Causes of Ordering

It should be noted that in the limit of very large heats of chemisorption one may form surface compounds, oxides or carbides, for example. In this circumstance ordering of the new surface phase may require the relocation of the substrate atoms as well as the adsorbate atoms. Such chemisorption-induced reconstructions have been observed for several systems and its presence makes the conditions necessary for ordering in the surface layer very difficult to analyze indeed. Some of these systems will be discussed later in this paper. 

Xhe interatomic forces responsible for the binding of adsorbates at surfaces and for the ordering of overlayers are of various types. Xhe binding of adsorbates to substrates is frequently due to the strong covalent chemical forces, as a result of the presence of electron orbitals overlapping both the substrate and the adsorbate. Some adatoms (notably the rare gases) and many molecules will only weakly stick to substrates. 

The binding force is then predominantly due to the Van der Waals interaction and we have physisorption. 

The chemisorption case is exemplified by oxygen and sulfur on metals, the physisorption case by krypton and xenon on metals and graphite. Intermediate cases do exist for example, undissociated CO on metals is not physisorbed but chemisorbed and nevertheless it seems in many cases to be able to produce close-packed hexagonal overlayers. Also, some metal surfaces [for example, Pt(lOO), Ir(lOO), Au(100)] reconstruct into different lattices, exhibiting the effect of adsorbate-adsorbate interactions (here the adsorbate is just another metal atom of the same species as in the substrate). 

The theory of the binding of single adsorbates to substrates is today understood to some extent (cf. Sect. IV and VI), while the theory of adsorbate-adsorbate interactions and especially of large-scale ordering is in its infancy. 

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