Fine structure of the exciton band and anisotropic optical constants in scheelite PbWO4 crystals -: art. no. 195105

Polarized reflectivity spectra of PbWO(4) crystals with scheelite structure have been measured using synchrotron radiation in the energy range up to 30 eV. The measurements are undertaken on (011) cleaved surfaces for the polarizations parallel and perpendicular to the crystal a axis. The spectra exhibit remarkable dichroism in the fundamental absorption region. The dielectric constants for the principal crystal a and c axes are derived from the reflectivity spectra with the application of a Kramers-Kronig relation and an equation of dielectric ellipsoid for uniaxial crystals. Polarization dependence of the reflectivity spectra is investigated in detail in the exciton-band region. It is confirmed that the exciton band peaking at 4.25 eV consists of two components with separation of about 0.1 eV for both polarizations Eparallel toa and Eparallel toc. The intensity of this doublet structure is stronger for Eparallel toa than for Eparallel toc. Another band appears at around 5.3 eV for Eparallel toc. Origin of these fine structures in the exciton-band region is discussed on the basis of the recent electronic band calculations of PbWO(4). For the polarization Eperpendicular toa, an additional structure is observed on the high-energy side of the exciton band, which is attributed to the longitudinal exciton-polariton mode.