Quantum yield

Quantum yield is the fraction of absorbed photons that produce a specified outcome, such as fluorescence, photochemical conversion, or charge separation.

Photoluminescence: Light emission from a substance following absorption of photons or other excitation. Fluorescence and phosphorescence are common outcomes whose photon yields can be measured.
Photon: A quantum of electromagnetic radiation that carries energy and momentum. Quantum yield uses photons as the units counted in its denominator and often its numerator.
Fluorescence spectroscopy: A technique that analyzes light emitted by a sample after excitation. Fluorescence yield helps explain signal strength and compare emitting samples.
Quantum efficiency: A measure of output quanta per input quanta in a specified physical process. The term may denote device or detector performance, while quantum yield counts a defined outcome per absorbed photon.
Fluorescence: Prompt light emission from an excited state, typically occurring within nanoseconds of photon absorption. Fluorescence quantum yield counts emitted photons against photons absorbed.
Absorption spectroscopy: A method that measures how much light a sample absorbs across wavelengths. Absorbed-photon counts are needed to determine a yield rather than an incident-light ratio.
Fluorescent dye: A compound that absorbs light at one wavelength and emits light at another. Its fluorescence quantum yield is a key measure of how efficiently absorbed light becomes emission.
External quantum efficiency: The fraction of incident photons that produce collected charge carriers in a device. Its denominator is incident photons, unlike absorbed-photon quantum yield.
Phosphorescence: Delayed light emission from an excited state, commonly following a transition to a triplet state. Its yield measures how often triplet-state relaxation emits light rather than losing energy nonradiatively.
Excited state: A state of an atom or molecule with more energy than its ground state. Absorbed photons populate excited states from which measured outcomes compete.

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