Exciton formation/trapping

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

For Ag, the decay time values were found similar to those reported in ref. [1, 2] providing information about the electron-phonon scattering. For Fe203, several other phenomena could cause the OD changes at the ultrafast time scale. The sub-picosecond and picosecond decay times allow to take into account hot electron thermalization [4] and subsequent fast relaxation processes such as exciton formation or surface traps filling [6]. 

The STH resonance is enhanced in ODMR spectra by the addition of parts per million of known electron trapping dopants such as Ir3+ [176], Ni2 + [177], and Rh3+ [178]. This is probably because these dopants enhance distant pair formation at the expense of exciton formation [179,180]. Extrinsic self-trapped hole species have also been observed in the ODMR spectra of AgClj Br samples (see below) [111, 181,182],... 

Besides differing efficiencies of hole and electron injection at the electrode/CP interface, hole and electron mobilities within the CP also differ substantially, with electron mobilities being impeded through trapping by impurities such as oxygen. Additionally, if electron-hole recombination occurs near one of the electrodes, quenching is more likely to occur. Thus, it is preferred to somehow be able to keep electron-hole capture, i.e. exciton formation, away from the electrodes and in the interior of the device. 

The EE and phE mechanisms for neat polymers proposed by ourselves and others all involve the consequences of breaking bonds during fracture. Zakresvskii et al. (24) have attributed EE from the deformation of polymers to free radical formation, arising from bond scission. We (1) as well as Bondareva et al. (251 hypothesized that the EE produced by the electron bombardment of polymers is due to the formation of reactive species (e.g., free radicals) which recombine and eject a nearby trapped electron, via a non-radiative process. In addition, during the most intense part of the emissions (during fracture), there are likely shorter-lived excitations (e.g., excitons) which decay in a first order fashion with submicrosecond lifetimes. The detailed mechanisms of how bond scissions create these various states during fracture and the physics of subsequent reaction-induced electron ejection need additional insight. 

Compared with the momentum of impinging atoms or ions, we may safely neglect the momentum transferred by the absorbed photons and thus we can neglect direct knock-on effects in photochemistry. The strong interaction between photons and the electronic system of the crystal leads to an excitation of the electrons by photon absorption as the primary effect. This excitation causes either the formation of a localized exciton or an (e +h ) defect pair. Non-localized electron defects can be described by planar waves which may be scattered, trapped, etc. Their behavior has been explained with the electron theory of solids [A.H. Wilson (1953)]. Electrons which are trapped by their interaction with impurities or which are self-trapped by interaction with phonons may be localized for a long time (in terms of the reciprocal Debye frequency) before they leave their potential minimum in a hopping type of process activated by thermal fluctuations. 

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