Stokes Shift

Stokes shift

ticular molecular structure. If a material has a direct bandgap in the range of visible light, the light shining on it is absorbed, causing electrons to become excited to a higher energy state. The electrons remain in the excited state for about 10−8 seconds. This number varies over several orders of magnitude depending on the sam- ple, and is known as the ﬂuorescence lifetime of the sam- ple. After losing a small amount of energy in some way (hence the longer wavelength), the molecule returns to the ground state and energy is emitted.

2 References

Absorption and emission spectra of Rhodamine 6G with ~25 nm [1] Gispert, J.R. (2008). Coordination Chemistry. Wiley- Stokes shift VCH. p. 483. ISBN 3-527-31802-X. [2] Albani, J.R. (2004). Structure and Dynamics of Macro- Not to be confused with Stark shift. molecules: Absorption and Fluorescence Studies. Elsevier. p. 58. ISBN 0-444-51449-X.

Stokes shift is the difference (in wavelength or frequency [3] Lakowicz, J.R. 1983. Principles of Fluorescence Spec- units) between positions of the band maxima of the troscopy, Plenum Press, New York. ISBN 0-387-31278- absorption and emission spectra (fluorescence and Raman 1. being two examples) of the same electronic transition.[1] [4] Guilbault, G.G. 1990. Practical Fluorescence, Second It is named after Irish physicist George G. Stokes.[2][3][4] Edition, Marcel Dekker, Inc., New York. ISBN 0-8247- When a system (be it a molecule or atom) absorbs a 8350-6. photon, it gains energy and enters an excited state. One [5] Kitai, A. (2008). Luminescent Materials and Applications. way for the system to relax is to emit a photon, thus los- John Wiley and Sons. p. 32. ISBN 0-470-05818-8. ing its energy (another method would be the loss of heat energy). When the emitted photon has less energy than [6] Rost, F.W.D. (1992). Fluorescence Microscopy. Cam- the absorbed photon, this energy difference is the Stokes bridge University Press. p. 22. ISBN 0-521-23641-X. shift. If the emitted photon has more energy, the energy difference is called an anti-Stokes shift;[5] this ex- tra energy comes from dissipation of thermal phonons 3 See also in a crystal lattice, cooling the crystal in the process. Yttrium oxysulfide doped with gadolinium oxysulfide is a • Stokes line common industrial anti-Stokes pigment, absorbing in the near-infrared and emitting in the visible portion of the • Jablonski diagram spectrum. Photon upconversion is another anti-Stokes • process. Kasha’s rule

1 Stokes ﬂuorescence

Stokes ﬂuorescence is the re-emission of longer wavelength photons (lower frequency or energy) by a molecule that has absorbed photons of shorter wavelengths (higher frequency or energy).[6] Both absorption and radiation (emission) of energy are unique characteristics of a par-

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