Local electric fields above individual surface atoms in the presence of field-adsorbed rare gas atoms: an additional field enhancement

Absolute field ion appearance energy measurements for He+ and Ne+ ions were carried out at steps of Rh(012) plane. Data of local electric fields close to the surface atoms, Fhklloc, are derived from appearance energies and compared with external field strengths, Fhkl0, determined by electron energy spectroscopy and i–V (Fowler-Nordheim) measurements. In both cases the step-site surface atoms were covered with field-adsorbed gas atoms, He and Ne, respectively. A field ionization flicker experiment with a Ne/Ar gas mixture probing Rh(111) was carried out using mass-to-charge separation of ions in the magnetic field. If Ne was ionized above the field-adsorbed Ar atoms, the appearance energy of Ne+ ions increased, providing the direct evidence for additional local field enhancements above the adsorbed rare gas atoms. The experimental results are discussed in terms of additional local field enhancement above the field-adsorbed image gas atoms and in view of recent self-consistent calculations of the spatial local field distribution in the proximity of protruding surface atoms. It is shown that the presence of a certain field enhancement is necessary to achieve the field adsorption of rare gas atoms under usual FIM conditions