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 monol ayers adsorbed on gold are reported as a contribution to study in detail th e structure of the layers, through an analysis of the interaction mechanism s between the electrons and these organic ultrathin films. To show the diff erent spatial distribution of scattered electrons from dipole and impact in teractions with the monolayer, angular distributions of elastic and inelast ic scattered electrons were measured for various primary energies. The abso lute electron scattering loss probabilities from both dipole and impact int eractions were estimated for different vibrational modes. The study of thes e different mechanisms was first hampered by the Door order of the layers a dsorbed on evaporated gold. It became therefore highly desirable to perform the experiments on a single-crystal substrate. Using Au(111) and Au(100) s ingle crystals as substrates, it was possible to separate the spectral resp onse from impact and dipole interacting electrons. A negative ion resonant behavior has been observed in the energy dependence of the electron scatter ing differential cross sections.