Electronic excitation in sputtered atoms

Sputtering is the result of collision cascades initiated when primary high energy ions impact a target. In collision events, whether between a primary ion and a lattice atom or between two lattice atoms, a fraction of the kinetic energy of the colliding partners is converted into electronic excitation. The latter inelastic process is referred to as electronic stopping as compared with elastic or nuclear stopping processes resulting in atomic displacements. Eventually, the collision cascade propagates to the surface where sufficient momentum is occasionally imparted to a surface atom in a final or ''causal'' collision to overcome its surface binding energy, resulting in an ejection or ''sputtering'' event. Electronic stopping manifests itself most directly in the sputtering of a very tiny fraction of atoms (10-4-10-6) in highly excited electronic states (>2 eV) from which fluorescence can be detected. Such ''secondary photon'' emission has been extensively studied for a very long time and is proposed as a method for surface analysis