Electronic Spectrum and Ultraviolet Optical Properties of Crystalline MgO

The electronic spectrum of single-crystal magnesium oxide, i.e., the real (${\epsilon }_1$) and imaginary (${\epsilon }_2$) 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°K and from 5-11.5 eV at 77°K. Structure in ${\epsilon }_2$ 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±0.01 and 7.76±0.01 eV, was attributed to the ${\Gamma }_{\frac{3}{2}}$ and ${\Gamma }_{\frac{1}{2}}$ 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±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.