Current Status of Sputtered Ion Emission Models

Significant progress appears to have been made in recent years in understanding the sputtered ion emission process. Theoretical efforts have followed two diverse paths. One approach assumes a highly simplified model of a metal surface; the ionization probability of a moving atom near the surface can now be calculated analytically. Given this capability, one then attempts to extend the model to real surfaces without losing the attractive simplicity of the analytical treatment. The second approach attempts to deal directly with the complexities of real surfaces, in particular those of complex materials. Here, the possibility of analytical solution appears remote, but the goal of the work is to construct appropriate potential energy diagrams, and thereby to indicate in a general way the form that an analytical approach would have to take. In parallel with these theoretical developments there have been advances in experimental technique. Studies on clean metal surfaces have reached the point where direct, and successful, comparisons with theoretical predictions can be made. On complex, oxyqenated surfaces, techniques are now available which allow quantification of the surface oxygen content, and correlation with ionization probability of the sputtered species. From such studies an idea of the form of the potential energy diagram appropriate for sputtered ion emission is beginning to emerge, and it is apparent that simple one-dimensional potential energy diagrams will not be adequate.