Outer-shell excitation in sputtered atoms

Inelastic effects that occur during energetic ion impact on solid surfaces include excitation and ionization of scattered and sputtered particles. Although these effects are being applied as surface analytical techniques to provide information on the composition of solid surfaces, little is understood about their underlying mechanisms. In order to gain some understanding of the excitation and ionization processes in sputtering, measurements have been undertaken of the absolute photon yield and the velocity distribution of the excited atoms. The experimental results show that the photon yield is lower than the secondary ion yield. They also show that the velocities and threshold energies for excited particles are much higher than those of ground state particles. In addition, the population of upper levels for various atomic transitions have been measured and the results suggest the non-existence of a Local Thermodynamic Equilibrium inside a solid undergoing sputtering. Moreover, the high energies of the sputter-excited atoms and the wide spread of these energies provide evidence against the existence of a local thermodynamic equilibrium. Based on these and other experimental observations such as the enhancement of both photon and secondary ion yields with oxygen, a unified model for excitation and ionization is proposed. This model is based on the level-crossing concept in atomic collisions and it explains qualitatively the similarities and differences between photon and secondary ion emissions