Photoconductivity primary

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

Postcolumn photochemical reactions are another approach to the detection problem. High-intensity UV light, generally provided by a Hg or Zn lamp, photolyzes the HPLC effluent, which passes through a Teflon (47) or quartz tube. The photolysis reaction determines the nature of the subsequent detection. If the compound has a UV chromophore, such as an aromatic ring, and an ionizable heteroatom, such as chlorine, then the products of the reaction can be detected conductometrically. Busch et al. (48) have examined more than 40 environmental pollutants for applicability to detection with photolysis and conductance detection. Haeberer and Scott (49) found the photoconductivity approach superior to precolumn derivatization for the determination of nitrosoamines in water and waste water. The primary limitation of this detection approach results from the inability to use mobile phases that contain ionic modifiers, that is, buffers and... 

The earliest all-organic photoconductive compositions consisted of poly-vinyl(carbazole) (PVK, 1) and 2,4,7-trinitrofluorenone (TNF, 2) [25], In spite of its historical importance, the PVK/TNF system is an unusual one, because it is one of the few photoconductors which acts simultaneously as a photogenerator of primary charges and the medium in which the charge transport is accomplished. It is much more usual to employ a separate charge generator and a distinct transport medium. 

All photoeffects involve the absorption of photons to produce an excited state in the absorber or liberate electrons directly. With the direct release of electrons, photoemission may occur from the surface of solids. While the excited state may revert to the ground state, it may proceed further to a photochemical reaction to provide an electron-hole pair (exciton) as the primary photoproduct. The exciton may dissociate into at least one free carrier, the other generally remaining localized. In an externally applied electric field, photoconduction occurs. Photomagnetic effects arise in a magnetic field. Absorption of photons yield photoelectric action spectra which resemble optical absorption spectra. Photoeffects are involved in many biological systems in which charge transfer takes place (e.g., as observed in the chlorophylls and carotenoids) [14]. 

When there is incomplete collection of charge, the primary photoconductivity is complicated by the presence of trapped space charge which distorts the electric field (see Section 10.1.2). Although the primary photoconductor is usually the best structure for a light detector, the recombination mechanisms are more commonly studied... 

The reverse bias p-i-n sensor is a primary photoconductor in which injection from the contacts is prevented by the junction. A secondary photoconductor allows charge to flow in from the contacts and offers the possibility of photoconductive gain. Gain occurs when the... 

Zinc is also a primary metal in the penny coin, which used to be made of copper. In addition, zinc can be mixed with copper to form brass, which is a very useful alloy because of its durability and hardness. Another useful compound is zinc oxide. Made by burning zinc vapor in air, zinc oxide is used to make white paints. Used as an ointment, zinc oxide makes a good sunscreen because it blocks the Suns harmful ultraviolet rays that damage the skin. Zinc oxide also has another important property it is photoconductive, which means that it conducts electricity better when exposed to light. For example, a photocopier contains a photoconductive plate that is sometimes made with zinc oxide. When this plate is electrically... 

In the treatment of photoconductivity in Sect. 8.4.1, we simply mentioned in passing the primary process the generation of the excess charge carriers using the internal photoeffect. In this section, we will treat the details of this process and choose again as an example the anthracene crystal. 

Related to the problem of detector operating temperature is that of its electrical power dissipation. A photoconductive detector must always carry a primary current / and therefore dissipate l R of electrical power, whereas a photovoltaic detector can be operated without a bias. The trend toward mechanical or electrical cooling, occurring for practical reasons, makes it necessary that a detector dissipate a minimum of electrical power and thereby heat itself as little as possible. 

The more notable and widely used infrared detectors can be divided into three basic classes which are indicative of the primary effect produced by the photon-detector interaction, i.e., thermal, photoconductive, photovoltaic, and photoemissive. Chapters 3, 4, and 5 offer a detailed treatment of each of these important processes. 

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