On the binding strength of surface metal atoms in a high electric field—face-specific appearance energy measurements of field evaporated rhodium ions

Abstract

To examine the effect of a high external electrostatic field on the strength of atomic binding at metal surfaces, we carried out absolute appearance energy measurements of rhodium ions field evaporated at 103 K from the relatively open Rh(100) plane and from the close packed Rh(111) plane. The crystal facets were characterized in a probe hole field ion microscope equipped with an external gimbal system for manipulation of the emitter apex with five degrees of freedom. Using a newly developed retarding potential technique, absolute values for rhodium field ion appearance energies were derived from measurements of the high energy onset of ion retardation curves. Appearance energies of ions field evaporated from Rh(100) were 0.6 eV greater than for field evaporation from Rh(111). Applying a thermionic cycle, the binding energy for Rh was 7.0 eV at Rh(100) step sites, and 6.4 eV at Rh(111) step sites, in both cases being greater than the cohesive energy for rhodium (5.75 eV). Conclusively, the external electric field of, in our case, approximately 41 V nm −1 enhances the binding strength of Rh surface atoms especially at less densely packed rhodium surfaces. The data are discussed within the framework of an earlier reported model involving the short-range, field-induced diffusion of a kink site atom prior to field evaporation.