Photoluminescence

Photoluminescence is light emitted by a material after it absorbs photons. It includes fluorescence and phosphorescence, distinguished by how excited states release energy.

Electronic excitation: The promotion of an electron to a higher-energy state by absorption of energy. Absorbed photons create excited electronic states that can later emit light.
Photon: A quantum of electromagnetic radiation that carries energy and momentum. Photons provide the excitation energy and are released during radiative emission.
Fluorescence microscopy: An imaging method that detects light emitted by fluorescent labels or samples after excitation. It uses photoluminescence to reveal selected structures in cells and materials.
Chemiluminescence: Light emission produced by a chemical reaction rather than direct optical excitation. Its excitation energy comes from chemistry, not absorbed photons.
Fluorescence: Light emission that typically stops almost immediately after excitation ends. It is the prompt-emission branch of photoluminescence.
Energy level: A permitted value of energy for a quantum system. Differences between material energy levels set the energies of absorbed and emitted photons.
Photoluminescence spectroscopy: A spectroscopic method that analyzes light emitted after optical excitation. It measures emission spectra to probe material states and defects.
Electroluminescence: Light emission caused by an electric field or electrical current in a material. Electrical excitation distinguishes it from photon-driven photoluminescence.
Phosphorescence: Delayed light emission from metastable excited states, often involving a spin-forbidden transition. Its long-lived excited states produce emission that persists after illumination stops.
Exciton: A bound state of an electron and a hole in a material. Excitons often serve as the emitting excited states in semiconductors.

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