Thermoreflectance in Alkali Halides

Abstract

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 $n=4\mathrm{}$ of the ${\Gamma }_1$ exciton. It has been possible, then, to determine the energy gap ($E_G=6.36\mathrm{}$ eV), the exciton binding energy ($E_0=0.56\mathrm{}$ eV), and the electron effective mass (${m_e}^{*}\cong 0.57m_e$) for RbI.