Abstract
Absorption, luminescence and excitation spectra of LiI crystals have been observed in a wide temperature range. Being extremely hygroscopic, LiI crystals were cleaved in liquid helium inside a cryostat before optical measurements. With this method of crystal cleavage, the luminescence spectrum of LiI is drastically changed. Experimental results have led to following conclusions: (1) the steepness constant of the exciton absorption (Urbach) tail is 0.87, (2) at least one self-trapped exciton state stably exists, (3) from the state one luminescence band peaking at 3.4 eV occurs and (4) the potential barrier between the free and the self-trapped exciton states is 55 meV. A possibility of the self-trapped exciton state in LiI to be of three-center type is discussed.
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