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\ifmmode^\circ\else\textdegree\fi{}K for RbI has put in evidence the Rydberg series up to $n=4$ of the ${\ensuremath{\Gamma}}_{1}$ exciton. It has been possible, then, to determine the energy gap (${E}_{G}=6.36$ eV), the exciton binding energy (${E}_{0}=0.56$ eV), and the electron effective mass (${{m}_{e}}^{*}\ensuremath{\cong}0.57{m}_{e}$) for RbI.