Collective Effects in the Optical Properties of Cs above the Plasma Frequency

Optical and dielectric constants of Cs have been obtained above the plasma frequency by measuring reflectance as a function of angle at a Cs-substrate interface. We find a strong, broad peak centered at 5 eV in the conductivity $\sigma$. The peak is attributed primarily to plasmon-assisted transitions, though direct transitions may contribute to the overall absorption in this energy region.