Optical Properties of Single-Crystal Magnesium Oxide
M. W. Williams, E. T. Arakawa
The reflectance of single crystals of MgO was measured at a 15° angle of incidence for photon energies between 4 and 29 eV and the optical constants obtained by a Kramers-Kronig analysis of the reflectance data. In the energy range 4-14.5 eV, structure in the reflectance is in good agreement with that already reported. Reflectance measurements have not been previously reported above 14.5 eV. Peaks in the optical conductivity locate the Γ exciton at 7.65 eV and interband transitions at 11.0, 13.2, 14.9, 17.0, ∼19, 20.8, and ∼24 eV. The absorption edge for band-to-band transitions is estimated to be at 7.9 eV. The calculated energy-loss function -Im(ε+1)⁻¹, where ε is the dielectric constant, shows a maximum at 21.3 eV while the calculated loss function -Imε⁻¹ shows a maximum at 22.4 eV and a subsidiary maximum at 25.4 eV. It is suggested that these maxima in -Imε⁻¹ correspond to volume plasma resonances involving 6 and 8 electrons per molecule, respectively. The volume plasma energies calculated on the assumption of 6 and 8 free electrons per molecule are 21 and 24 eV, respectively. Use of a sum rule enables us to explain qualitatively the shifts of the plasma resonances from the free electron values. Similarly, the proximity of the peaks in -Im(ε+1)⁻¹ and -Imε⁻¹, locating surface and volume plasma losses can be explained. These results, from optical measurements, are used to identify characteristic electron energy losses, obtained from the literature, as being due to single-electron transitions or to collective oscillations. © 1967 The American Institute of Physics.
