Analysis of the electronic structure in Cs halides by reflectance and thermoreflectance studies

Measurements of the reflectance and thermoreflectance of CsI, CsBr, and CsCl in the 5-9-eV region at several temperatures are reported. The spectra are compared and analyzed with particular reference to their dependence on temperature and chemical composition. The experimental results show that the principal features of the Onodera's energy-band system for CsI might be valid in the case of CsBr and CsCl, too, taking into account relativistic effects. Through an analysis of line shapes and temperature coefficients, the complex structure of the fundamental optical spectra of the three compounds can be explained by excitons associated with the $s$-like and $d$-like conduction bands at the $\Gamma$ and $M$ points of the Brillouin zone.