Direct Observation of Individual Adatoms: Nitrogen on Tungsten

Abstract

The utility of the field ion microscope for adsorption studies has been explored by examining the interaction of nitrogen with an atomically smooth tungsten surface. Individual nitrogen adatoms are shown to be visible, and it is demonstrated that adsorption fails to perturb the lattice permanently. In probing the room‐temperature distribution of adsorbed material, the closely packed (110) is found to remain bare, even when exposed to nitrogen at p=2×10^—6 mm for 2 hr. All the other planes, with the possible exception of the {100} and {130}, are irreversibly covered. Nitrogen, presumably in the form of atoms, is bound on the (110) only at T<190°K. Adsorption on this plane at higher temperatures is limited not by an activation barrier but rather by thermodynamics: the binding energy on the (110) is only 5.1 ev, compared with 6.7 ev elsewhere on the surface.