phosphorescence

phosphorescence
/fos'feuh res"euhns/, n.
1. the property of being luminous at temperatures below incandescence, as from slow oxidation in the case of phosphorus or after exposure to light or other radiation.
2. a luminous appearance resulting from this.
3. any luminous radiation emitted from a substance after the removal of the exciting agent.
[1790-1800; PHOSPHORESC(ENT) + -ENCE]

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Emission of light from a substance exposed to radiation and persisting as an afterglow after the exciting radiation has been removed.

Unlike fluorescence, in which the absorbed light is emitted about 10-8 second after excitation, in phosphorescence the extra energy absorbed is stored in metastable states and reemitted later. Phosphorescence may last from about 10-3 second to days or even years. The term phosphorescence is often applied to luminescence of living organisms, as well.

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      emission of light from a substance exposed to radiation and persisting as an afterglow after the exciting radiation has been removed. Unlike fluorescence, in which the absorbed light is spontaneously emitted about 10-8 second after excitation, phosphorescence requires additional excitation to produce radiation and may last from about 10-3 second to days or years, depending on the circumstances.

  To understand the difference between phosphorescence and fluorescence it is helpful to refer to the energy level diagrams in the Figure—>, in which the relative distance of a line, or level, above a base line (the ground level) denotes the energy of an electron occupying that level. In A, representing fluorescence, an electron is raised from the ground level to an excited level by a light photon or other radiation. Transition of the electron back to the ground level can occur spontaneously with radiation of the same energy as that which was absorbed. According to electromagnetic theory, the return is almost coincident, occurring within 10-8 second or so. The case for phosphorescence is illustrated in the Figure—> by B. There, interposed between the ground level and the excited level, is a level of intermediate energy, called a metastable (metastable state) level, or electron trap, because a transition between the metastable level and other levels is forbidden (highly improbable). Once an electron has fallen from the excited level to the metastable level (by radiation or by energy transfer to the system), it remains there until it makes a forbidden transition or until it is further excited back to the transition level. This excitation may come about through thermal agitation of the neighbouring atoms or molecules (called thermoluminescence) or through optical (e.g., infrared) stimulation. The time spent in the metastable level, or electron trap, determines the length of time that phosphorescence persists.
 

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Universalium. 2010.


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