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 KC₈, LiC₆, and KHgC₄ 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_eff(ω). For LiC₆ we report 0.5--3.0-eV reflectivity spectra with scrE{→}{∥}c{→}. All three compounds show significant departures from rigid-band behavior, which for LiC₆ is well represented by a band-structure-derived calculation described in the accompanying paper. Intercalant-derived states contribute strongly to the absorption in KHgC₄. The free-carrier plasma frequencies and fitted scattering times are in good agreement with the dc conductivities, and the relatively weak optical anisotropy of LiC₆ 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_eff(ω), which for all three compounds sums to the correct value at 40 eV.