Optical Properties of Graphite

The complex dielectric constant ε(ω)=ε₁+iε₂ and associated functions are derived by application of the Kramers-Kronig relation to reflectance data for graphite obtained in the energy range to 26 eV. It is possible to divide the optical properties into two spectral regions. In the range 0 to 9 eV, intra- and interband transitions involve mainly the π bands. At higher energies, a broad absorption peak near 15 eV is associated with interband transitions involving the 3 σ electrons per atom. This viewpoint is strongly supported by evaluation of the sum rules for n_eff. Plasma resonances which produce peaks in the energy-loss function -Imε^-1 at 7 and 25 eV are identified and described physically. At low energies, structure in the reflectance curve near 0.8 eV is attributed to the onset of transitions between the E₂ and E₃ bands at the point K. This yields a value for γ₁ of ≈0.4 eV.