Phosphorescent substances

Fluorescent and phosphorescent substances are excited into an unstable energy state by UV light. When they return to the ground state they release a part of the energy taken up in the form of radiation. The emitted radiation is less energetic than the light absorbed and usually lies in the visible part of the spectrum. Since absorption (excitation) and emission obey a linear relationship over a certain range a reduction in absorption leads to a reduction in the luminescence, too. 

A scintillation counter makes use of the fact that phosphors—phosphorescent substances such as sodium iodide and zinc sulfide (see Section 15.14)—give a flash of light—a scintillation—when exposed to radiation. The counter also contains a photomultiplier tube, which converts light into an electrical signal. The intensity of the radiation is determined from the strength of the electronic signal. 

Spectroscopists interested in elucidation of the molecular energy schemes studied the phosphorescence emission of over 200 compounds, of which 90 were tabulated by Lewis and Kasha in 1944. They classified phosphorescing substances in two classes, based on the mechanism of phosphorescence production. The first group comprises minerals or crystals named phosphors, where the individual molecule is not phosphorescent as such, but emits a shining associated with the presence of some impurity localized in the crystal. This type of phosphorescence cannot be attributed to a concrete substance. The second type of phosphorescence emission is attributed to a specific molecular species, being a pure substance in crystalline form, adsorbed on a suitable surface or dissolved in a specific rigid medium [22],... 

One day Kunckel proudly exhibited to a friend in Hamburg—much as a modern chemist might show a specimen of hafnium or rhenium— a phosphorescent substance. To his great surprise, the friend had not only seen this substance before, but offered to take Kunckel to the home of the medical alchemist, Dr. Brand, to see a still more remarkable substance that shines spontaneously in the dark. Brand, they found, had given away his entire supply, but he took Kunckel to the home of a friend to see the wondrous element. 

Priestley mentioned that Jacopo Bartolomeo Beccari and other scientists of Bologna in 1711 took a great deal of pains with the chymical analysis of this fossil, by which they thought they discovered in it some sulphur and also an alkaline salt" (59). Before testing his phosphors, Beccari used to remain for some time in a dark, portable booth, or cell. When the pupils of his eyes had become sufficiently dilated, he was able to observe the dim, cold light which the phosphorescent substances emitted (89). 

A few months after Roentgen announced his discovery of X rays, the French physicist Antoine Henri Becquerel (1852—1908) experimented to see if they were emitted by phosphorescent substances—those that glow in the dark after being exposed to bright light. One substance that appeared to confirm the idea that phosphorescence resulted in X rays was uranium. When... 

A scintillation counter makes use of the fact that phosphors, phosphorescent substances such as sodium iodide and zinc sulfide, give a flash of light—... 

Many uranium salts, like other fluorescent and phosphorescent substances, are tribolunoinescent— that is, they emit light when crushed (see p. 324). 

It may be prepared by dissolving any of the oxides or hydroxides of uranium in nitric acid and concentrating the solution or by double decomposition of uranyl chloride with silver nitrate. It crystallises in lemon-yellow prisms, the form of which varies according to the nature of the solvent. The crystals exhibit a yellowish-green fluorescence which does not increase, as is usually the case with phosphorescent substances, even at such a low temperature as —190° C. They also... 

One wraps a Lumiere photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the Sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the Sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. (Becquerel Comptes Rendus)... 

One of the major sources of background in a scintillation counter is the dark current of the phototube (see Sec. 6.6.1). Other background sources are naturally occurring radioisotopes, cosmic rays, and phosphorescing substances. 

The term phosphorescence refers to delayed emission of light as a result of deexcitation of atoms or molecules. Phosphorescent half-lives may extend to hours. This source of background may originate in phosphorescent substances contained in the glass of the phototube, the walls of the sample holder, or the sample itself. 

In general, fluorescence emission occurs very soon (10" to 10" sec) after a species reaches the excited state. Thus, it is impossible for the eye to perceive a fluorescent substance once the source of ultraviolet radiation has been removed. Phosphorescence emission occurs more slowly (> 10" sec) and with a greater variation in the lifetimes of the phosphorescence emission. Thus, while many phosphorescent substances, including most organic molecules, also cannot be perceived by the... 

To permit visualization of the spots, the plate is often sprayed with something e.g., silica gel plates show spots for most organic substances when the plates are sprayed with a methanol solution of sulfuric acid and then baked in an oven. A permanent record can be made by photographing the plates in color before the colors fade. An alternative method for detecting spots consists of mixing about 1% of a fluorescent or phosphorescent substance (sold by companies which handle TLC accessories) with the adsorbent... 

Following exposure to light, phosphorescent substances glow for several minutes as the excited phosphors return to the ground state very slowly. 

Entrepreneurs wasted no time in finding other commercial applications for tadium. Mixed with zinc sulphide, it produced a phosphorescent substance that was used to make luminous watch faces and aircraft instrumentation. Since radium dissipated stadc chai in surrounding air, it also reduced sparks, which made it useful in factories where flammable vapours could be ignited. Public f ination with the new wonder substance created a market for commercial applications of more dubious utility. Radium paint was used to create glow-in-the-daik slipper buttons, roulette wheels, and fish bait, and farmers were sold radioactive manures that had waste products fiom radium refineries mixed into them. 

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