Interpretation of Exciton Induced Luminescence in KI/T1

Details of the exciton induced luminescence in KI/T1 1) are discussed. It is pointed out that the absorption coefficient at the long wavelength tail of the first characteristic absorption band of a single crystal of KI, is much smaller than that of an evaporated polycrystalline film of KI, and then the complex shapes of excitation spectra of the exciton induced luminescences from T1 + ion and a certain impurity are explained. Furthermore, a new weak absorption band is found in the long wavelength tail of the first exciton absorption band, which is infered to be due to the triplet-like state of the exciton. Some discussions are given on the problem of migration of excitons and the life time of the exciton is estimated to be much longer than 10 -8 sec.