Electron recombination

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

The hole current in this LED is space charge limited and the electron current is contact limited. There are many more holes than electrons in the device and all of the injected electrons recombine in the device. The measured external quantum efficiency of the device is about 0.5% al a current density of 0.1 A/cm. The recombination current calculated from the device model is in reasonable agreement with the observed quantum efficiency. The quantum efficiency of this device is limited by the asymmetric charge injection. Most of the injected holes traverse the structure without recombining because there are few electrons available to form excilons. 

Electrostatic fields applied slightly above the onset of the saturation region had little effect on product distribution (37). This presumably indicates that homogeneous ion-electron recombination is the same as neutralization at the electrodes, or that one is simply not observing the products of neutralization in either case. 

The low concentration and high energy of the electrons in this region reduces the probability of ion-electron recombination processes. 

In the planning of a FAMS for operation at 1.0-atm pressure, the advantages of turbulent flow as summarized above would be extremely useful. The radial mixing of neutrals added to the tube would be very fast, and ions within the turbulent core would be protected from contact with the wall. Under these conditions, ion-ion or ion-electron recombination reactions, alone, would provide the only physical... 

Even though the TRAPI and the PHPMS both involve a pulsed high-pressure ion source, they are fundamentally different methods in that they are based on different underlying principles. In the TRAPI method, ions are transported through the sampling aperture by convective flow with the buffer gas, rather than by diffusion through the buffer gas. Also, second-order ion-ion or ion-electron recombination... 

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

In contrast to liquid water, a detailed mechanistic understanding of the physical and chemical processes occurring in the evolution of the radiation chemical track in hydrocarbons is not available except on the most empirical level. Stochastic diffusion-kinetic calculations for low permittivity media have been limited to simple studies of cation-electron recombination in aliphatic hydrocarbons employing idealized track structures [56-58], and simplistic deterministic calculations have been used to model the radical and excited state chemistry [102]. While these calculations have been able to reproduce measured free ion yields and end product yields, respectively, the lack of a detailed mechanistic model makes it very difficult... 

The stronger absorption at lower temperatures can be attributed to several factors. One is that the equilibrium between quasi-free and trapped electrons shifts to favor trapped electrons as the temperature is lowered for these liquids. Another is that homogeneous recombination of electrons with positive ions is slower at the lower temperatures and therefore occurs to a lesser extent during the pulse, which, for the pulse radiolysis studies, was typically 10 to 20 nsec. The rate constant, k, for electron recombination with positive ions, in most nonpolar liquids, is given by ... 

The breakdown of the diffusion theory of bulk ion recombination in high-mobility systems has been clearly demonstrated by the results of the computer simulations by Tachiya [39]. In his method, it was assumed that the electron motion may be described by the Smoluchowski equation only at distances from the cation, which are much larger than the electron mean free path. At shorter distances, individual trajectories of electrons were simulated, and the probability that an electron recombines with the positive ion before separating again to a large distance from the cation was determined. The value of the recombination rate constant was calculated by matching the net inward current of electrons... 

In nonpolar materials, most of the electrons recombine with the counter cation radicals as shown by Eq. (46). RH is one of the main products produced through the recombination. 

Consider the hole movement first In the //-type of material, holes are generated when electrons from the valence bandjump to acceptor atoms. These holes can random walk across the junction into the //-type of material [Fig. 7.21(b)]. Conversely, holes from thep side can random walk into the //-type of material, where they are consumed in a hole-electron recombination process (the reverse of a hole-generation process). Both electrons and holes have considerable mobility (Table 12). 

Fig. 14. Some Auger processes involving one-free carrier (boles as illustrated) The case of two trapped electrons on the same center is shown in (a), and the situation for trapping on nearby centers is shown in (b). The case of an exciton (isoelec-tronic) type center, with electron recombination to the trapped hole is shown in (c), and recombination with a free hole in (d) [note that in practice these two processes have to be considered in parallel (see, for example, Neumark, 1973)].

However, under some circumstances, the rate is not only leveled but it is also inhibited as the substrate concentration increases. In a comprehensive study on 3-chloro-4-hydroxybenzoic acid and chlorophenols, Cunningham and colleagues [35] deduced that the rate can be strongly inhibited in consequence of (1) chemisorption-induced depletions of surface —OH groups, (2) adsorbate-enhanced hole-electron recombination on the TiO2 surfaces, (3) mass transport limitations within the TiOz particle aggregates. They reported that, depending on... 

Semiconductor band-gap luminescence results from excited electrons recombining with electron vacancies, holes, across the band gap of the semiconductor material. Electrons can be excited across the band gap of a semiconductor by absorption of light, as in photoluminescence (PL), or injected by electrical bias, as in electroluminescence (EL). Both types of luminescence have been used in chemical sensing applications [1,3]. 

Changes in intensity of semiconductor PL or EL can be used to detect molecular adsorption onto semiconductor surfaces [1,3]. PL occurs most efficiently when ultra-band-gap radiation excites electrons from the valence band to the conduction band of a direct-band-gap semiconductor and the electrons recombine radiatively with the holes left behind in the valence band. 

C) Recombination luminescence. Still another kind of delayed emission observed in dye solutions in low temperature glasses is by cation-electron recombination mechanism. Under high intensity irradiation and in rigid media, a dye molecule can eject an electron which is trapped in suitable sites. When this electron recombines with the dye cation, an Sj state is generated and a photon is emitted as fluorescence. 

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