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A luminescence excited in a body by some form of electromagnetic radiation incident on the body. The term photoluminescence is generally limited to cases in which the incident radiation is in the ultraviolet, visible, or infrared regions of the electromagnetic spectrum.
Photoluminescence may be either a fluorescence or a phosphorescence, or both. Energy can be stored in certain luminescent materials by subjecting them to light or some other exciting agent, and can be released by subsequent illumination of the material with light of certain wavelengths. This type of photoluminescence is called stimulated photoluminescence. See Fluorescence, Luminescence, Phosphorescence
luminescence excited by light. The simplest case of photoluminescence is the resonance radiation of atomic vapors, which occurs when electromagnetic radiation with the same frequency as the exciting radiation is emitted.
In accordance with Stokes’ law of luminescence, the radiation emitted during the photoluminescence of molecules and other complex systems has a lower frequency than the exciting light. Stokes’ law is often violated, and both the Stokes and the anti-Stokes components of the spectrum are observed; the anti-Stokes component consists of radiation whose frequency is higher than that of the exciting light. In more complex molecules, light absorption is followed by a redistribution of energy among the molecules. As a result, the emission spectrum is independent of or only weakly dependent on the excitation frequency.
As a result of molecular interactions—and, in complex molecules, of intramolecular processes—electron excitation energy may be converted into the energy of vibrational, rotational, or translational molecular motion, that is, into thermal energy. The energy conversion processes are known as quenching, and they lead to a situation in which the quantum yield, that is, the ratio of the number of emitted photons to the number of exciting photons, is less than unity.
In general, the effficiency of photoluminescence depends in a complex manner on the wavelength of the exciting light. In 1924, S. I. Vavilov established a law for the photoluminescence of molecules in a liquid or solid medium. The law, which may be regarded as a generalization of Stokes’ law, states that the quantum yield of photoluminescence is constant over a broad range of excitation wavelengths—and is, for Stokes excitation—but decreases sharply at wavelengths lying within the radiation spectrum of the molecules—that is, for anti-Stokes excitation.
More complex regularities are observed during the photoluminescence of crystal phosphors in cases where both excitation and photoionization occur when light is absorbed. In such cases, photoluminescence arises as a result of the recombination of electrons and ionized luminescence centers. The yield and other properties of the photoluminescence depend on whether the exciting light is absorbed in the luminescence centers or in the crystal lattice of the host substance.
REFERENCESLevshin, V. L. Fotoliuminestsentsiia zhidkikh i tverdykh veshchestv. Moscow-Leningrad, 1951.
Antonov-Romanovskii, V. V. Kinetika fotoliuminestsentsii kristallofosforov. Moscow, 1966.