Electrons annihilation process

At present it is universally acknowledged that TTA as triplet-triplet energy transfer is caused by exchange interaction of electrons in bimolecular complexes which takes place during molecular diffusion encounters in solution (in gas phase -molecular collisions are examined in crystals - triplet exciton diffusion is the responsible annihilation process (8-10)). No doubt, interaction of molecular partners in a diffusion complex may lead to the change of probabilities of fluorescent state radiative and nonradiative deactivation. Nevertheless, it is normally considered that as a result of TTA the energy of two triplet partners is accumulated in one molecule which emits the ADF (11). Interaction with the second deactivated partner is not taken into account, i.e. it is assumed that the ADF is of monomer nature and its spectrum coincides with the PF spectrum. Apparently the latter may be true when the ADF takes place from Si state the lifetime of which ( Tst 10-8 - 10-9 s) is much longer than the lifetime of diffusion encounter complex ( 10-10 - lO-H s in liquid solutions). As a matter of fact we have not observed considerable ADF and PF spectral difference when Sj metal lo-... 

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

Pair production has a threshold energy of 1.022 MeV because two particles are created, one electron and one positron. Thus, some energy is stored in or used to create the mass of the pair. Notice the total electric charge is conserved because the electron charge is — le and the positron charge is +le. One of the unique features of this process is that the energy that went into the creation of the two particles will be released when the positron comes to rest and annihilates with an electron. The annihilation process is... 

The total positron scattering cross section, erT, is the sum of the partial cross sections for all the scattering channels available to the projectile, which may include elastic scattering, positronium formation, excitation, ionization and positron-electron annihilation. Elastic scattering and annihilation are always possible, but the cross section for the latter process is typically 10-2O-10-22 cm2, so that its contribution to erT is negligible except in the limit of zero positron energy. All these processes are discussed in greater detail in Chapters 3-6. 

The wave function of the ion that remains after annihilation is a superposition of eigenstates of the Hamiltonian of the ion, the relative probabilities of which may be determined from the wave function used in the calculation of Zeg. The annihilation process takes place so rapidly, compared with normal atomic processes, that it is reasonable to assume the validity of the sudden approximation. Consequently, the wave function of the residual ion when the positron has annihilated with electron 2 at the position r = r2 is... 

Recent progress in the study of positron scattering and positron annihilation processes is reviewed in Refs. [11,170-172]. Experimentally, the positron sources from radioisotopes and from electron accelerators are quite weak and have a broad spectrum of energies as compared with electron sources, making precise measurements of positron collision processes extremely difficult. Such measurements, however, have become possible recently due to the... 

V. Nucleophilic and Electron-Transfer Processes in Ion-Pair Annihilation.. 96... 

NUCLEOPHILIC AND ELECTRON-TRANSFER PROCESSES IN ION-PAIR ANNIHILATION... 

The generalized mechanism for ion-pair annihilation as presented in Scheme 11 involves the rather circuitous route for radical-pair production [involving Eqs. (55) and (56), certainly in comparison with the direct electron-transfer pathway (Scheme 8)]. In other words, why do ion pairs first make a bond and then break it, when the simple electron transfer directly from anion to cation would achieve the same end The question thus arises as to whether electron transfer between Fe(CO)3L+ and CpMo(CO)3 is energetically disfavored. The evaluation of the driving force for the electron transfer process obtains from the separate redox couples, namely,... 

The survey of the investigations and results covers the release of water from salts and hydroxides, the calcination of carbonates and oxalates, the reactions of metallic oxides and carbonates with SO2, and reactions on the surface of carbon. The application of the non-isothermal method to the thermal decomposition of carboxylic acids and polymeric plastics as well as to the pyrolyses of natural substances, in particular bituminous coal, is explained. Finally, chemical reactions in a liquid phase, the desorption of gases from solids, annihilation processes in disturbed crystal lattices and the emission of exo-electrons from metallic surfaces are discussed. 

The first anti-particle discovered was the anti-electron, the so-called positron, in 1933 by Anderson [3] in the cloud chamber due to cosmic radiation. The existence of the anti-electron (positron) was described by Dirac s hole theory in 1930 [4], The result of positron—electron annihilation was detected in the form of electromagnetic radiation [5]. The number and event of radiation photons is governed by the electrodynamics [6, 7]. The most common annihilation is via two- and three-photon annihilation, which do not require a third body to initiate the process. These are two of the commonly detected types of radiation from positron annihilation in condensed matter. The cross section of three-photon annihilation is much smaller than that of two-photon annihilation, by a factor on the order of the fine structure constant, a [8], The annihilation cross section for two and three photons is greater for the lower energy of the positron—electron pair it varies with the reciprocal of their relative velocity (v). In condensed matter, the positron—electron pair lives for only the order of a few tenths to a few nanoseconds against the annihilation process. 

Problem 1.1 Anti-matter and matter interact via the annihilation process, which is represented by a delta function or zero distance between them (see Chapter 2), while the electromagnetic interaction is represented by the reciprocal of the distance. Electromagetically, the positron attracts an electron it is suggested to call the positron as the counter electron analogy to a counter ion to an ion from chemistry perspective. 

In this section we consider annihilation at two distinct levels. First we examine the essence of the annihilation process itself, starting from one electron and one positron and ending with two photons. Next we consider the process that precedes it, in which the annihilating pair meet each other under the influence of a many-electron environment and the manifold of energy levels provided by an atom or molecule. Finally the relaxation of the post-annihilation system is discussed. 

Almost a linear dependence between pore size and positrons lifetime can be observed which was not clearly obtained in previous studies. This relationship is expected because when the pores are wider the probability of interaction between the positrons and the surface electron density in the pore walls decreases. This results in a lower rate of positrons annihilation with the surrounding electrons and then a higher lifetime. A simple model for the annihilation process can be constructed assuming that the positron is trapped in a spherical pore of radius R of constant potential. The resolution of the Schroedinger equation shows that the lifetime of positrons is a function of R [5]. 

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