Excitonic Effects in the Electroreflectance of Lead Iodide

The reflectance and electroreflectance spectra of single crystals of lead iodide (Pb${\mathrm{I}}_2$) were measured between 2.2 and 5.0 eV at 4.5 and 77°K, respectively. A Kramers-Kronig dispersion analysis of the data was made, and the real and imaginary parts of the dielectric constant were calculated together with their field-induced modulation. The first three lines of the hydrogenic exciton series associated with the absorption edge are well resolved in reflectivity. In the same energy range, the electoreflectance spectrum is interpreted as the Stark shift of the first exciton line from which the binding energy of the exciton is deduced. In the region of fundamental absorption, the existence of several metastable exciton states gives rise to a new and large contribution to the electroreflectance spectra.