Exciton scattering-related

In high-quality ZnO epitaxial films grown by MBE on GaN templates, biexciton formation was also reported and radiative recombination of the biexciton state was observed at 3.35 eV for excitation densities lower than the threshold for exciton-exciton scattering-related SE [156]. The spectral shape of the biexciton PL, Jxx( ). is generally expressed by an inverse Boltzmarm distribution function. 

Even when the intramolecular vibrations do not mix with the phonons, they are coupled by the intermolecular potential, through its dependence on the internal molecular coordinates. This coupling determines the band structure of the vibrational excitons or vibrons in molecular crystals, which can be studied by inelastic neutron scattering (see Fig. 12). In the case of TCB crystals the vibron band structure has been observed recently by laser phosphorescence [59, 104]. This is a rather special achievement since, normally, optical techniques probe only the g = 0 excitations. It is related to the long lifetime of triplet electronic excitons in TCB, which are scattered into different q states. By phosphorescent de-excitation q 0 triplet excitons to the electronic ground state it is possible to detect the g 0 vibrons, with intensities controlled by the (very low) temperature and by the phosphorescence delay time. 

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