Stokes shift is the difference (in wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 or frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 units) between positions of the band maxima of the absorption and emission spectra

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....

 (fluorescence

Fluorescence

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...

 and Raman

Raman spectroscopy

Raman spectroscopy is a spectroscopic technique used to study vibrational, rotational, and other low-frequency modes in a system.It relies on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range...

 being two examples) of the same electronic transition. It is named after Irish physicist George G. Stokes

George Gabriel Stokes

Sir George Gabriel Stokes, 1st Baronet FRS , was an Irish mathematician and physicist, who at Cambridge made important contributions to fluid dynamics , optics, and mathematical physics...

.

When a system (be it a molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 or atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

) absorbs a photon

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

, 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

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 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; 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

Gadolinium oxysulfide

Gadolinium oxysulfide , also called gadolinium sulfoxylate or GOS, is an inorganic compound, a mixed oxide-sulfide of gadolinium. Its CAS number is .-Uses:...

 is a common industrial anti-Stokes pigment, absorbing in the near-infrared and emitting in the visible portion of the spectrum. Photon upconversion

Photon upconversion

Photon upconversion is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength. It is a anti-Stokes type emission. An example is the conversion of infrared light to visible light. Materials by which...

is another anti-Stokes process.

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). 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. The electron returns to the ground state and energy is emitted.