Comparative studies on field ionization at surface sites of Rh, Ag and Au—differences in local electric field enhancement

Field ion imaging conditions were studied for clean Rh, and for Rh epitaxially covered with Ag and Au, respectively. Significant, metal-specific decreases of the image voltages have been observed. This is caused by different enhancements of local fields in the vicinity of protruding Rh, Ag and Au atoms due to their different electronic structures. The interpretation is based on self-consistent calculations of local electrostatic field distributions in the vicinity of single atoms adsorbed on a metal surface. For different facets of a surface, a close correlation between local field distributions and the degree of localization of the field ionization process is establihed from the measured full width at half maximum (FWHM) of field ion energy distributions and is discussed in association with possible field ionization mechanisms