Local electric fields at individual atomic surface sites: field ion appearance energy measurements

Abstract

A new method of determining local electric fields is reported. Under conditions of highest field ion image contrast, absolute appearance energy 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 Rh(113) facet. Data of local fields, F hkl loc , are derived from appearance energies. A comparison is made with external electric field strengths, F hkl 0 , determined by electron energy spectroscopy and i- V (Fowler-Nordheim) measurements. Significantly different enhancement factors, F hkl loc / F hkl 0 , 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 are discussed in view of recent self-consistent calculations of local-field distributions in the proximity of protruding surface atoms. The occurence of field ionization of image gas atoms, released from field-adsorbed states at step sites by atom-collision processes, is in accord with the present observations.