Stokes shift

The Stokes shift is the difference between the wavelengths of light absorbed and emitted by a substance, usually with emission at a longer wavelength.

Fluorescence: Fluorescence is light emission from an excited substance after it absorbs electromagnetic radiation. The shift is commonly measured between a fluorophore’s excitation and fluorescence emission.
Photon energy: Photon energy is the energy carried by a single photon, proportional to its frequency and inversely proportional to its wavelength. A longer emission wavelength means a lower-energy photon.
Fluorescence microscopy: Fluorescence microscopy forms images by detecting light emitted from fluorescent labels or specimens. Separated excitation and emission wavelengths enable selective imaging.
Anti-Stokes shift: An anti-Stokes shift occurs when emitted light has a shorter wavelength and higher photon energy than absorbed light. It reverses the usual direction of the Stokes shift.
Jablonski diagram: A Jablonski diagram represents electronic states and radiative and nonradiative transitions in a molecule. It maps the relaxation pathways that separate absorption from emission.
Electromagnetic spectrum: The electromagnetic spectrum comprises electromagnetic radiation across its range of wavelengths and frequencies. Absorption and emission wavelengths locate the shift within this spectrum.
Fluorescence spectroscopy: Fluorescence spectroscopy measures light emitted by a sample after optical excitation. The shift is a basic feature used to characterize fluorescent samples.
Stokes scattering: Stokes scattering is inelastic scattering in which outgoing photons have lower energy than incident photons. It shares the name and red-shift direction but is not fluorescence.
Vibrational relaxation: Vibrational relaxation is the loss of excess vibrational energy within an electronic state, typically as heat. It lowers excited molecules’ energy before they emit photons.
Absorption spectrum: An absorption spectrum shows how strongly a material absorbs light as a function of wavelength or frequency. Its absorption band supplies one side of the wavelength difference.

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