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A plasma afterglow (also afterglow) is the radiation emitted from a plasma after the source of ionization is removed The external electromagnetic fields that sustained the plasma glow are absent or insufficient to maintain the discharge. A plasma afterglow can either be a temporal, due to an interrupted (pulsed) plasma source, or a spatial one, due to a distant plasma source. In the afterglow, plasma-generated species de-excite and participate in secondary chemical reactions that tend to form stable species. Depending on the gas composition, super-elastic collisions may continue to sustain the plasma in the afterglow for a while by releasing the energy stored in rovibronic degrees of freedom of the atoms and molecules of the plasma. Especially in molecular gases, the plasma chemistry in the afterglow is significantly different from the plasma glow.

Flowing afterglow
A flowing afterglow is an ion source that is used to create ions in a flow of inert gas, typically helium or argon. Reagents are added downstream to create ion products and study reaction rates. Detection of ions is accomplished using a mass spectrometer or by optical spectroscopy. Flowing afterglow ion sources can be coupled with a selected-ion flow-tube for selection of reactant ions.

Flowing-afterglow mass spectrometry uses a flowing afterglow to create protonated water cluster ions in a helium or argon carrier gas in a flow tube that react with sample molecules that are measured by a mass spectrometer downstream. These systems can be used for trace gas analysis. These systems have also been shown to be an effective means of sterilization.

Remote plasma
A remote plasma is also called an afterglow plasma because it is a plasma processing method in which processing occurs in the afterglow of the plasma rather than in the plasma itself.