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Luminescence and Fluorescence spectroscopy

Defining light emissive phenomena is often difficult. Researchers from different backgrounds frequently call it by different names due to its diverse nature and the fact that it cannot be singularly defined. In this note we will try to present the simple and clear classification of general types of luminescence based upon the source and the nature of light excitation (see the chart below).

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Strictly speaking, Luminescence is a light emission which represents an excess over the thermal radiation and lasts for a time exceeding the period of electromagnetic oscilation. This actually indicates two things. First of all, excess over thermal radiation brings the distinction between Luminescence and the light emitted from warm or hot incandescent bodies. Visible light from thermal radiation begins emitting at minimum temperatures of a few hundred degrees K, while Luminescence can be observed at any temperature. That is why Luminescence is sometimes labeled a cold light. The second condition, in which Luminescence should last for a time exceeding the period of electromagnetic oscilation, distinguishes it from reflected and stray light. In Luminescence there are intermediate processes between absorption and emission duration which exceeds the period of a single electromagnetic oscilation. As a result, Luminescence looses correlation between phases of absorbed and emitted light, in contrast to reflected and stray light, in which the phase correlation can always be observed.

Luminescent analysis is performed using the intrinsic Luminescence of materials under observation, or special markers (luminophors) are added when the material itself does not demonstrate luminescent properties. Luminescence applications are so numerous and diverse that contemporary reviews and books are unnable to accommodate them all.

Photoluminescence, a luminescence stimulated by light absorption in UV-Vis-NIR spectral region, represents any process in which material absorbs electromagnetic energy at a certain wavelength and then emits part of it at a different (usually longer) wavelength. Therefore, only a part of the absorbed energy is transformed into luminiscent light. The rest of it ends up as molecular vibrations, or simply as a heat. Photoluminescence is the most popular type of Luminescence because a large selection of reliable and inexpensive excitation sources are available and also because the effect can often be observed with the naked eye. Usually an excitation source emits in UV and the Photoluminescence occurs in Vis or NIR.

There is always a delay between the moment the material has absorbed the higher energy photon and the moment the secondary lower energy photon is re-emitted. This delay is defined by the lifetime of excitation states, or simply by how long atoms or molecules are able to stay in excited high-energy conditions. Delay time can vary many orders of magnitude for different materials. Based on practical observations, two types of Photoluminescence were historically established - Fluorescence and Phosphorescence. Technically, delay time is the only difference between them. It is shorter for Fluorescence (10-12 to 10-7 s) and much longer for Phosphorescence (up to a few hours and even days).

Fluorescence is a "fast" Photoluminescence. The effect is widely used in such everyday practical applications as industrial and residential lightning (neon and fluorescent lamps) as an analytical technique in science and as a quality and process control method in industry.

Phosphorescence is a "slow" Photoluminescence. In contrast to Fluorescence, it demonstrates itself as a glowing that lasts long after the excitation light is gone. Phosphorescent materials are usually called "glow-in-the-dark". This effect is generally used by the Department of Transportation to attract drivers' attention to road signs, in adertising campains to produce glowing stickers and promotional materials, as well as in numerous industries to notify people of potential hazards and dangers.

Electroluminescence is a Luminescence excited in gases and solids by applying an electromagnetic field. Molecules are excited upon creation of any form of electric discharge in a material.

Electroluminescence of gases is used in discharge tubes. The electroluminescence effect, which readily occurs in semiconductors and light emitting diodes (LEDs), is the most well-known application. Natural blue diamond emits light when electrical current is passed through it.

Triboluminescence occurs when a material is scratched, crushed, rubbed or stressed mechanically in any way. When a material is subjected to mechanical stress spatially separated, electrical charges are produced. Upon recombining these charges, a flash of light emerges as a result of electric discharge, ionizing the surrounding space. Since electrical discharge is in the foundation of Triboluminescence, it can be classified as a part of Electroluminescence. Blue or red Triboluminescence can be observed when sawing a diamond during the cutting process. Another example includes sugar crystals, which produce tiny electrical sparks while crushing. Other substances exhibiting Triboluminescence include minerals fluorite (CaF2) and sphalerite (ZnS).

