When a beam of ions hits a solid surface part of the projectiles will be scattered back into the vacuum after one or more collisions with target atoms of the top few layers. Measurement of the energy of the backscattered particles can be used to identify the mass of the these atoms. The technique is called Ion Scattering Spectroscopy (ISS). The term encompasses actually several techniques depending on the energy of the primary ion beam. LEIS (Low Energy Ion Scattering) spectroscopy is referred to primary energies in the range of 100 eV to 10 keV, Medium Energy Ion Scattering (MEIS) to a range from 100 to 200 keV, and High Energy Ion Scattering (HEIS) to energies between 1 and several MeV. Often the LEIS technique is called Ion Scattering Spectroscopy (ISS), the term we will use below meaning LEIS, while HEIS technique is best known as Rutherford Backscattering Spectroscopy (RBS).