Substrate atom sharing interactions

If an adsorbed species, such as a H atom, is adsorbed in a bridged site between two substrate atoms, a second adatom adsorbed in a bridge with one substrate atom common to the first will produce a direct bonding interaction. This effect has been theoretically examined by Grimley and Torrini [62] and may be energetically dominant. It is clearly a specific, short-range interaction. 

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In the formation of electron-pair sharing, we need to create a radical state in the interacting atom or molecule in order to form the bond to the substrate. The different interactions between an adsorbate and metal surface are summarized in Fig. 12.4. For a molecular adsorbate the bond-prepared radical state can be obtained upon internal (partial) bond-breaking where the fragments will have unpaired electrons that can interact with unpaired electrons in the metal surface. We can also consider virtual processes through orbital interactions or an excitation process that creates a bond-prepared state. 

Chemisorption occurs when the attractive potential well is large so that upon adsorption a strong chemical bond to a surface is fonued. Chemisorption involves changes to both the molecule and surface electronic states. For example, when oxygen adsorbs onto a metal surface, a partially ionic bond is created as charge transfers from the substrate to the oxygen atom. Other chemisorbed species interact in a more covalent maimer by sharing electrons, but this still involves perturbations to the electronic system. 

A local electronic influence is illustrated by B. The important feature in this case is the presence or absence of an electron (or a hole in the complementary vacancy). In the absence of the electron, the defect will be identical with that of A, but the difference will be that in B the substrate interacts by sharing an electron with the defect as a whole (or donating one to it), while in A the substrate interacts individually with two or more of the atoms of the defect, presumably by polarization. 

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