Recombination processes, free

In an electron scattering or recombination process, the free center of the incoming electron has the functions Wi = ui U u, and the initial state of the free elechon is some function v/ the width of which is chosen on the basis of the electron momentum and the time it takes the electron to aiTive at the target. Such choice is important in order to avoid nonphysical behavior due to the natural spreading of the wavepacket. 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

In conclusion we may state that there is evidence for multiple ion-pair recombination in spurs yet a theoretical analysis of free-ion yield and scavenging at low-LET based on the geminate ion-pair picture is meaningful in view of the similarity of the recombination process in the geminate and multiple ion-pair cases. However, if this analogy holds, the geminate ionization yield has to be somewhat less than the true ionization yield. 

One of the most important experimental methods of studying the electron-ion recombination processes in irradiated systems are measurements of the external electric field effect on the radiation-induced conductivity. The applied electric field is expected to increase the escape probability of geminate ion pairs and, thus, enhance the number of free ions in the system, which will result in an enhanced conductivity. 

In the limit of an infinitely long mean free path, the bulk electron-ion recombination may be described using the energy diffusion model [43,44]. This model is especially relevant to the electron-ion recombination processes in the gas phase. 

The chain fragments formed by the recombination of free radicals can be reconverted into radicals by a variety of reinitiation processes, some of which are listed in Table 1. Such reactions can occur in the gas phase via electron collision and on the polymer surface by impact of charged particles or photon absorption. Reinitiation may also be induced in both the gas phase and on the polymer surface by hydrogen transfer reactions. These last processes are similar to the chain transfer processes which occur during homogeneous polymerization. Expressions for the rates of reinitiation are given by Eqns. 20 through 23. 

As the temperature of the semiconductor is increased electrons are thermally removed from interatomic bonds to the conduction band, simultaneously creating a positive hole and a free electron. Under the influence of an applied potential, the hole and electron move away from each other in opposite directions giving rise to an electric current. Occasionally an electron and a hole will meet in a recombination process and the electron will fall back into the interatomic bond. Upon heating to a sufficiently high temperature any insulator is expected to show this intrinsic conduction behavior. 

The recombination process triggered by the initial scissions made by exoV and mediated by the recA enzyme are not sufficient for recombination between interacting chromosomes. An additional enzyme, resolvase, is required for efficient separation of the recombining chromosomes in the recA-chromosome complex. This protein is encoded by the ruvC gene in E. coli and is absolutely required for homologous recombination in the bacterium. Its effectiveness has also been demonstrated in a cell-free in vitro system. 

Fig. 8. Outline of the time-scale of the processes observed during an electron transfer reaction observed through thermal lensing. Processes which occur in times below ca. 0.5 ps are very fast , beyond the temporal resolution of the thermal lensing technique they would appear as a step function in the kinetics of heat release. The slowest processes which would be observed in this case are the second-order recombinations of free ions, which take place in time scales of ps to several ms.

Many experimental data concerning the M.I.R. pertain to the back e.t. between geminate ions, prior to separation in polar solvents [53]. In some cases direct spectroscopic measurements can be made, although these depend on the correctness of the assignment of the absorption spectra. In principle the geminate ion recombination is expected to follow first order kinetics, whereas the diffusional recombination of free ions follows second order kinetics, so there is here another possibility to distinguish between the two processes [54], In some cases, the rate... 

It should be noted that in the above examples significant energy migration can occur between thermal stimulation and recombination since the electron is free to diffuse throughout the sample. Other electron-hole recombination processes occur, however, in which the excited states of the charge carrier are not to be found in the delocalized bands. An example of this, recently noted in the literature, concerns the TL emission from oligoclase feldspar in which the thermal stimulation involves the transition of the electron to a localized, intermediate state - see Figure 8... 

In the absence of any deliberately added solution phase charge scavenger, the majority of trapped holes on Ti02 particles recombine with free conduction band or trapped photogenerated electrons however, a small fraction of the (>Ti-OH )+ sites do react to form thermodynamically stable >TiOOH or >TiOOTi< sites (vide infra) [119]. Measurement of the rate constants for these processes by photoelectrophoresis has already been reviewed elsewhere [47]. 

The recombination width can be minimized by the confinement of the recombination process at the interface of two organic materials as typically occurs in double- and multi-layer organic LEDs [2] (see also Chapter 5). The penetration depths of holes and electrons can then be identified with thicknesses of hole and electron transporting layers, respectively, and their mobilities used to calculate the recombination width. A good example of such a situation is the recombination process in the most studied double-layer LED, ITO/TPD/Alq3/Mg/Ag. The free carrier kinetics at the TPD/Alq3 interface after an abrupt switch of the voltage off takes on a simple form... 

This resemblance is highly significant if one considers that 10,359 structural isomers exist for saturated hydrocarbons with 16 C atoms (Lederberg, 1972). Apparently the meteoritic hydrocarbons were made by FTT reactions, or some other process of the same extraordinary selectivity. The Miller-Urey reaction, incidentally, shows no such selectivity. Gas chromatograms of hydrocarbons made by electric discharges in methane show no structure whatsoever in the region around Cjg (Ponnamperuma et al., 1969). Apparently all 10 possible isomers are made in comparable yield, as expected for random recombination of free radicals. 

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