Radiationless annihilation,

The positron was subsequently discovered by Anderson (1933) in a cloud chamber study of cosmic radiation, and this was soon confirmed by Blackett and Occhialini (1933), who also observed the phenomenon of pair production. There followed some activity devoted to understanding the various annihilation modes available to a positron in the presence of electrons radiationless, single-gamma-ray and the dominant two-gamma-ray processes were considered (see section 1.2). The theory of pair production was also developed at this time (see e.g. Heitler, 1954). 

Fig. 1.1. Feynman diagrams of the lowest order contributions to (a) radiationless, (b) one-gamma, (c) two-gamma, (d) three-gamma-ray annihilation. A2+ and A+ denote the charge states of the remnant atomic ion.

This selection rule does not appear to exclude radiationless annihilation and annihilation into a single gamma-ray, but these modes of annihilation are nevertheless forbidden for free positronium. 

Shimizu, S., Mukoyama, T. and Nakayama, Y. (1965). Search for radiationless annihilation of positrons. Phys. Lett. 17 295-296. 

If the exothermicity of the annihilation of the given ions is still smaller than the energy of the excited triplet states, the reaction is generally not of interest, although it has been shown that such systems may still produce light. This last case, however, corresponds formally to radiative electron transfer from R to R+ (the E-route). It should be described in terms of the competition between radiative and radiationless transition in the inverted Marcus region. 

The creation of the core hole should be taken into account in studying the process of its annihilation if the electron generated by the core level ionization is involved in the annihilation of the created hole. That is to say, the excited atom goes into an intermediate state with a core hole and a secondary electron. Next, the radiationless annihilation of the hole created occurs with the participation of the same secondary electron and an electron of the valence state. The amplitude of... 

With the intensities obtained in Eqs. (84)-(86) we evaluate the probability of hole annihilation due to the second-order process. The probability of the ionization of the atom by an incident electron is determined by the corresponding integral intensity of the first-order process. An estimate of this probability has been made already. Then the probability of the radiationless annihilation of the electron-hole pair created in the atom upon interaction with the incident electron is determined by the ratio between the integral intensities of the first- and second-order processes, Ja /Ja This probability is determined as /Ja 0( rianpC, up to a constant that depends only slightly on the type of the wave function of the core electron. With the expressions obtained, we have determined the value of the constant of proportionality 0.6,0.4, and 0.5 for... 

Spectroscopic experiments pronounce it as delayed fluorescence. In such an experiment, a species is first excited by pulse irradiation to the 5i state, and emission is delayed until all of the 5i states have decayed. This decay process may occur either by radiating to the ground state or by radiationless processes such as intersystem crossing to the T state, emission after such a delay results from transformation of the triplets to excited singlets. Note that the spectroscopy of delayed fluorescence is not observed for some cases. Though, such systems can be dealt by the formation of triplets in ECL on account of adjusting the energetics of annihilation reaction, where TTA is then observed. Thus, states are sometimes accessible in ECL that is not available spectroscopically [3]. 

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