Localized surface bond

When the combined system of adatom and substrate is in its ground state, so that the lowest energy levels are each doubly occupied with an a- and a /2-spin electron, then, if two electrons are in a localized level, a localized surface bond will be formed. However, if the localized level is unoccupied, then an adbond formation will be achieved without localization of the bonding electrons (i.e., only delocalized electrons will be involved in the adbond). In the case of electron localization, the type of adbond is determined by the charge order of the adatom state, namely,... 

Local surface structure and coordination numbers of neighbouring atoms can be extracted from the analysis of extended X-ray absorption fine structures (EXAFS). The essential feature of the method22 is the excitation of a core-hole by monoenergetic photons modulation of the absorption cross-section with energy above the excitation threshold provides information on the distances between neighbouring atoms. A more surface-sensitive version (SEXAFS) monitors the photoemitted or Auger electrons, where the electron escape depth is small ( 1 nm) and discriminates in favour of surface atoms over those within the bulk solid. Model compounds, where bond distances and atomic environments are known, are required as standards. 

If the wave function for a localized state is such that the probability of encountering the electron on the foreign atom is the same as that of encountering it in the crystal then, when such a state is doubly occupied, we have a purely homopolar surface bond. The quantity R, defined in terms of the wave-function coefficients by the equation... 

The occurrence of homopoiar localized states for the one-dimensional model has been discussed in some detail because the possibility of such states is a natural consequence of the molecular orbital approach to the problem of the surface bond. It is clear, however, that the conditions for their existence are rather stringent. Most sets of interaction parameters do not give a purely homopoiar state but give states with ionic character to a greater or lesser degree. 

Figure 1.9. The local aligned-bridge adsorption sites of the formate (HCOO-) species on Cu(110) and Cu(100). Also shown is the cross-bridge site on Cu(100) originally proposed as a new type of surface bond but subsequently shown to be incorrect.

Perhaps one of the important conclusions of these studies that points to the unique chemistry of surface irregularities, steps, and kinks, which appear to be active sites, is the controlling influence of the local atomic structure, local surface composition, and local bonding between adsorbates and surface sites. The microstructure of the metal surface controls bond scission and thus the rate and path of chemical reactions. Calculations taking into account this local bonding picture should help to unravel the elementary bond-breaking steps in catalytic surface reactions. 

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