but the total momentum of the atom and ion is conserved. Therefore, as the initially "stationary" atom recoils, some kinetic energy is lost from the scattered ion, and the quantity of lost energy depends on the relative masses of the atom and ion.
Experimental geometry for ion scattering spectroscopy.
An ion scattering spectrum typically contains one peak for each element in the sample, with their separation in energy relating to the relative atomic masses of those elements. In some cases, different isotopes of the same element can be resolved, although this usually requires a primary ion heavier than He+. Strong scattering signals are limited to the topmost layer of atoms, so ISS is extremely surface-sensitive. Therefore, the sample must be clean, and even a small quantity of surface contaminant can significantly influence the ion scattering spectrum.
The absolute kinetic energy (Es) of each peak also depends on the energy of the scattered ion beam (Eo) because ISS is an energy-loss technique. The relevant quantity when dealing with ISS is the ratio of the scattered and incident ion energies, E/Eo. The angle through which the ions are scattered (θ) is also important. Where the ion source and detector are fixed, the angle is a parameter that must be included in the calculations.