Infrared optical properties of graphite

The complex dielectric constant $\epsilon \left(\omega \right)={\epsilon }_1+i{\epsilon }_2$ 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 $R\to 1$ for $\hslash \omega <0.01\mathrm{}$ eV. The results are decomposed into intraband and interband terms. The free-electron contribution to $\epsilon$ can be quantitatively described by Drude theory with plasma frequency $\hslash {\omega }_p=0.44\mathrm{}$ eV and relaxation time ${\tau }_c=2\mathrm{}\times {10}^{-13\mathrm{}}$ sec. Strong interband transitions set in at $\hslash \omega \sim 0.01$ eV and are associated with a peak in ${\epsilon }_2$ at 0.02 eV.