Phosphorescence

Phosphorescence is delayed light emission that persists after excitation ends, typically because an excited triplet state returns slowly to the ground state.

Triplet state: A molecular electronic state with two unpaired electron spins aligned in the same direction. Phosphorescent emitters typically occupy this state before releasing light.
Electronic excited state: A higher-energy electronic configuration occupied after a system absorbs energy. Phosphorescence begins with an excited electronic state.
Glow-in-the-dark material: A material that stores excitation energy and emits visible light after illumination stops. Persistent afterglow is a familiar macroscopic use of phosphorescence.
Bologna stone: A barium sulfide mineral preparation that became famous for glowing after exposure to sunlight. Its persistent afterglow was an early, widely discussed phosphorescent phenomenon.
Intersystem crossing: A nonradiative transition between electronic states of different spin multiplicities. It can populate the triplet state from an initially excited singlet state.
Singlet state: An electronic state in which electron spins are paired to give zero total spin. It commonly precedes triplet formation and supports prompt fluorescence.
Phosphorescent paint: Paint containing pigments that emit light for a time after absorbing light. Its visible afterglow comes from persistent excited states in the pigment.
Vincenzo Cascariolo: An Italian alchemist associated with the discovery of the luminous Bologna stone in the early seventeenth century. His preparation brought persistent mineral afterglow to scientific attention.
Spin selection rule: A quantum-mechanical rule restricting transitions that change total electron spin. It makes triplet-to-singlet emission weak and therefore often delayed.
Photoluminescence: Light emission following the absorption of photons. Phosphorescence is one delayed form of photoluminescence.

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