Information about pair interactions between adatoms at specified sites on a surface can be readily obtained from the probabilityP(R) of finding two atoms at a separationRat equilibrium on a surface at temperatureT. Observations of two adatoms in such a system at different separations, using the field ion microscope, yield a map of the free energy of interactionF(R), inasmuch as the probabilityP(R) is proportional to explsqb;minus;F(R)/kTrsqb;. Only pairs of chemically different metal adatoms have so far been examined in this way, and measurements have therefore been extended to chemically similar atoms. Observations with two iridium adatoms in many different configurations on W(110) reveal behavior similar to that previously found with Rendash;Pd and Wndash;Pd: interactions extending over long distances, ge;10 Aring;, showing a pronounced orientational anisotropy. With the iridium pair oriented along the close packed lsqb;11macr;1rsqb; direction, interactions are generally attractive at close distances; along lsqb;001rsqb; and lsqb;11macr;0rsqb;, repulsion is the rule. However, the strength of the attractions is greater than in the heteropairs previously studied. The behavior of rhenium atoms is quite different, in that pair interactions are repulsive at short distances. However, at larger ranges, interactions become attractive and are again dependent upon the orientation of the pair on the surface. The extent to which threehyphen;atom effects are important in cohesion has been tested for both iridium and rhenium by comparing the experimentally measured dissociation energy of trimers with values of the cohesion estimated from pair interactions. Manyhyphen;atom contributions turn out to play a significant role in cohesion, not just for rhenium but also for iridium trimers.
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