The imaging process in field ion microscopy

Abstract

Imaging by field ionization is a complex process. Only recently has the original concept of field desorption led to high resolution self-imaging of a surface. The better established use of field ionization in field-ion microscopy revealed experimental facts which in existing quantum mechanical discussions of the basic process are still quite incompletely understood. Problematic are the sharply localized origin of the ions a few Å away from the metal surface, regional and local brightness features indicating chemical specificity within the b.c.c., f.c.c. and h.c.p. lattice groups of the pure metals, and selective imaging of constituents of alloys. A new aspect of the ionization process was introduced with the discovery, by the atom-probe, of field adsorption of image gas atoms at the apex of the substrate atoms. It is proposed that field ionization probability is greatly enhanced when the electron can tunnel through the intermediate apex-adsorbed atom. This hypothesis is applied to the qualitative explanation of several experimental features.