LOCAL ELECTRIC-FIELDS AT INDIVIDUAL ATOMIC SURFACE SITES - FIELD-ION APPEARANCE ENERGY MEASUREMENTS

A new method of determining local electric fields is reported. Under c onditions of highest field ion image contrast, absolute appearance ene rgy measurements were carried out at a weakly corrugated step of a Rh( 001) plane and at strongly corrugated step across the atom rows of a R h(113) facet. Data of local fields, F(loc)hkl, are derived from appear ance energies. A comparison is made with external electric field stren gths, F0hkl, determined by electron energy spectroscopy and i-V (Fowle r-Nordheim) measurements. Significantly different enhancement factors, F(loc)hkl/F0hkl, were found, 1.27 for steps at the (001) plane, and 1 .54 for steps across the rows of Rh(113). In both cases the step-site atoms were covered with field-adsorbed Ne. The experimental results ar e discussed in view of recent self-consistent calculations of local-fi eld distributions in the proximity of protruding surface atoms. The oc currence of field ionization of image gas atoms, released from field-a dsorbed states at step sites by atom-collision processes, is in accord with the present observations.