Exciton Processes, Energy Conduction

Electronic excitation energy can be conducted within molecular crystals by excitons from one location to another. This is a very important and characteristic process in molecular crystals. In this section, we show how this energy conduction can be investigated and what consequences it has. See also [30]. 

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In an excitation process, the electron and the hole can remain bound, producing an exciton state just below the conduction band. Indeed, the mass of an inner hole is considered as infinite and the exciton binding energy is thus almost zero with reference to the absorption threshold energy. If the resonance lines were excitonic type transitions, the emission spectmm should be exactly the reverse of absorption. We would see that this is not the case although a localized excited Mjy state has a large probability of existing, sometimes the resonance Mjy lines are absent, whereas the resonance My lines are the most intense of the spectrum (77). 

Energy conduction processes by exdtons and various excitonic phenomena also play an important role in other molecular systems besides molecular crystals especially in biological systems. Molecular crystals provide an exemplary field from which a fundamental understanding of these processes can be gained. 

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. 

In a metal, there are excited states for electrons that lie below the ionization energy. This can be conceived as an electron in a "conduction band" and a "hole" that interact so that the combination is neutral but not of lowest energy. Such an excited state is called an exciton. Excitons may move by diffusion of the electron-hole pair or by transfer of a molecular exciton to another molecule. Reversion of the exciton to a lower energy state may be slow enough for the lifetime to be longer that of lattice relaxation processes. 

A wide range of condensed matter properties including viscosity, ionic conductivity and mass transport belong to the class of thermally activated processes and are treated in terms of diffusion. Its theory seems to be quite well developed now [1-5] and was applied successfully to the study of radiation defects [6-8], dilute alloys and processes in highly defective solids [9-11]. Mobile particles or defects in solids inavoidably interact and thus participate in a series of diffusion-controlled reactions [12-18]. Three basic bimolecular reactions in solids and liquids are dissimilar particle (defect) recombination (annihilation), A + B —> 0 energy transfer from donors A to unsaturable sinks B, A + B —> B and exciton annihilation, A + A —> 0. 

Alternatively, another process called excitation can occur by which a valence band electron is excited to an energy level lower than the conduction band. The electron remains bound to the hole in the valence band. This neutral electron-hole pair is called an exciton, and it can move through the crystal also. Associated with the exciton is a band of energy levels called the exciton band (see Fig. 18.19). 

Fig. 3.17 Schematic representation of some photophysical and photochemical processes in and on a semiconductor (SC) particle (for example Ti02). bg- Band gap energy VB valence band CB conduction band h electron hole ( defect electron ) in the valence band e photoelectron in the conduction band LT lattice trap ST surface trap A ds, Dads chemical species adsorbed on the surface of the SC particle with A being an electron acceptor and D an electron donor. Formation of an electron-hole pair (exciton) by irradiation SC-i-hv ecb + hvb (modified according to Serpone, 1996 and Bottcher 1991).

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