SIMS/XPS studies of clean and reacted metal surfaces

The techniques of XPS and SIMS have been used to probe the structural and chemical properties of solid surfaces. Early studies involved single crystal surfaces with overlayers of reasonably well characterized geometry, in order to evaluate the information inherent in the molecular cluster ions detected in SIMS. The XPS measurements yield preliminary bonding information and indicate approximate surface coverages. For Ni(100), for example, we find that the initial room temperature O₂ chemisorption to form a p(2×2) overlayer occurs by a dissociative adsorption of O₂. The large O₂^-/O^- ratio observed by SIMS at low coverages also indicates that the dissociated atoms remain fixed near their original adsorption sites. Major progress has been made in interpreting the intensity of the cluster ions in SIMS using classical trajectory calculations to study the motion of atoms around the primary ion impact point. For simple metal/oxygen systems the cluster emission mechanism involves an atom-ion formation over the surface but within interaction range of the solid. With the Ni(100)/oxygen system and with FeRu alloys, using XPS to calibrate surface concentrations, we demonstrated that considerable local atomic order is inherent in the make-up of the clusters. Several important applications of these techniques to the elucidation of the surface chemistry of catalyst materials are also considered.