Thermoreflectance in Alkali Halides

We present room-temperature thermoreflectance spectra of NaCl, KCl, NaBr, KBr, RbBr, KI, and RbI crystals. Extension of these measurements at 90^∘{}K for RbI has put in evidence the Rydberg series up to $n=4$ of the Γ₁ exciton. It has been possible, then, to determine the energy gap (EG=6.36 eV), the exciton binding energy (E₀=0.56 eV), and the electron effective mass (mₑ*0.57mₑ) for RbI.