Infrared optical properties of graphite

The complex dielectric constant $\ensuremath{\epsilon}(\ensuremath{\omega})={\ensuremath{\epsilon}}_{1}+i{\ensuremath{\epsilon}}_{2}$ for graphite is derived in the energy range 1 to 0.001 eV by Kramers-Kronig analysis of reflectance data with a suitable extrapolation of the curve to $R\ensuremath{\rightarrow}1$ for $\ensuremath{\hbar}\ensuremath{\omega}<0.01$ eV. The results are decomposed into intraband and interband terms. The free-electron contribution to $\ensuremath{\epsilon}$ can be quantitatively described by Drude theory with plasma frequency $\ensuremath{\hbar}{\ensuremath{\omega}}_{p}=0.44$ eV and relaxation time ${\ensuremath{\tau}}_{c}=2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ sec. Strong interband transitions set in at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\sim}0.01$ eV and are associated with a peak in ${\ensuremath{\epsilon}}_{2}$ at 0.02 eV.