text: While obtaining qualitative information about the elemental composition of a surface is relatively straightforward, it is necessary to understand the statistical cross-section of interaction between ion and surface atoms in order to obtain quantitative information.  Stated another way, it is easy to find out if a particular species is present, but much more difficult to determine how much of this species is there.
  The two-body collision model fails to give quantitative results as it ignores the contributions of coulomb repulsion as well as the more complicated effects of charge screening by electrons.  This is generally less of a problem in MEIS and RBS experiments but presents issues in LEIS.  Coulomb repulsion occurs between positively charged primary ions and the nuclei of surface atoms.  The interaction potential is given as:
  Where  and  are the atomic numbers of the primary ion and surface atom, respectively,  is the elementary charge,  is the interatomic distance, and  is the screening function.   accounts for the interference of the electrons orbiting each nucleus.  In the case of MEIS and RBS, this potential can be used to calculate the Rutherford scattering cross section (see Rutherford scattering) :
  As shown at right,  represents a finite region for an incoming particle, while  represents the solid scattering angle after the scattering event.  However, for LEIS  is typically unknown which prevents such a clean analysis.  Additionally, when using noble gas ion beams there is a high probability of neutralization on impact (which has strong angular dependence) due to the strong desire of these ions to be in a neutral, closed shell state.  This results in poor secondary ion flux.  See AISS and TOF-SARS below for approaches to avoiding this problem.