Helium ion and fast atom scattering from polycrystalline copper surfaces

Low-energy ion scattering from surfaces is an established technique that gives unique information from the first one or two atomic layers on a surface. The standard technique employing an electostatic analyser, however, has some disadvantages. It detects only scattered ions whose yield from surfaces is relatively low and thus high incident ion fluxes must be used. A high flux of ions will change the nature of the surface in a number of ways. It will lead to significant preferential sputtering and to charging effects, particularly in insulating and semiconducting samples. It has been shown also that ions produce more chemical damage in surfaces that do neutrals of the same energy and species. Many of these disadvantages may be overcome if neutrals are used as the bombarding particles instead of ions. Conventional electrostatic methods cannot be employed to detect neutral sacttered spectra. Hence, a time-of-flight mass spectrometer with a pulsed ion/fast atom source (energy range 100–5000 eV) has been developed. This instrument has been used to measure the energy of particles scattered from a polycrystalline copper surface bombarded with either ions or fast atoms. In this study, spectra of He scattered from both ‘ion-cleaned’ and ‘ion-cleaned and heated’ copper surfaces were monitored for incident angles ranging from 5° to 75° with a constant scattering angle of 90°. For the ‘ion-cleaned’ surfaces, spectra of both ion and neutral projectiles show that the technique is sensitive to the outermost surface atomic layer, but that fast atom bombardment seems the more surface specific. For the ‘ion-cleaned and heated’ surface, spectra for both projectiles are qualitatively identical and first-order effects can be accounted for by the single binary collision model; however, small but significant differences in peak energy were observed and these may be explained in terms of inelastic collision processes.