fluorescence

Fluorescence is a type of glow of some kinds of material (a phosphor) when it receives electromagnetic radiation (EMR), due to molecules' spontaneous emission of a photon subsequent to absorption of a photon. In many cases, the emitted photon has a longer wavelength than the absorbed photon, a fact used by fluorescent lamps as a means of producing visible light from an ultraviolet light source. Such wavelength-increase occurs in phosphors that can be excited to many closely-spaced energy levels, e.g., from many levels of vibration between the molecule's individual atoms, which allows the molecule to absorb photons over a range of somewhat-shorter wavelengths. Before the molecule spontaneously emits a photon, it generally falls to a lower state of excitation among those of the electron shell, which happens on a quicker timescale than that spontaneous emission, often reaching the lowest energy level for the electron shell (i.e., with no vibration). The resulting difference in the wavelength emitted from that absorbed is termed the Stokes shift. The energy that does not leave via the photon heats the phosphor. The produced EMR is often not monochromatic, but distributed over a spectral band much like the band that the material absorbed, but with longer wavelengths. The resulting emitted spectral energy distribution is narrower than that of a black-body spectrum, which is how fluorescent lamps efficiently produce visible light (i.e., less infrared, etc.).

If the phosphor is a monatomic gas, it has no levels of vibration and it tends to absorb only the same wavelength that it emits, which is termed resonance fluorescence. Some very complex (organic) molecules also exhibit this behavior.

Phosphorescence is the same as fluorescence, except that it takes a much longer time to spontaneously emit a photon, due to quantum rules, specifically the necessity of a spin flip to emit the photon. The emission can occur seconds or minutes later rather than within a tiny fraction of a second. Both are types of photoluminescence (aka luminescence).

Fluorescence can occur in gas (and plasma) such as clouds, atmospheres, and accretion disks, and comet tails. Biofluorescence is fluorescence within lifeforms, such as on planets.