Excitonic processes

Excitons can, as we have seen in Sect. 6.9.2, react with one another and also with other excitation states. The most important processes are exciton annihilation or fusion and exciton splitting or fission. The annihilation of two triplet excitons, which leads to delayed fluorescence, takes place more precisely via the alternate reaction mechanism 

the symbols T and S each refer to a particular vibronic state of an electronically-excited triplet or singlet state, so that energy conservation is fulfilled. 

In order to obtain the dependence of the phosphorescence intensity Ip and the intensity of delayed fluorescence, lup, on the incident power Is or the absorption coefficient a, the kinetics of the triplet state must be investigated. From the solution of the balance equations for mono- and bimolecular decay, one finds for the steady state in the limiting cases of strong and weak excitation the following expressions [32]  

Studies of the delayed fluorescence are usually carried out in the range of strong excitation. The phosphorescence is in this case unimportant. 

Depending on the excitation density [Ti], either the monomolecular depopulation (-fer[Til) directly from Tj or the bimolecular (-y,ot [I i] ) da armihilation predominates. 

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M. Ueta, H. Kanzaki, K. Kobayashi, Y. Toyozawa and E. Manamura, Excitonic Processes in Solids (Springer, Berlin, 1986). 

Conductivity phenomena should not be considered in isolation the interplay between these and excitonic processes underlying photoconduction and luminescence must be taken into account. The reader should, therefore, possess a prior knowledge of those aspects of solid-state science that deal with periodic structures. Adequate explanations are given in numerous recent reviews and texts dealing with the band theory of solids, in general, and excitonic behaviour... 

Absorption of the X-ray makes two particles in the solid the hole in the core level and the extra electron in the conduction band. After they are created, the hole and the electron can interact with each other, which is an exciton process. Many-body corrections to the one-electron picture, including relaxation of the valence electrons in response to the core-hole and excited-electron-core-hole interaction, alter the one-electron picture and play a role in some parts of the absorption spectrum. Mahan (179-181) has predicted enhanced absorption to occur over and above that of the one-electron theory near an edge on the basis of core-hole-electron interaction. Contributions of many-body effects are usually invoked in case unambiguous discrepancies between experiment and the one-electron model theory cannot be explained otherwise. Final-state effects may considerably alter the position and strength of features associated with the band structure. 

Therefore, at least in some CPs, the lowest singlet exciton is a g state. If recombination in such a CP is by the exciton process, no EL, or only a very weak one, is expected. The case of direct (nonexciton) recombination has not been considered yet, but one may expect that if a low-lying g state exists, it will be formed by recombination just as well, and no EL will be emitted. 

Ueta M., Kanzaki H., Kobayashi K., Toyozawa Y. and Hanamnra E. (1986, eds.), Excitonic Processes in Solids, Springer-Verlag, Berlin. 

Kim and S.E. Wehher, Effect of molecular weight on triplet exciton processes. 4. Delayed emission of solid poly(2 vinylnaphthalene), Macromolecules 13, 1233 (1980). 

In polymers, also, excitons are frequently the lowest-lying excited states. They can therefore play an important role for photoprocesses in polymers. This is also true of the key substance of genetics, the DNA helix. Here, again, the excitation energy can be conducted to reactive side groups via excitonic processes. 

We initially restrict ourselves to the simplest process the linear intrinsic photogeneration of charge-carrier pairs. Their production rate is proportional to the absorbed intensity of photons of the excitation light in the crystal, and requires neither excitonic processes at the crystal surface nor at the contacts, nor does it involve biexcitonic processes. 

The first report on coordination of exciton processes with organometallic polymers was for polymers 33 and 61 [38]. The model compounds are the corresponding M(CN-t-Bu) " complexes (M = Cu (62), Ag (63)), assuming that the steric and electronic environment are comparable to the corresponding polymers 33 and 61 as illustrated in Eigure 4.14. The emission maxima for 62 and 63 are 490 and 474nm, respectively (solid state at 77 K), comparatively at 517 and 500 nm for 61 and 33, respectively. The 26-27 nm red shifts indicate the presence of interactions in the triplet excited states. 

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