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Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (excitation by photons), hence the prefix ''photo-''.〔IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "(photochemistry )".〕 Following excitation various relaxation processes typically occur in which other photons are re-radiated. Time periods between absorption and emission may vary: ranging from short femtosecond-regime for emission involving free-carrier plasma in inorganic semiconductors〔Hayes, G.R.; Deveaud, B. (2002). "Is Luminescence from Quantum Wells Due to Excitons?". ''physica status solidi (a)'' 190 (3): 637–640. (doi:10.1002/1521-396X(200204)190:3<637::AID-PSSA637>3.0.CO;2-7 )〕 up to milliseconds for phosphorescent processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours. Observation of photoluminescence at a certain energy can be viewed as indication that excitation populated an excited state associated with this transition energy. While this is generally true in atoms and similar systems, correlations and other more complex phenomena also act as sources for photoluminescence in many-body systems such as semiconductors. A theoretical approach to handle this is given by the semiconductor luminescence equations. == Forms of photoluminescence == Photoluminescence processes can be classified by various parameters such as the energy of the exciting photon with respect to the emission. Resonant excitation describes a situation in which photons of a particular wavelength are absorbed and equivalent photons are very rapidly re-emitted. This is often referred to as resonance fluorescence. For materials in solution or in the gas phase, this process involves electrons but no significant internal energy transitions involving molecular features of the chemical substance between absorption and emission. In crystalline inorganic semiconductors where an electronic band structure is formed, secondary emission can be more complicated as events may contain both coherent such as resonant Rayleigh scattering where a fixed phase relation with the driving light field is maintained (i.e. energetically elastic processes where no losses are involved) and incoherent contributions (or inelastic modes where some energy channels into an auxiliary loss mode),〔Kira, M.; Jahnke, F.; Koch, S. W. (1999). "Quantum Theory of Secondary Emission in Optically Excited Semiconductor Quantum Wells". ''Physical Review Letters'' 82 (17): 3544–3547. (doi:10.1103/PhysRevLett.82.3544 )〕 The latter originate, e.g., from the radiative recombination of excitons, Coulomb-bound electron-hole pair states in solids. Resonance fluorescence may also show significant quantum optical correlations.〔〔Kimble, H. J.; Dagenais, M.; Mandel, L. (1977). "Photon Antibunching in Resonance Fluorescence". ''Physical Review Letters'' 39 (11): 691–695. (doi:10.1103/PhysRevLett.39.691 )〕〔Carmichael, H. J.; Walls, D. F. (1976). "Proposal for the measurement of the resonant Stark effect by photon correlation techniques". ''Journal of Physics B: Atomic and Molecular Physics'' 9 (4): L43. (doi:10.1088/0022-3700/9/4/001 )〕 More processes may occur when a substance undergoes internal energy transitions before re-emitting the energy from the absorption event. Electrons change energy states by either resonantly gaining energy from absorption of a photon or losing energy by emitting photons. In chemistry-related disciplines, one often distinguishes between fluorescence and phosphorescence. The prior is typically a fast process, yet some amount of the original energy is dissipated so that re-emitted light photons will have lower energy than did the absorbed excitation photons. The re-emitted photon in this case is said to be red shifted, referring to the reduced energy it carries following this loss (as the Jablonski diagram shows). For phosphorescence, absorbed photons undergo intersystem crossing where they enter into a state with altered spin multiplicity (see term symbol), usually a triplet state. Once energy from this absorbed electron is transferred in this triplet state, electron transition back to the lower singlet energy states is quantum mechanically forbidden, meaning that it happens much more slowly than other transitions. The result is a slow process of radiative transition back to the singlet state, sometimes lasting minutes or hours. This is the basis for "glow in the dark" substances. Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as GaAs and InP and for quantification of the amount of disorder present in a system. Several variations of photoluminescence exist, including photoluminescence excitation (PLE) spectroscopy. Time-resolved photoluminescence (TRPL) is a method where the sample is excited with a light pulse and then the decay in photoluminescence with respect to time is measured. This technique is useful for measuring the minority carrier lifetime of III-V semiconductors like gallium arsenide (GaAs). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Photoluminescence」の詳細全文を読む スポンサード リンク
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