Reflectivity spectra and dielectric function of stage-1 donor intercalation compounds of graphite

We have determined the frequency-dependent scrE→⊥c→ dielectric function ε(ω) for the stage-1 graphite intercalation compounds ${\mathrm{KC}}_8$, ${\mathrm{LiC}}_6$, and ${\mathrm{KHgC}}_4$ in the range 0–40 eV, using a combination of normal-incidence reflectivity (0–12 eV) and previously published electron-energy-loss spectra (EELS) (10–40 eV). This provides a better approximation to ε(q=0, ω) than EELS alone, particularly in the spectral region encompassing the free carrier and π valence plasmons. Successive Kramers-Kronig analyses were performed to obtain the unscreened plasma frequencies for the conduction and π valence electrons and the frequency-dependent effective electron densities $n_{\mathrm{eff}\left(\mathrm{\omega }\right)}$. For ${\mathrm{LiC}}_6$ we report 0.5–3.0-eV reflectivity spectra with scrE→∥c→. All three compounds show significant departures from rigid-band behavior, which for ${\mathrm{LiC}}_6$ is well represented by a band-structure-derived calculation described in the accompanying paper. Intercalant-derived states contribute strongly to the absorption in ${\mathrm{KHgC}}_4$. The free-carrier plasma frequencies and fitted scattering times are in good agreement with the dc conductivities, and the relatively weak optical anisotropy of ${\mathrm{LiC}}_6$ agrees with theory and therefore can be explained without invoking intercalant-derived or s-like graphitic states at the Fermi energy. Significant shifts in oscillator strength relative to rigid bands are seen in $n_{\mathrm{eff}\left(\mathrm{\omega }\right)}$, which for all three compounds sums to the correct value at 40 eV.