luminescence
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luminescence, emission oflight by certain materials when they are relatively cool. It is in contrast to lightemitted from incandescent bodies, such as burningwood orcoal, molteniron, and wire heated by anelectric current. Luminescence may be seen inneon andfluorescent lamps;television,radar, andX-rayfluoroscope screens; organic substances such asluminol or theluciferins infireflies andglowworms; certain pigments used in outdoor advertising; and also natural electrical phenomena such as lightning and theaurora borealis. In all these phenomena, light emission does not result from the material being above room temperature, and so luminescence is often called cold light. The practical value of luminescent materials lies in their capacity to transform invisible forms of energy into visible light.
Sources and process
Luminescence emission occurs after an appropriate material has absorbed energy from a source such as ultraviolet or X-rayradiation, electron beams, chemical reactions, and so on. The energy lifts the atoms of the material into an excited state, and then, because excited states are unstable, the material undergoes another transition, back to its unexcited ground state, and the absorbed energy is liberated in the form of either light or heat or both (all discrete energy states, including the ground state, of an atom are defined asquantum states). The excitation involves only the outermost electrons orbiting around the nuclei of the atoms. Luminescenceefficiency depends on the degree of transformation of excitation energy into light, and there are relatively few materials that have sufficient luminescence efficiency to be of practical value.
Luminescence andincandescence
As mentioned above, luminescence is characterized by electrons undergoing transitions from excited quantum states. The excitation of the luminescent electrons is not connected with appreciable agitations of the atoms that the electrons belong to. When hot materials become luminous and radiate light, a process called incandescence, the atoms of the material are in a high state of agitation. Of course, the atoms of every material are vibrating at room temperature already, but thisvibration is just sufficient to produce temperature radiation in the far infrared region of the spectrum. With increasing temperature this radiation shifts into the visible region. On the other hand, at very high temperatures, such as are generated in shock tubes, the collisions of atoms can be so violent that electrons dissociate from the atoms and recombine with them, emitting light: in this case luminescence and incandescence become indistinguishable.
Luminescentpigments anddyes
Nonluminescent pigments and dyes exhibitcolours because they absorb white light and reflect that part of the spectrum that is complementary to the absorbed light. A small fraction of the absorbed light is transformed into heat, but no appreciable radiation is produced. If, however, an appropriate luminescent pigment absorbs daylight in a special region of its spectrum, it can emit light of a colour different from that of the reflected light. This is the result of electronic processes within the molecule of the dye or pigment by which evenultraviolet light can be transformed to visible—e.g., blue—light. These pigments are used in suchdiverse ways as in outdoor advertising, blacklight displays, and laundering: in the latter case, a residue of the “brightener” is left in the cloth, not only to reflect white light but also to convert ultraviolet light intoblue light, thus offsetting any yellowness and reinforcing the white appearance.
Early investigations
Although lightning, the aurora borealis, and the dim light of glowworms and of fungi have always been known to mankind, the first investigations (1603) of luminescence began with asynthetic material, whenVincenzo Cascariolo, an alchemist and cobbler in Bologna, Italy, heated a mixture ofbarium sulfate (in the form of barite, heavy spar) and coal; the powder obtained after cooling exhibited a bluish glow at night, and Cascariolo observed that this glow could be restored by exposure of the powder to sunlight. The namelapis solaris, or “sunstone,” was given to the material because alchemists at first hoped it would transform baser metals into gold, the symbol for gold being the Sun. The pronounced afterglow aroused the interest of many learned men of that period, who gave the material other names, including phosphorus, meaning “light bearer,” which thereafter was applied to any material that glowed in the dark.
Today, the namephosphorus is used for thechemical element only, whereas certain microcrystalline luminescent materials are called phosphors. Cascariolo’sphosphor evidently was a barium sulfide; the first commercially available phosphor (1870) was “Balmain’s paint,” a calcium sulfide preparation. In 1866 the first stable zinc sulfide phosphor was described. It is one of the most important phosphors in modern technology.
One of the first scientific investigations of the luminescence exhibited by rotting wood or flesh and by glowworms, known from antiquity, was performed in 1672 byRobert Boyle, an English scientist, who, although not aware of the biochemical origin of that light, nevertheless established some of the basic properties ofbioluminescent systems: that the light is cold; that it can beinhibited by chemical agents such as alcohol,hydrochloric acid, and ammonia; and that the light emission is dependent on air (as later established, on oxygen).
In 1885–87 it was observed that crude extracts prepared from West Indianfireflies (Pyrophorus) and from the boringclam,Pholas, gave a light-emitting reaction when mixed together. One of the preparations was a cold-water extract containing acompound relatively unstable to heat,luciferase; the other was a hot-water extract containing a relatively heat-stable compound, luciferin. The luminescent reaction that occurred when solutions of luciferase and luciferin were mixed at room temperature suggested that all bioluminescent reactions are “luciferin–luciferase reactions.” In view of the complex nature of bioluminescent reactions, it is not astonishing that this simple concept ofbioluminescence has had to be modified. Only a small number of bioluminescent systems have been investigated for their respective luciferin and the corresponding luciferase, the best known being the bioluminescence of fireflies from theUnited States, a little crustacean living in the Japanese sea (Cypridina hilgendorfii), and decaying fish and flesh (bacterial bioluminescence). Although bioluminescent systems have not yet found practical applications, they are interesting because of their high luminescence efficiency.
The first efficientchemiluminescent materials were nonbiological syntheticcompounds such asluminol (with the formula 5-amino-2,3-dihydro-1.4-phthalazinedione). The strong bluechemiluminescence resulting from oxidation of this compound was first reported in 1928.
Phosphorescence andfluorescence
The name luminescence has been accepted for all light phenomena not caused solely by a rise of temperature, but the distinction between the terms phosphorescence and fluorescence is still open to discussion. With respect to organic molecules, the term phosphorescence means light emission caused by electronic transitions between levels of different multiplicity (explained more fully below), whereas the term fluorescence is used for light emission connected with electronic transitions between levels of like multiplicity. The situation is far more complicated in the case of inorganic phosphors.
The term phosphorescence was first used to describe thepersistent luminescence (afterglow) of phosphors. The mechanism described above for the phosphorescence of excited organic molecules fits this picture in that it is also responsible for light persistence up to several seconds. Fluorescence, on the other hand, is an almost instantaneous effect, ending within about 10−8 second after excitation. The term fluorescence was coined in 1852, when it was experimentally demonstrated that certain substances absorb light of a narrow spectral region (e.g., blue light) and instantaneously emit light in another spectral region not present in the incident light (e.g., yellow light) and that this emissionceases at once when the irradiation of the material comes to an end. The name fluorescence was derived from the mineralfluorspar, which exhibits a violet, short-duration luminescence on irradiation by ultraviolet light.