Optical Properties and Band Structure of Trigonal Selenium

The reflectivity spectra of single crystals of trigonal selenium have been measured at 300 and 20°K between 1.0 and 5.9 eV for light polarized in directions parallel and perpendicular to the $c$ axis. The Kramers-Kronig relation has been applied to obtain the wavelength dependence of the optical constants. At 20°K, the imaginary part of the dielectric constant for perpendicular polarization shows one sharp peak at 1.95 eV which has been interpreted as being due to exciton absorption. Another sharp peak in the parallel polarization spectrum at 3.09 eV has also been attributed to an exciton absorption. All other observed peaks have been interpreted in terms of transitions at symmetry points in $k$ space. Based on the double-group selection rule and the observed transition energies, a band structure for trigonal selenium is proposed and is compared with published theoretical calculations.