The principle of secondary-ion mass spectrometry. The primary Cs+ beam hits the sample and sputters the surface. Atoms and molecular fragments are ejected from the sample surface; during this process a fraction of the secondary particles are ionized. The identity of the secondary particles, determined by mass spectrometry, indicates the atoms or atomic clusters from the molecules in the sample that have been hit by the primary Cs+ beam. The figure shows only the types of atoms and ions that are relevant to this article; other particles formed by sputtering are not represented. Cs, cesium.