NEW ASPECTS OF FIELD ADSORPTION AND ACCOMMODATION IN-FIELD ION IMAGING

Field ionization (FI) experiments characterized by field ion imaging a nd by field ion appearance spectroscopy have been performed of Ne on R h(012) at emitter temperatures of 79 K, and slightly above at 107 K, a s well as of Ne with admired Ar on Rh(111) at 79 K, and also of CO on Rh(111) in the temperature range of 79-300 K. The results obtained for rare gas FI give evidence of weakly bound image gas atoms on the inte rior sites of crystal facets playing an active role in imaging gas acc ommodation. These atoms are as necessary for localized FI as are the e arlier suggested actively involved field-adsorbed atoms on the imaged surface step sites of the emitter. FI of CO was found to proceed by fi eld desorption. From analyses of the appearance energies of CO+, argum ents have been derived that the earlier model of localized FI, at whic h at the instant of FI a touching of imaging gas at the sites of stron g field adsorption was assumed, is of minor importance and should be r eplaced in favour of a collision-induced exchange process prior to FI being sufficient and necessary for highest contrast field ion images.