Temperature and Pressure Dependence of the Optical Absorption and Luminescence of a Defect in TlCl

The luminescence associated with an absorption band at about 3.0 eV in chalcogen-doped TlCl has been observed. The temperature and pressure coefficients of the positions of the absorption and emission peaks have been found to have "anomalous" negative signs. The temperature coefficient of the absorption peak is dominated by the effect of thermal expansion, the band shifting toward the blue as the lattice expands. The emission peak exhibits very little temperature dependence. Some arguments are presented in favor of interpreting this band as an exciton band associated with an anion vacancy ($\alpha$ band) instead of an $F$ band. Lattice relaxation may cause the emission to occur at $\mathrm{Cl}_2^{}{}_{}{}^{-}$ molecule-ions near the vacancy.