Optical properties and electronic structure of alkali halides by thermoreflectivity

We report detailed measurements of thermoreflectivity spectra of NaCl, KCl, RbCl, NaBr, KBr, RbBr, NaI, KI, and RbI in the 5-9-eV region at several temperatures. Through an analysis of the thermoreflectivity line shapes and temperature coefficients, we try to find a general interpretation of the optical-spectra structures for all crystal families in terms of $\Gamma$, $L$, and $X$ transitions, on the basis of present understanding of band calculations. The spin-orbit splitting of the $\Gamma$ exciton is determined to be 0.13 ± 0.02 eV in chlorides, 0.49 ± 0.02 eV in bromides, and 1.10 ± 0.02 eV in iodides.