Results of Surface Crystallography

In this section, we present and discuss the accumulated results of surface structure determination. We give a comprehensive tabulation of published results. Only those structures which provide three-dimensional geometrical 

A variety of surface analytical techniques have contributed to these results, as detailed in the tables. Their names, acronyms and characteristics were summarized in Table I and further described in sections 2 and 3. 

We first discuss some basic aspects of surface structures, including ordering principles and notations, followed by an overview of the number and types of surface structures that have been investigated. Finally we shall highlight a few major trends emerging from the structural results. 

The driving force for ordering originates, just as with three-dimensional crystal formation, in the mutual atomic interactions. With adsorbates, an important distinction must be made between adatom-adatom and adatom-substrate interactions. In chemisorption the adatom-adatom forces are usually 

The surface coverage of an adsorbate is another important parameter in ordering. We shall use the common definition of coverage where one monolayer corresponds to one adsorbate atom or molecule for each unit cell of the clean, unreconstructed substrate surface. Thus, if an adsorbed undissociated carbon monoxide molecule bonds to alternating top-layer metal atoms exposed at the Ni(100) surface, we have a coverage of a half monolayer. 

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We shall be concerned principally with electron diffraction, but recently some important results have been obtained using neutral atom (He) beam diffraction, to which we make brief reference in Sect. 3. The applications of interest are to the determination of the crystallography of clean surfaces and also of surface—adsorbate systems. 

Some experimental techniques [e.g., low-energy electron diffraction (LEED)-surface crystallography] can detect the structural changes that occur on both sides of the surface chemical bond. However, most currently used techniques are only capable of detecting the structural changes that occur on the adsorbate side (e.g., infrared spectroscopy) or on the substrate side (e.g., electron microscopy). As a result, we often gain only incomplete information about the surface chemical bond, leading to a one-sided molecule-centric or surface-centric view of the adsorbate-surface compound that is produced. 

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