Adsorbate-induced relaxation

In contrast to the adsorption geometry there is some discrepancy concerning the adsorption energies as calculated by the cluster calculations. The values of 45 and 28 kJ/mol estimated for the two TDS peaks from the Redhead formula are considerably larger than the calculated values of about 27 and 15 kJ/mol. As has been pointed out in connection with NiO, it has to be expected that the calculated values are too low since neither adsorbate-induced relaxations of the... 

Another complication is that the substrate often does not possess a rigid structure but responds to the forces exerted by the adsorbate by some local deformation. Such an adsorbate-induced relaxation of the topmost substrate layer (with a corresponding change of lattice parameters but no change of lattice symmetry) or adsorbate-induced reconstruction (with a corresponding change of lattice symmetry of the topmost substrate layer) will not be discussed further here. 

As noted in Section 1.2, accurate determination of adsorbate-induced changes in surface-normal structure, i.e. the Adj2 interplanar spacing between the first and the second atomic layers, can be achieved by measuring the CTRs [1—4, 10, 35]. Previous reviews summarized adsorbate-induced relaxation and reconstruction on well-defined Pt(hkl) and Pt-bimetallic surfaces in aqueous electrolytes at electrode potentials at which a maximum surface coverage of adsorbed species is established [28, 29]. The data revealed that either close to the hydrogen evolu-... 

U. Starke, M. Van Flove, and G. Somoijai, Adsorbate-induced relaxations of close-packed fee and hep metal surfaces. Progress in Surface Science, vol. 46, pp. 305-319, 1994. 

Relaxations must be referenced to another structure, for which the obvious choices are the ideal bulklike positions or the already relaxed clean-substrate positions. Since the clean-substrate positions are not uniquely known (each analysis gives somewhat different results), we use as an unambiguous reference for relaxations the ideal bulk-like positions obtained by a mathematical termination of the bulk lattice. However, we also include in this chapter, for direct comparison with the adsorbate-induced relaxations, tables of the published structural results determined for relaxed clean surfaces. 

For low-symmetry adsorption, ad hoc descriptions are given in each individual case, and in these situations the adsorbate-induced relaxations often become quite complicated (asymmetrical) and are not listed in detail it is suggested to consult the Surface Structure Database [99W] for further information. 

Trends in adsorbate-induced relaxations on hcp(OOOl) are less clear than on fcc(lll), cf. Table 5. Perhaps the effects seen on fcc(l 11) do not carry over to hcp(OOOl), or the effects on hcp(OOOl) are more sensitive to details of the atomic arrangements within the various unit cells. A more exhaustive analysis and comparison of the results on a single metal, Ru(OOOl) [97M1], also suggests that structural trends may be difficult to extract for this one metal, and thus even more so for the class of hcp(OOOl) surfaces. 

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