokes shift. If the emitted photon has more energy, the energy difference is called an anti-Stokes shift.

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 (this number varies widely depending on the atom or molecule in question and is known as the fluorescence lifetime of the sample) then, assuming all of the excess energy is not lost to the conduction band, the electron returns to the ground state. Energy is emitted during the electrons' return to their ground state. Emitted light always has a longer wavelength than the absorbed light due to limited energy loss by the molecule prior to emission. This effect would be on a much smaller energy scale than any hyperfine structure of atomic energy, or else atomic energy would not be discrete.

Further reading

• Lakowicz, J.R. 1983. Principles of Fluorescence Spectroscopy, Plenum Press, New York. ISBN 0-387-31278-1.

• Guilbault, G.G. 1990. Practical Fluorescence, Second Edition, Marcel Dekker, Inc., New York. ISBN 0-8247-8350-6.

See also

• Stokes line
• Jablonski diagram
• anti-Stokes fluorescence