Crystalloluminescence is a type of Luminescence generated during crystallization, used to determine the critical size of the crystal nucleus. There is a theory that the light from crystalloluminescence emerges through the micro-fracture of growing crystallites. Separation of electrical charges may occur on the fracture facets on the surface of micro-fractures and their following recombination. This effectively classifies Crystalloluminescence as a type of Triboluminescence and a subtype of Electroluminescence. Let us note that electrically charged micro-fractures may be developed due to multiple processes such as the movement of charged dislocations, piezoelectrification, etc.

Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound. It is believed that when a bubble starts imploding, extremely high pressures inside the bubble cause the water to form ice-like structures. At the moment when the opposite sides of an imploding bubble collide, the very strong mechanical stress causes the ice to fracture. The growth of ice micro-fractures results in separation of electrical charges and their following recombination, which generates light. Therefore, Sonoluminescence is a part of the Triboluminescence phenomenon.

Sonoluminescence light flashes from a single bubble and lasts from a few tens to a few hundred picoseconds. It is emitted at relatively short wavelengths, which can reach into the ultraviolet. The emitting bubble size is averaged at about 1 mkm in diameter. The addition of a small amount of noble gas (such as helium, argon, or xenon) to the gas in the bubble enhances the intensity of the emitted light dramatically. A possible reason for this is an increase in the ice fracturing ability.

Chemoluminescence is conversion of chemical energy directly into light as a result of a chemical reaction. In brief, reactants A and B are transformed into an excited intermediate I. The decay of the excited intermediate I to a lower energy level is responsible for the emission of light.

[A] + [B] --> [I] --> [Products] + Light

Theoretically, each molecule of the reactant should produce one photon of light, or Avogadro's number of photons-per-mole. In practice, non-catalytic reactions usually generate about 1 photon-per-100 reactant molecules with a quantum efficiency of about 1%. Standard laboratory applications of Chemoluminescence include the forensic test for locating blood, even if it has been cleaned or removed. The chemical substance luminol emits blue light upon contact with the iron in haemoglobin if blood is present. The glow lasts for about 30 seconds. Lightsticks are another well-known Chemoluminescence application.

Bioluminescence is Chemoluminescence produced by living organisms. Bioluminescence observed at the surface of the sea is produced by microscopic plankton. Other examples of bioluminescence include glow-worms, fireflies, and various fungi and bacteria found on rotting wood or decomposing flesh.

Radioluminescence (Scintillation) is a Luminescence resulting from excitation by high-energy particles or radiation. Excitation sources include a-particles - helium nuclei (a-Luminescence), beta-particles - electrons and positrons (Cathodoluminescence), accelerated protons and neutrons, ions (Ionoluminescence), ?-radiation and X-Ray radiation (X-Ray Luminescence). Radioluminescence is widely used in medical physics, dosimetry, and television and radar screens.

Thermoluminescence light emerges from a heated material as a result of high-energy electrons previously trapped within the material being released. In other words, heat frees the electrons, which produces light. The intensity of Thermoluminescence is proportional to the energy absorbed by the mineral as a result of its previous exposure to ionizing radiation. A very important application of Thermoluminescence is archeological and geological dating. The natural flux of ionizing radiation - both from cosmic radiation and natural radioactivity - creates excited states in crystalline structures.

A very small fraction of the radiation energy can stay in these excited states for a long time. When such material is heated, the stored energy is released as weak light or Thermoluminescence. After cooling, re-heating the material will not generate light, as no excited states remain. This method is used when materials do not contain carbon atoms, therefore eliminating the possibility of radiocarbon dating. It is frequently used for authenticating the age of old ceramic wares, for which it gives the approximate date of the last firing.

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