Exciton hopping process

Such a mechanism would be consistent with the absence of reaction by a two-photon absorption process, because such a process does not produce excitons in high concentration and, therefore, reduce blexcitonic process. The temperature effect is simply a manifestation of reduced exciton hopping at lower temperature. 

Figure 1. One-exciton band (energy vs wave vector) for a 15-srte lattice described by the Hamiltonian in Eq. (1), with only nearest-neighbor exciton hopping, 7, and constant intermolecular spacing, r = 1. Left and right panels refer to negative and positive 7, respectively. The arrows mark one-photon absorption and emission processes. The emission process for 7 > 0 is forbidden...

Recent work by Venikouas and Powell (1977) has provided a more quantitative insight in the energy transfer processes in YVOr-Eu . At low temperatures there is only acting an (inefficient) one-step transfer process from vanadate to Eu . Transfer within the vanadate host lattice is unimportant. At higher temperatures, however, thermally activated exciton hopping occurs yielding an efficient Eu phosphor at room temperature. Activator-induced host traps play an important role in the transfer process to the Eu ion. 

Scheme 3 summarizes this problem with a minimum number of sites and competing processes. In this scheme, two sites, square-well type (X) and spherical-well type (Y), are available for the residence of reactant molecules (A). For the sake of convenience, molecules residing at sites X and Y are labeled Ax and AY. Excitation of these molecules gives rise to A and A. Photoreactivity of molecules excited in each site will be identical if they equilibrate between X and Y before becoming photoproducts. In media with time-independent structures, such as crystals, equilibration requires diffusion of molecules of A in media with time-dependent structures, such as micelles and liquid crystals, equilibration can be accomplished via fluctuations in the microstructure of the reaction cavities as well as translational motion of A (Scheme 4). An additional mechanism for site selective reactions or equilibration of A and A molecules can be achieved via energy migration (e.g., energy hopping, exciton migration, or Forster energy transfer).

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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