Phosphors phosphorescence

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. 

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

A common uses of phosphors (phosphorescent materials) is in televisions and computer monitors small dots of red, green, and blue phosphors are grouped together on the inner surface of a cathode-ray tube. When electrons generated in the back of the tube hit the phosphors, they absorb the energy and then emit light. 

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

Cadmium Silicates. Cadmium orthosihcate [15857-59-2] Cd2SiO, (mp 1246°C d = 5.83 g/ inL) and cadmium metasihcate [13477-19-5] CdSiO, are both prepared by direct reaction of CdO and Si02 at 390°C under 30.4 MPa (300 atm) or at 900°C and atmospheric pressure ia steam. The materials are phosphors whea activated with Mn (IT) ioa and are both fluorescent and phosphorescent. 

Phosphor-eszierung, /. phosphorescence, -fleischsaure, /. phosphocarnic acid. 

Phosphorescence quenching la 35 -, detection limits la 15 -, time dependance la 34 Phosphoric acid la 179,185,242,278,430 Phosphoric acid esters la 44,170 Phosphoric acid insecticides lb 115,332, 339,340... 

Scientific research in the field of phosphors is almost 140 years old. In 1866, the young French chemist Theodore Sidot prepared, by a sublimation method, tiny ZnS crystals that manifested phosphorescence in the dark.4 After the experiment was repeated and confirmed it was presented in a note to the Academy of Sciences of Paris the note was then published by Becquerel.5 From present knowledge of phosphors it seems likely that Sidot s ZnS contained a small quantity of copper as an impurity, and was the precursor for ZnS-type phosphors. 

The delayed light emission as observed from the Bolonian stone is now classified as phosphorescence. We know now that these stones contain barium sulfate with traces of bismuth and manganese, and that the corresponding reducing process concerns the transformation of sulfate into sulfur. It is now well known that alkaline earth metal sulfates emit phosphorescence that strongly increases when traces of heavy metals are present. The so-called inorganic multi-component compounds phosphor and crystallophosphor are in fact polycrystalline substances containing traces of some ionic activators of luminescence. 

In fact, an important advance in the phosphorescence theory was realized by Wiedemann in 1889, stating that a phosphor exists in two forms, a stable one, A, and an unstable one, B. Light absorption brings along conversion of form A to B, which then returns to A emitting light. This hypothesis was in agreement with the exponential decay law as postulated years before by Becquerel, but who did not provide any information about the nature of both forms [5],... 

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],... 

It is worth noting some historical aspects in relation to the instrumentation for observing phosphorescence. Harvey describes in his book that pinhole and the prism setup from Newton were used by Zanotti (1748) and Dessaignes (1811) to study inorganic phosphors, and by Priestley (1767) for the observation of electroluminescence [3], None of them were capable of obtaining a spectrum utilizing Newton s apparatus that is, improved instrumentation was required for further spectroscopic developments. Of practical use for the observation of luminescence were the spectroscopes from Willaston (1802) and Frauenhofer (1814) [13]. 

The photochemical process built into 7 was encountered previously with regard to 2, Le, the capability of 02 in quenching excited states of sufficiently long lifetimes. In the case of 7, the process is so efficient that ambient levels of 02 completely kill off phosphorescence, even if the phosphor is enveloped by p-cyclodextrin. 

As an extension of the fluorescent sensitizer concept, Forrest et al. have applied this approach to phosphorescent OLEDs, in which the sensitizer is a phosphorescent molecule such as Ir(ppy)3 [342]. In their system, CBP was used as the host, the green phosphor Ir(ppy)3 as the sensitizer, and the red fluorescent dye DCM2 as the acceptor. Due to the triplet and the singlet state energy transfer processes, the efficiency of such devices is three times higher than that of fluorescent sensitizer-only doped device. The energy transfer processes are shown in Figure 3.21. 

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