Separation of dipole and impact scatterings in high resolution electron energy loss spectroscopy:: Experiment from model organic material

The interpretation of High Resolution Electron Energy Loss Spectroscopy (HREELS) spectra recorded from polymers remains a difficult task because of (1) the relatively poor resolution achieved on these non-ordered systems, (2) the large number of vibrational features induced by the numerous chemical groups in the monomer unit, (3) the superposition of the dipole, impact and resonant scattering mechanisms. In this report, the study of the physical mechanisms responsible for the electron-molecular vibration coupling is investigated, using a simple model system consisting of a well-ordered film of eicosanoic acid adsorbed on a GeS substrate and an hypothesis supposing that dipole scattering is extinguished in non specular geometry. The ordered character of the molecular layer allowed to separate the dipole from the impact contributions in the signal recorded in specular geometry. The results support the idea that any vibrational mode of an organic compound induced by electron excitation contains both contributions with different relative extent, depending on the chemical nature of the molecular group involved in the vibration and their orientation. Cross section measurements for both interaction mechanisms are in agreement with theoretical prediction, confirming the validity of this new and original method suggested for the separation of the interaction mechanisms. Furthermore, we measured a resonance near 6 eV impact energy as the intensity of the nu(C-D) band is enhanced for this impact energy. This observation again is in agreement with our hypothesis that the impact scattering is the dominant interaction mechanism involved in the vibrational excitation of the C-D group. This study allowed for a model of the geometry of adsorption of the molecule on the substrate. It is confirmed that the eicosanoic acid adsorbs the COO group on the substrate, with the long hydrocarbon chain standing up and the CD3 terminating group pointing out of the surface. (C) 1999 Elsevier Science B.V. All rights reserved.