A beam of ions (usually He ions) is directed at the sample surface. When the ions strike the surface, there is a charge exchange, and an electron is emitted from the surface. The ion is neutralised in this exchange. The energy of the emitted electron can be analysed and the resulting spectrum is representative of the density of states at the surface. Hence the technique is complementary to UPS and XPS. INS is more surface sensitive than these techniques as the charge exchange occurs in the topmost atomic layer. A closely related technique to INS is MIES.