Biexciton emission

Apparently, in the near future there will be developed (a) a detailed theory of surface excitons not only at the crystal boundary with vacuum but also at the interfaces of various condensed media, particularly of different symmetry (b) a theory of surface excitons including the exciton-phonon interaction and, in particular, the theory of self-trapping of surface excitons (c) the features of surface excitons for quasi-one-dimensional and quasi-two-dimensional crystals (d) the theory of kinetic parameters and, particularly, the theory of diffusion of surface excitons (e) the theory of surface excitons in disordered media (f) the features of Anderson localization on a surface (g) the theory of the interaction of surface excitons of various types with charged and neutral particles (h) the evaluation of the role of surface excitons in the process of photoelectron emission (i) the electronic and structural phase transitions on the surface with participation of surface excitons. We mention here also the theory of exciton-exciton interactions at the surface, the surface biexcitons, and the role of defects (see, as example, (53)). The above list of problems reflects mainly the interests of the author and thus is far from complete. Referring in one or another way to surface excitons we enter into a large, interesting, and yet insufficiently studied field of solid-state physics. 

To explain the held dependence, we have to assume that the applied held has an influence on v hether the spin selection rules can be obeyed in the fusion process. A kinetic explanation by Merrifleld and Johnson [49, 50] assumes in the reaction Ti + Ti (Ti Ti) Si + So the existence of an intermediate pair state (Ti Ti), in v hich the tv o excitons repeatedly collide before they react. The possible spin correlations in this pair state have both triplet as well as singlet character. The triplet fraction in the pair state is also influenced by an applied magnetic field via the Zeeman interaction of the coupled individual spins with the field. The strength and direction of the field thus determine the relative fraction of triplet and singlet states in the pair. The singlet fraction leads to the states Si and So, and thus to delayed emission. Therefore, the intensity and lifetime of the emission can be modulated by an applied magnetic field. This holds for all biexcitonic processes in which two triplet states participate. 

FIGURE 22.4 Energy levels, optical transitions, and emission bands associated with singlet and triplet excitons and PP excitations, respectively. The symbols lAg, IBu, mA, and BX are the ground state, lowest allowed exciton, most strongly coupled even-parity exciton, and biexciton level, respectively T is the lowest triplet level Full and... 

In the intermediate excitation density regime emissions due to biexcitonic, exciton-exciton, and exdton-carrier interactions may be observed. The inelastic collision between the exdtons results in an exciton exdted into a higher state and a photon with energy [145] ... 

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