EXPERIMENTAL AND THEORETICAL APPROACHES TO THE IONIZATION PROCESS IN SECONDARY-ION EMISSION

Despite some disagreement in experimental findings, especially regarding the secondary energy spectrum, and the lack of well-tried theoretical methods, moderate progress has been made in the understanding of the ionization process in secondary ion emission. Experimentally, conclusive evidence has been obtained for the existence of at least two excitation mechanisms: a collisional mechanism, which prevails in the production of multicharged ions, and a mechanism originating in the separation of the outgoing particle from its solid environment. The role of foreign atoms (host atoms in alloys, associated species in covalent and ionic compounds, surface contaminants such as oxygen and caesium), although often considerable, is not fully understood owing to the complex influence of the environment, and its description is at present limited mainly to some empirical rules of relative validity. Theoretically, if many approaches have been proposed, few prove to rely on valid physical foundations. Among the latter, an earlier theory which explains the emission of multicharged ions rather well, describes ionization in terms of Auger decay outside the target of collision-induced core hole excitations. In a second kind of approach, probably of more general significance, it is postulated that ionization results from the perturbation of outer atomic electrons during the separation of the outgoing particle from the surface. Its mathematical treatment has been made possible by recent progress in the many-body theory of localized time-dependent perturbations, but much work remains to be done in this direction. A third kind of approach, regarding ionization as induced by diffuse excitation of the surrounding electron gas, remains in the field of conjecture. © 1979.