Optical properties from reflection electron energy loss spectroscopy

The optical constants of graphite (highly oriented pyrolitic graphite) and Kapton (Dupont de Nemours) polymer, pyromellitic dianhydride-oxy dyaniline (PMDA-ODA), over the 0-40 eV energy range, have been obtained by reflection electron energy loss spectroscopy (REELS) measurements. Raw data handling procedures, such as background substraction, elastic peak elimination and the evaluation of the double-scattering contribution, are discussed. The influence of surface plasmons on the experimental line shapes is also considered in order to deduce the importance of surface effects in the REELS experiment. The pure Im(-1/epsilon) loss function is therefore obtained by a conventional Kramers-Kronig analysis giving the optical constants of the samples. The REELS-deduced optical constants are compared with those obtained by a conventional wide-range reflectivity spectrum, showing a very good agreement. No relaxation of dipole selection rules has been observed for the primary energy used (1.2 keV), thus justifying the retention of the optical limit assumption (q --> 0).