The sample is bombarded with noble gas ions of a few keV. The energy of a noble gas ion that is backscattered by a surface atom is determined by the laws of conservation of energy and momentum, for a given primary energy and scattering angle.
The energy spectrum of the scattered ions is equivalent to a mass spectrum of the surface atoms.
A LEIS analysis of a surface consisting of three different types of atoms results in a spectrum with three peaks. Such a spectrum can be produced within one minute.