Fluorescence is generally defined as a luminescence emission that is caused by the flow of some form of energy into the emitting body, this emission ceasing abruptly when the exciting energy is shut off. In attempts to make this definition more meaningful it is often stated, somewhat arbitrarily, that the decay time, or afterglow, of the emission must be of the order of the natural lifetime for allowed radiative transitions in an atom or a molecule, which is about 10-8 s for transitions involving visible light. Perhaps a better distinction between fluorescence and its counterpart, phosphorescence, rests not on the magnitude of the decay time per se, but on the criterion that the fluorescence decay is temperature-independent.
In the literature of organic luminescence, the term fluorescence is used exclusively to denote a luminescence which occurs when a molecule makes an allowed optical transition. Luminescence with a longer exponential decay time, corresponding to an optically forbidden transition, is called phosphorescence, and it has a different special distribution from the fluorescence. See Phosphorescence
The decay time of fluorescent materials varies widely, from the order of 5 × 10-9 s for many organic crystalline materials up to 2 s for the europium-activated strontium silicate phosphor. Fluorescent materials with decay times between 10-9 and 10-7 s are used to detect and measure high-energy radiations, such as x-rays and gamma rays, and high-energy particles such as alpha particles, beta particles, and neutrons. These agents produce light flashes (scintillations) in certain crystalline solids, in solutions of many polynuclear aromatic hydrocarbons, or in plastics impregnated with these hydrocarbons. The so-called fluorescent lamps employ the luminescence of gases and solids in combination to produce visible light. See Luminescence
a luminescence that decays in a time τ ~ 10–8 –10–9 sec. The division of luminescence into fluorescence and phosphorescence is out of date, since it is an arbitrary distinction based on a qualitative measurement of the duration of luminescence. In terms of the mechanism for converting the excitation energy into visible emission, fluorescence, as a rule, is spontaneous luminescence; hence τ is determined by the lifetime of an atom in an excited state.
Resonance fluorescence, the frequency of which coincides with the frequency of the exciting radiation, is observed in atomic vapors (seeRESONANCE RADIATION). Molecules may fluoresce in highly rarefied vapors; an increase in the vapor pressure or the addition of foreign impurities may quench the fluorescence. Many organic substances, particularly aromatic compounds, fluoresce in liquid and solid solutions as well as in the crystalline state.
The spectra, polarization, and kinetics of fluorescence are associated with the structure and symmetry of molecules and with the nature of molecular interactions and depend, for example, on the concentration of solutions and the type of excitation. Fluorescence may be used, for example, to study crystal structure and excition processes in crystals (seeSPECTROSCOPY, CRYSTAL), the energy levels of molecules, the structure and interaction of molecules, and the processes by which excitation energy is transferred. Fluorescence is used in luminescence analysis, scintillation counters, and mineralogical research.
The decay time of fluorescence is measured by means of fluorometers.
of minerals, a luminescence that is excited in minerals by light, X rays, or electrons and that decays rapidly (within 10–2–10–1 sec) after the excitation ceases. The rapid decay of the luminescence distinguishes fluorescence from phosphorescence and thermoluminescence. The physical phenomenon of the fluorescence of minerals was first observed in fluorite, from which the term “fluorescence” is derived.
Fluorescence is characteristic of dielectric minerals and semiconductors that are transparent to visible light and to near-ultraviolet and infrared radiation. The fluorescence of minerals is associated with impurities or occasionally with intrinsic ions or complexes; such impurities, ions, or complexes form luminescence centers. Fluorescence is sometimes completely or partially quenched by certain isomorphic impurities, such as ions of bivalent iron.
The fluorescence of minerals is used in luminescence analysis to detect such minerals as scheelite, zircon, apatite, and uranites in mining excavations and to analyze microscopic impurities consisting of rare or dispersed elements, such as uranium or rare earths. It is also used for ore dressing by means of the identification of the useful component—for example, diamonds, fluorspar, or scheelite—on the basis of the component’s luminescence.
B. S. GOROBETS