Optical spectra and electronic structures of lead halides

Polarized reflectivity spectra of single crystals of $\alpha -{\mathrm{PbF}}_2,$ $\beta -{\mathrm{PbF}}_2,$ ${\mathrm{PbCl}}_2,$ and ${\mathrm{PbBr}}_2$ have been measured at 10 K in the energy range up to 30 eV using synchrotron radiation as a light source. The spectra are well resolved in comparison with those reported previously. Optical constants have been derived through the Kramers-Kronig analysis. The effect of phase transition on the optical spectra of ${\mathrm{PbF}}_2$ is also examined. With use of the discrete variational $X\alpha$ method, the electronic structures of the clusters consisting of a ${\mathrm{Pb}}^{2+}$ ion and neighboring halogen ions have been calculated for better understanding of the optical properties of Pb halides. The calculation indicates that the lowest excitons in all lead halides are attributed to the cationic transitions. The electronic structures of lead halides are discussed on the basis of a comparison of experiment and calculation.