Optical Properties and Collective Excitations in MoS 2 and NbSe 2 in the 1.7 to 30 eV Range

Optical functions of two layer type compounds MoS 2 and NbSe 2 are calculated in the 1.7 to 30 eV range from reflectivity data by means of a Kramers‐Kronig analysis. The positions of the plasmon peaks given by the energy loss function, — Im \documentclass{article}\pagestyle{empty}\begin{document}$ \tilde \varepsilon _ \bot ^{ - 1} $\end{document} , are in good agreement with those evaluated directly by electron energy loss experiments. The optical effective number of electrons saturates at the high energy boundary of the experimental interval, for a value corresponding to all the valence electrons.