Optical properties of selenium

The permittivity, reflectivity, absorption coefficient, and electron-energy-loss function of trigonal Se are calculated from 0 to 25 eV with the use of the self-consistent nonmuffin-tin Hartree-Fock-Slater band-structure results obtained with the orthogonalized-plane-wave method. The momentum matrix elements for the Ehrenreich-Cohen permittivity tensor are rigorously calculated in the bulk of the first Brillouin zone. The calculated spectra agree closely with the experimental ones. The interpretation of all the main peaks and anisotropies in the spectra are given in terms of the transitions between the band groups. The analysis provides new insight into the origin of the main optical features of Se above {~}5 eV (i.e., above the energy range of the first main peak in the optical spectra). The results are also used to interpret the imaginary part of the permittivity of amorphous Se. The effect of the local fields on the spectra is estimated to be small. The difference between the calculated and the experimental spectra of Se at lower energies is compared with the calculated local-field and continuum-exciton corrections for C, Si, and TlCl. The possible reasons for the differences in the ultraviolet region between the calculated one-electron spectra and the experimental ones of Se, Te, ZrSe₂, Si, Ge, GaP, GaAs, InAs, InSb, ZnS, and ZnSe are discussed.