The Adsorption Geometry of Atoms

Turning to metal substrates, in most cases of atomic adsorption on metal surfaces where the adsorption geometry has been determined (cf. Table 6.1), only one adsorption site is involved, i.e., all adatoms have identical surroundings [the exceptions are Ni(l 11) 

Adsorption sites (or registries ) on metals differ mainly in the number of nearest metal neighbors (the coordination number) and the two-dimensional symmetry. One might expect the adatom valence to influence the number of nearest metal neighbors and therefore the adsorption site. However there is little evidence for such a behavior, 

If one now also takes the second and deeper substrate layers into consideration, one may in particular wonder whether the adsorbate atoms choose an adsorption site consistent with a continuation of the substrate lattice. It appears from the available 

Multilayers of Cd on Ti(0001) have been studied as well, indicating a Cd crystal growth according to the sequence. . acacABAB... the Cd film has the expected hep structure known for the bulk material. In this case the Ti and Cd lattice constants are sufficiently close to allow growth of the film in registry with the substrate mesh. 

A further question regarding the adsorption registry is whether it depends on adsorption coverage, i.e., on density of adatoms this is relevant to the effects of adatom-adatom interactions. The situation is illustrated by a limited set of results, namely those for quarter-monolayer and half-monolayer adsorption of 0, S, Se and Te on Ni(lOO) in p(2 X 2) and c(2 X 2) periodicities the adsorption site is found not to depend on coverage in these cases (the nearest adatom-adatom distances are 4.90 and 3.46 A for the two coverages, respectively, compared with the largest adatom diameter of about 2.7 A for Te). 

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