Atom/ion sources

In this chapter we describe the analytical information that can be obtained by using ions to probe the material surface. Since ions have a mass which is several orders of magnitude greater than either electrons or photons, the potential for damage using even the lightest ion, the hydrogen ion or proton, is considerable. However, ions with very low kinetic energy can be reflected from a surface without displacing any of the surface atoms from their lattice positions so that it is possible to probe a surface with ions while causing little or no surface damage. This method of probing a surface is known as ion scattering spectroscopy (ISS). In addition, because an ion cannot penetrate into a surface without causing damage, this technique detects only the outermost atoms in the material and is the most surface-sensitive of those currently available. In general, though, ions are used to dislodge surface atoms, which are ejected as neutral atoms and atom clusters, positive or negative ions and ion clusters together with electrons. These ejected ions are mass-analysed to identify the clusters using secondary ion mass spectroscopy (SIMS). The large number of neutral atoms ejected when the surface is bombarded can be ionised after ejection and detected by the technique of sputtered neutral mass spectroscopy (SNMS). Ions and electrons can be induced to leave a surface by applying a large electromagnetic field and this involves the use of methods to promote electron tunnelling. Here we will also address techniques which employ this method to desorb and analyse surface atoms such as the atom probe, field ion microscopy (FIM) and field emission microscopy (FEM). In the case of the first two of these techniques ions are not used as a probe but are ejected, albeit by very high potential fields, and the concept of using ions to study a surface is maintained. Electron tunnelling is also fundamental to the technique of scanning probe microscopy (SPM) and so this relatively new, but rapidly developing, technique has been included here. Finally the technique of proton-induced X-ray emission (PIXE) is described although it could equally have been described in previous chapters.