Direct observations of pair interactions on a metal: Heteropairs on W(110)

Abstract

Quantitative information about the range and strength of interactions between adatoms on surfaces is still scarce. However, through the use of the field ion microscope it is possible to pinpoint the location of individual metal atoms on a crystal, and therefore to establish by direct observation the probability P(R) of finding two atoms at sites separated from each other by the vector R. If equilibrium is established at the surface, this probability is related to F(R), the free energy of interaction, by P(R) ∝ exp[-F(R)/kT]; measurements of the probability therefore provide a quantitative estimate of the interaction between adatoms at specified sites on the surface. Such measurements have been carried out with two metal atoms on the most densely packed plane of tungsten, the (110). To minimize the amount of data taking, initial studies have been made on the distribution of an adatom (Pd) mobile at the temperature of equilibration around a chemically different metal atom (either Re or W) stationary at the center of the W( 110) plane. These observations reveal a remarkable anisotropy. Palladium atoms are most frequently found at the nearest-neighbor position to the central atom, but the close by second and third-nearest neighbor sites are not populated. It is shown that this is due to a strong orientational dependence of interactions. Quantitative values for the free energy of interaction with the central adatom have been determined for Pd atoms at different sites on the surface. Along the close-packed direction [111], interactions are attractive out to ≈10 Å, and show an oscillatory dependence upon distance; along [001] and [110] they tend to be repulsive for both Re and W as the central atom. These effects are so far not predicted by theory. © 1991 American Institute of Physics.