Stokes shift

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Not to be confused with Stark shift.
Absorption and emission spectra of Rhodamine 6G with ~25 nm Stokes shift

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.[1] It is named after Irish physicist George G. Stokes.[2][3][4]


The Stokes shift is the result of two actions: Vibrational relaxation or dissipation and solvent reorganisation. A fluorophore is a dipole, surrounded by water molecules. When a fluorophore enters an excited state, its dipole moment will change, but water molecules will not be able to adapt this quickly. Only after vibrational relaxation, will there will be a realignment of their dipole moments.

Anti-Stokes Shift[edit]

If the emitted photon has more energy, the energy difference is called an anti-Stokes shift;[5] this extra energy comes from dissipation of thermal phonons in a crystal lattice, cooling the crystal in the process. Yttrium oxysulfide doped with gadolinium oxysulfide is a common industrial anti-Stokes pigment, absorbing in the near-infrared and emitting in the visible portion of the spectrum. Photon upconversion is another anti-Stokes process. An example of this later process is demonstrated by Upconverting nanoparticles.

Stokes fluorescence[edit]

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 particular 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 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.

See also[edit]


  1. ^ Gispert, J.R. (2008). Coordination Chemistry. Wiley-VCH. p. 483. ISBN 3-527-31802-X. 
  2. ^ Albani, J.R. (2004). Structure and Dynamics of Macromolecules: Absorption and Fluorescence Studies. Elsevier. p. 58. ISBN 0-444-51449-X. 
  5. ^ Kitai, A. (2008). Luminescent Materials and Applications. John Wiley and Sons. p. 32. ISBN 0-470-05818-8. 
  6. ^ Rost, F.W.D. (1992). Fluorescence Microscopy. Cambridge University Press. p. 22. ISBN 0-521-23641-X. Archived from the original on November 13, 2012.