Infrared optical properties of graphite

The complex dielectric constant ε(ω)=ε₁+iε₂ 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→1 for ω<0.01 eV. The results are decomposed into intraband and interband terms. The free-electron contribution to ε can be quantitatively described by Drude theory with plasma frequency ωₚ=0.44 eV and relaxation time τc=2×10^-13 sec. Strong interband transitions set in at ω~0.01 eV and are associated with a peak in ε₂ at 0.02 eV.