High-resolution electron energy loss spectroscopy investigation of the vibrational structure and order of an octadecanethiol monolayer on gold substrates

The vibrational spectra induced by electron beam from octadecanethiol monolayers adsorbed on gold are reported as a contribution to study in detail the structure of the layers, through an analysis of the interaction mechanisms between the electrons and these organic ultrathin films. To show the different spatial distribution of scattered electrons from dipole and impact interactions with the monolayer, angular distributions of elastic and inelastic scattered electrons were measured for various primary energies. The absolute electron scattering loss probabilities from both dipole and impact interactions were estimated for different vibrational modes. The study of these different mechanisms was first hampered by the Door order of the layers adsorbed on evaporated gold. It became therefore highly desirable to perform the experiments on a single-crystal substrate. Using Au(111) and Au(100) single crystals as substrates, it was possible to separate the spectral response from impact and dipole interacting electrons. A negative ion resonant behavior has been observed in the energy dependence of the electron scattering differential cross sections.