Excitons high-lying states

Because of their special polymerization process [136], PDA crystals should not contain any dopant or associated residual polaron or bipolaron concentration. These negative results give some constraints on other low-lying states, especially exciton states. Triplet-state absorption from the ground state would probably be too strongly spin-forbidden to have absorption coefficients as high as 1 cm-1 [129]. As for g states, the absorption coefficient depends sensitively on how far from the intense absorption they are (for the polyene case, see Ref. 127). That they are not found in these experiments means that they are either above the main transition at 2 eV or not far below it and buried in its tail. Indeed, evidence of a weak absorption = 0.1 eV below the main transition has recently been found at low temperature [118] it would be buried in the absorption tail at higher temperatures. 

With a sufliciently high total energy, this process can even cause the ejection of an electron from the crystal that is, the exciton annihilation leads to ionisation. In the case of anthracene, the ionisation limit of 5.75 eV lies lower than twice the Si energy, 2x3.15 = 6.30 eV. Measurement of the kinetic energy of the emitted photoelectrons permits the verification of the fusion process. In general, the fusion of two excitons allows higher excited states to be reached with smaller energy quanta. Table 6.4 contains numerical values of the rate constants for exciton annihilation processes. 

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