Self-trapped exciton in LiI

Measurements of absorption and luminescence in LiI crystals have been carried out as a function of temperature. Being extremely hygroscopic, LiI crystals are cleaved in liquid helium inside a cryostat before optical measurements. A drastic change of luminescence spectrum by the sample cleavage is found. From these results, it is concluded that the self-trapped exciton (STE) state in LiI is stable, and from the state one luminescence band peaking at eV is emitted. The STE state in LiI is likely to be of three-center type (I 3 +e) rather than of two-center type (V k +e).