Photo-effect external

Phosphorus-containing pesticides la 254 Phosphorus insecticides lb 83 Phosphorus pesticides lb 32 Photochemical activation lb 13 Photochemical reactions lb 15,17 Photodiodes la 24,29 Photo effect, external la 24 -, internal la 24, 29 Photo element la 24,29 Photography, exposure times la 137 -, instmmentation la 137 Photomultiplier la 25ff -, disadvantages la 27 -, energy distribution la 26 -, head on la 27 -, maximum sensitivity la 28 -, side on la 27 -, spectral sensitivity la 28 -, window material la 28 Photocells la 25 Phloxime lb 116... 

A role is also played by the temperature and frequency dependence of the photocurrent, the variable surface sensitivity at various parts of the cathode and the vector effect of polarized radiation [40]. All the detectors discussed below are electronic components whose electrical properties vary on irradiation. The effects depend on external (photocells, photomultipliers) or internal photo effects (photoelements, photodiodes). 

Photocells and photomultipliers (secondary electron multipliers, SEM) are mainly employed in photometry. These are detectors with an external photo-effect . 

A common practice for the determination of the intensity of irradiation is the use of physical devices that are based on either the internal or the external photo effect. The internal photo effect is exploited in semiconducting materials. Thus, photons absorbed by these materials cause a charge transfer from the valence band to the conducting band of the semiconductor. The resultant increase in conductivity is measured and is related to the number of photons impinging on the surface of the photodiode. Many types of photodiodes are known, the specific properties of which are tailored to their particular applications (for details, see Chapter 9). Thus, different sizes, response times, and signal-to-noise levels have been achieved. Moreover, such devices are inexpensive however, as will be discussed later, they... 

Photocells and photomultipliers (secondary electron multipliers, SEM) are mainly i are detectors with an external photo-effect .-------------------------------... 

PMTs use the emission of photoelectrons from a photocathode, i.e. the extemal photo effect", as a primary step of detection. The drawback of the external photo effect is that the photoelectrons are emitted in all directions, including back into the photocathode. Therefore the quantum efficiency, i.e. the probability that a photon releases an photoelectron, is smaller than 0.5. The best cathodes reach a quantum efficiency of about 0.4 between 400 and 500 nm [214]. 

Standard equipment makes use of photodiodes working with the internal photo effect. The charge current is linear proportional to the number of photons falling onto the sensitive area. These photodiodes allow an arrangement in arrays. This type of detector is used in modem simultaneous detection systems (see Sec. 3.2.) and in chromatography [1,18]. Photomultipliers use the external photo effect. Incident radiation catapults out of the sensitive surface (photocathode) electrons which are multiplied (scintillation) and accelerated between dynodes on their way to the anode. Their detectivity is better and prices are higher [23]. 

Photodetectors are more sensitive sensors than the human eye for qualitative detection, as well as quantitative determinations. Photomultipliers are mostly employed, which have replaced photocells in commercial optomechanical instruments, and depend on the external photo effect and are evacuated photocells incorporating an amplifier. The photocurrent is amplified by a factor of 10 to 10 using secondary electrodes (dynodes). Various types of photomultipliers, e.g. side on or head on , can be employed. 

Planck s hypothesis was confirmed and developed by Einstein s theory of an external photo-effect. 

It should be noted immediately that not all the frequencies absorbed by a semiconductor are photocatalytically active, but only those that are also photoelectrically active, i.e., that cause an internal photoelectric effect in the semiconductor. Note further that the sign and magnitude of the photo-catalytic effect depend on the past history of the specimen exposed to illumination i.e., they depend on the external influences to which the specimen in question was subjected in the course of the whole of its life, and also on the conditions of the experiment (temperature, intensity of illumination, etc.). For example, by introducing into the semiconductor an impurity of any concentration or by adsorbing foreign gases on its surface it is possible to render its catalytic activity more or less sensitive to illumination. 

Magnetic field effects have been used to study the photo-Eries rearrangement of 4-methoxyphenyl acetate [49] and 1-naphthyl acetate [50], These effects can be subdivided into internal and external. 

In this case the quantity n indicates how many times the number of reactive particles adsorbed per unit surface increases under illumination (other external conditions remaining the same). Evidently, the rate of the heterogeneous reaction in which these particles participate will be a function of fi and thus will be sensitive to illumination. If An = Ap = 0 (photo-electrically inactive absorption of light), then according to (41) m = 1, and illumination has no effect on the reaction rate. 

Fig. 3. Like a photoelectrochemical cell, such a powder includes sites for photo-induced oxidation and reduction, but no external current flow accompanies these transformations. Photoactivity is also maintained as the size of the particle decreases to the colloidal range although the absorption characteristics, the quantum efficiency of charge separation, and the kinetics of interfacial electron transfer may be influenced by the particle size. On sufficiently small particles, for example, the calculated space-charge width necessary for effective band bending may exceed the dimensions of the particle.

More recently, the use of a pyridinium mediator in an aqueous p-GaP photo-electrochemical system illuminated with 365 nm and 465 nm light has been reported [125], In this case, a near-100% faradaic efficiency was obtained for methanol production at underpotentials of 300-500 mV from the thermodynamic C02/methanol couple. Moreover, quantum efficiencies of up to 44% were obtained. The most important point here, however, was that this was the first report of C02 reduction in a photoelectrochemical system that required no input of external electrical energy, with the reduction of C02 being effected solely by incident fight energy. 

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