  Low energy ion scattering is a technique to detect the energy of ions which
are scattered from a surface. For noble gas ions, it is predominantly sensitive
to the topmost surface layer due to strong neutralisation processes. Depending
on the combination of projectile ion and target material, the scattering
spectra can exhibit contributions resulting from multiple scattering processes
in deeper layers when probing ions are re-ionised on the exiting trajectory.
These events cause a pronounced continuum located toward lower scattering
energies with respect to the direct scattering peak. In a previous work a
semi-empirical formula has been given which allows fitting and derivation of
quantitative information from the measured spectra [Nelson 1986 J. Vac. Sci.
Technol. A 4 1567-1569]. Based on the former work an improved formula is
derived which has less numerical artefacts and is numerically more stable.
