Electronic Spectrum and Ultraviolet Optical Properties of Crystalline MgO

Abstract

The electronic spectrum of single-crystal magnesium oxide, i.e., the real (${\ensuremath{\epsilon}}_{1}$) and imaginary (${\ensuremath{\epsilon}}_{2}$) parts of the dielectric response, has been obtained over the region 5-28 eV from normal-incidence reflectance spectra. Measurements were made on freshly cleaved crystals over the entire region at 295\ifmmode^\circ\else\textdegree\fi{}K and from 5-11.5 eV at 77\ifmmode^\circ\else\textdegree\fi{}K. Structure in ${\ensuremath{\epsilon}}_{2}$ was observed near 7.7, 10.8, 13.3, 16.8, 17.3, and 20.5 eV. The first peak, which was found to be a doublet with components at 7.69\ifmmode\pm\else\textpm\fi{}0.01 and 7.76\ifmmode\pm\else\textpm\fi{}0.01 eV, was attributed to the ${\ensuremath{\Gamma}}_{\frac{3}{2}}$ and ${\ensuremath{\Gamma}}_{\frac{1}{2}}$ spin-orbit split exciton. A Lorentzian fit to the exciton components gave oscillator strengths of 0.035 and 0.017 per molecule, respectively. Subtraction of the exciton structure from the remaining interband structure gave a direct interband edge at 7.77\ifmmode\pm\else\textpm\fi{}0.01 eV. A large plasma peak was observed in the energy-loss function near 22 eV, in agreement with recent energy-loss experiments. The remaining structure was attributed to interband transitions and will be discussed in terms of recent pseudopotential band-structure calculations.