Mechanisms of vibrational and electronic excitations of polystyrene films in high resolution electron energy loss spectroscopy

Abstract

Abstract 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 configuration of the experiment. Vibrational excitations were compared with the very strong bands of infrared and Raman spectra. Losses produced by impact interaction correspond mostly to Raman active modes; those produced by dipole interaction correspond mostly to infrared active modes. 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 configurations far from specular conditions, showing the existence of transient 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 assigned to interband transitions. Finally, structures at fixed kinetic energies, appearing in all spectra, are associated with the relaxation of slow electrons occupying conduction band levels with high density of states before escaping to the vacuum.