DETERMINATION OF CLUSTER SPACINGS IN THE FIM - IRIDIUM DIMERS ON W(110)

The field ion microscope normally does not provide a reliable indication of distances; nevertheless, by mapping out the binding sites for a single adatom and relying upon the known lattice spacings, it has in general been possible to circumvent this difficulty. Problems still remain, however, in arriving at unequivocal results for the interatomic spacings in clusters. A combination of methods, involving mapping of the exact sites at which a cluster is bound, computer-based image processing, together with diffusion measurements and observation of field effects, can usually yield concordant information about spacings, at least in clusters containing only a small number of atoms. These techniques are illustrated using as an example the spacing between atoms in Ir2 on W(110), about which there is still considerable disagreement.