Limitations to depth resolution in ion scattering experiments

The energy spectra of scattered ions in Rutherford backscattering and elastic recoil scattering experiments are commonly described using stopping power and straggling data. These, in turn, are based on statistical approximations and thus neglect the characteristics of individual ion-atom collisions. This simplified analysis is not justified when considering experiments with high-energy resolution. We have performed theoretical and experimental studies to understand the shape of high-energy resolution ion scattering spectra. The impact parameter dependent energy loss in single ion-atom collisions has been calculated using the semi-classical approximation (SCA). We show that limitations for extracting monolayer depth resolution become obvious from this non-statistical approach. Experiments were performed using a medium energy ion scattering (MEIS) system. For the case of 100 keV protons scattered on sulfur atoms, good agreement between the theoretical description and experimental data was found. The influence of thermal vibrations of the sample atoms on the backscattering spectra is discussed