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Stokes shift
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== Stokes fluorescence == [[File:Stokes shift- Rh6G.png|right|thumb|Absorption and emission spectra of [[rhodamine 6G]] with ~25 [[nanometre|nm]] Stokes shift]] Stokes fluorescence is the emission of a longer-wavelength photon (lower frequency or energy) by a molecule that has absorbed a photon of shorter wavelength (higher frequency or energy).<ref>Banwell C.N. and McCash E.M. ''Fundamentals of Molecular Spectroscopy'' (4th ed., McGraw-Hill 1994) p.101 and p.113 {{ISBN|0-07-707976-0}}</ref><ref>[[Peter Atkins|Atkins P.]] and de Paula J. ''Physical Chemistry'' (8th ed., W.H. Freeman 2006) p.431 {{ISBN|0-7167-8759-8}}</ref><ref>{{cite book |last=Rost |first=F. W. D. |url=https://books.google.com/books?id=RU_6doL1q0IC |title=Fluorescence Microscopy |date=1992 |publisher=[[Cambridge University Press]] |isbn=0-521-23641-X |page=22 |archive-url=https://web.archive.org/web/20121113051227/https://books.google.co.uk/books?id=RU_6doL1q0IC |archive-date=November 13, 2012 |url-status=dead}}</ref> Both absorption and radiation (emission) of energy are distinctive for a particular molecular structure. If a material has a direct bandgap in the range of visible light, the light shining on it is absorbed, which excites electrons to a higher-energy state. The electrons remain in the excited state for about 10<sup>β8</sup> seconds. This number varies over several orders of magnitude, depending on the sample, and is known as the ''fluorescence lifetime'' of the sample. After losing a small amount of energy through vibrational relaxation, the molecule returns to the ground state, and energy is emitted.
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