Optical Properties of Graphite

TLDR

Optical properties of graphite can be divided into two spectral regions. The dielectric constant and related functions were derived from reflectance data up to 26 eV using the Kramers–Kronig relation. The study finds that graphite’s optical response is dominated by π‑band transitions below 9 eV, a broad 15 eV σ‑band absorption peak, plasma resonances at 7 and 25 eV, and a low‑energy 0.8 eV feature linked to E2–E3 band transitions, with an estimated γ1 of ~0.4 eV.

Abstract

The complex dielectric constant $\ensuremath{\epsilon}(\ensuremath{\omega})={\ensuremath{\epsilon}}_{1}+i{\ensuremath{\epsilon}}_{2}$ 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 $\ensuremath{\pi}$ bands. At higher energies, a broad absorption peak near 15 eV is associated with interband transitions involving the 3 $\ensuremath{\sigma}$ electrons per atom. This viewpoint is strongly supported by evaluation of the sum rules for ${n}_{\mathrm{eff}}$. Plasma resonances which produce peaks in the energy-loss function $\ensuremath{-}\mathrm{Im}{\ensuremath{\epsilon}}^{\ensuremath{-}1}$ 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 ${E}_{2}$ and ${E}_{3}$ bands at the point $K$. This yields a value for ${\ensuremath{\gamma}}_{1}$ of \ensuremath{\approx}0.4 eV.