Interband dielectric function of ${\mathrm{C}}_{60}$ and ${\mathit{M}}_6$${\mathrm{C}}_{60}$ (M=K,Rb,Cs)

We report the room-temperature optical properties of ${\mathrm{C}}_{60}$ and ${\mathit{M}}_6$ ${\mathrm{C}}_{60}$ determined from the near-normal-incidence reflection and transmission experiments. The observed peak positions in ${\mathrm{\epsilon }}_2$(ω) for ${\mathrm{C}}_{60}$ and ${\mathit{M}}_6$ ${\mathrm{C}}_{60}$ compare well with recent band-structure calculations and suggest that the ${\mathit{M}}_6$ ${\mathrm{C}}_{60}$ compounds exhibit a weak hybridization between the ${\mathrm{C}}_{60}$ and M states, consistent with the Raman-scattering studies of ${\mathit{M}}_6$ ${\mathrm{C}}_{60}$. The onset of absorption across the energy gap between the highest occupied molecular orbital (MO) and the lowest unoccupied MO in ${\mathrm{C}}_{60}$ is measured to be 1.7 eV.