MECHANISMS OF VIBRATIONAL AND ELECTRONIC EXCITATIONS OF POLYSTYRENE FILMS IN HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY

High resolution electron energy loss spectroscopy (HREELS) was used to study vibrational and electronic excitations induced by electrons in polystyrene thin films cast on silicon substrates. In order to deduce the nature of the interaction mechanisms involved, differential cross- sections of vibrational and electronic excitations were measured as a function of the incident electron energy and of the geometrical config uration of the experiment, Vibrational excitations were compared with the very strong bands of infrared and Raman spectra, Losses produced b y impact interaction correspond mostly to Raman active modes; those pr oduced by dipole interaction correspond mostly to infrared active mode s. Electronic excitations leading to triplet or singlet excited states and having constant widths around 0.7 eV were detected. In this case, pronounced resonance mechanisms are involved, mainly in geometrical c onfigurations far from specular conditions, showing the existence of t ransient negative ions centred at the maxima found for the excitation functions. The resonant mechanisms decline as specular conditions are reached. Losses having widths increasing with incident energy are assi gned to interband transitions. Finally, structures at fixed kinetic en ergies, appearing in all spectra, are associated with the relaxation o f slow electrons occupying conduction band levels with high density of states before escaping to the vacuum. (C) 1997 Elsevier Science B.V.