Indirect adatom interactions

Adatom lateral interactions are dominated by through metal interactions, as direct steric interaction is absent. When the adatom shares bonding to a surface metal atom with another adatom, their interaction will be repulsive. This is because a metal atom decreases coordinative unsaturation when other atoms attach. The resulting repulsive interaction will be a few times 10 kj mol . On the other hand, when two adatoms interact indirectly so that each adatom-metal atom bond is separated by one metal-metal atom distance the lateral interaction wiU be attractive. This is the consequence of bond weakening of bonds directed to a metal atom, when its coordinative unsaturation decreases. 

The above 1-dimensional model may be extended to 3-dimensions, in a straightforward manner, and yields a substrate whose surface is completely covered by adatoms. The TBA again leads to a difference equation and boundary conditions which can be solved directly (Grimley 1960). We do not intend to discuss the 3-dimensional case here and, instead, direct the reader to the loc. sit. articles. However, in passing, we note that, even when there is no direct interaction between the atoms in the adlayer, an important indirect interaction occurs between them via the substrate by a delocalization of the bonding electrons in directions parallel to the surface (Koutecky 1957, Grimley 1960). This topic is discussed in Chapter 8. 

In another interesting development, Gumhalter and Brenig (1995a,b) proposed that, in H-Ni(llO) and H-Cu(llO) systems, adatom-induced reconstruction of the surface gives rise to quasi-one-dimensional surface states, which mediate the indirect interaction. The greatly reduced dimensionality... 

The interaction energy, AIT, is the contribution (positive or negative) to AE, due to the (indirect) interaction between the two adatoms. In other words, AIT is the difference between the chemisorption energy AE for the doubleadsorption system and the sum of the chemisorption energies A (A = a or b) for the two individual single-adsorption systems, i.e.,... 

Summarizing, it is clear that the indirect interaction between adatoms has a significant effect on the chemisorption properties of the system. Most noticeably, the chemisorption energy has a damped, oscillatory dependence on the adatom separation, as first quantified in (8.1) by Grimley. Thus, multi-atom adsorption occurs preferentially with the atoms at certain sites relative to one another. 

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