Determining the stopping power of low energy helium in alkanethiolates with Neutral Impact Collision Ion Scattering Spectroscopy (NICISS)

Abstract

We have determined the energy loss of helium projectiles with a primary energy of 1.2 and 4.5 keV in self-assembled monolayers of alkanethiolates on silver (1 1 1). The energy loss is determined by comparing spectra obtained with Neutral Impact Collision Ion Scattering Spectroscopy (NICISS) of clean silver (1 1 1) and the same surfaces but covered with self-assembled monolayers of alkanethiolates. The film thickness is measured with XPS and the quality of the films with Metastable Induced Electron Spectroscopy (MIES). We found that the energy loss depends on the angle of incidence of the helium projectiles on the investigated surfaces. The dependence of the energy loss on the angle of incidence is interpreted as an transparency effect in the meaning of a reduced number of scattering centers for the helium projectiles in directions which are close to the average orientation of the alkanethiolate molecules. This interpretation is supported by computer simulations and by determining the energy loss in a self-assembled monolayer on a polycrystalline gold foil. The energy loss in directions far from the average orientation of the alkanethiolate molecules is regarded as representative for a disordered system.