text: Several different types of events may take place as a result of the ion beam impinging on a target surface.  Some of these events include electron or photon emission, electron transfer (both ion-surface and surface-ion), scattering, adsorption, and sputtering (i.e. ejection of atoms from the surface).  For each system and each interaction there exists an interaction cross-section, and the study of these cross-sections is a field in its own right.  As the name suggests, LEIS is primarily concerned with scattering phenomena.