Pair interaction of metal atoms on a metal surface

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Abstract
Pair interactions of tungsten and iridium adatoms on the W {110} plane are studied by measuring two-dimensional pair distributions with two adatoms on a plane. Each distribution contains from 600 to 950 field-ion-microscopy observations. Pair energies over a distance range of ∼2.5 to ∼50 Å are derived by comparing the experimentally measured pair distributions with the calculated pair distributions for two noninteracting atoms. It is found that Ir-Ir pair interaction exhibits an attractive region at ∼5 Å and a repulsive region around 8 Å. If an oscillatory structure exists, its amplitudes decay already to less than ∼10 meV beyond 10 Å. The plane edge seems to repel Ir adatoms with a weak long-range force. The W-Ir interaction at a short range is weaker than the Ir-Ir interaction. However, the interaction extends to larger distances. From ∼950 observations at 330 K with two adatoms, we derive a pair energy which exhibits two attractive and two repulsive regions, thus strongly suggesting an oscillatory structure. The pair energies derived beyond 25 Å are erratic for both Ir-Ir and W-Ir interactions, most probably because of the limited amount of data available. However, this work represents the first time statistically reliable amounts of data have been obtained for two-dimensional pair distributions with only two adatoms on a plane. The nonmonotonic behaviors of adatom-adatom interaction on the smooth W {110} plane are clearly established.