Direct observation of the directional walk of single adatoms and the adatom polarizability

Using the field-ion microscope, it is possible to observe the directional walk of an adatom on an atomically perfect metal surface. The driving force arises from the polarization binding in a nonuniform electric field on a crystal plane. From the average velocity of the walk the polarizability of the adatom can be calculated. A systematic study of single $5-d$ transition-metal atoms adsorbed on the (110) plane of W was made and the adatom polarizabilities obtained. These values were found to be significantly less than the free-atom polarizabilities. It will be shown that the polarizability of chemisorbed atoms arises mainly from a field-induced charge transfer between the adatom and the substrate plane, and is thus very closely related to the electronic density of states of the adatom. For a W adatom adsorbed on the W (110) plane the average density of states in the range from —5.3 to —7.2 eV is estimated to be ∼0.75 e${\mathrm{V}}^{-1\mathrm{}}$ per adatom.