Adatom sites

The relative importance of the two mechanisms - the non-local electromagnetic (EM) theory and the local charge transfer (CT) theory - remains a source of considerable discussion. It is generally considered that large-scale rough surfaces, e.g. gratings, islands, metallic spheres etc., favour the EM theory. In contrast, the CT mechanism requires chemisorption of the adsorbate at special atomic scale (e.g. adatom) sites on the metal surface, resulting in a metal/adsorbate CT complex. In addition, considerably enhanced Raman spectra have been obtained from surfaces prepared in such a way as to deliberately exclude one or the other mechanism. 

Fig. 1.1. Chemisorption model, where aa(Pa) is adatom site (bond) energy, as(a) surface (chain) atom site energy and (3 chain bond energy.

Fig. 16.10. Topograph and local spectroscopy of Si(lll) with oxygen. Left, the STM image of a region of the oxygen-exposed Si( 111 )-7 X 7 surface. Right, the local tunneling spectra at different sites. Spectra A, B, and C are those of unreacted restatom, corner atom, and center adatom, respectively. Spectra D, E, and F are obtained over oxygen-induced dark, bright, and perturbed (gray) adatom sites, respectively. (Reproduced from Avouris, Lyo, and Bozso, 1991, with permission.)...

Fig. 4.57 (a) shows a hydrogen field ion image of an iron tip. When N2is admitted into the system, many atoms in lattice sites are displaced to adatom sites, as shown in the N2 gated image of (ft). It is from these adsorption sites that NH, and intermediates are desorbed as shown in (c). 

Keywords Atomic scale characterization surface structure epoxidation reaction 111 cleaved silver surface oxide STM simulations DFT slab calculations ab initio phase diagram free energy non-stoichiometry oxygen adatoms site specificity epoxidation mechanism catalytic reactivity oxametallacycle intermediate transition state catalytic cycle. 

Analysis of INS (33) LEED (29a, 33, 40), and UPS data 32 For the same adatom sites, coordination for binding to Ni atoms may change depending on S coverage... 

For instance, an adatom site ads on the surface is transferred to an unoccupied or free" adsorption site after dissolution. A surface atom" site is changed to a vacant site vac on the surface, accordingly. In both cases each single ion transfer changes the number of sites on the surface by unity ... 

Fig. 5.1. Highly coordinated adatom sites at an fcc(lll) or an hcp(OOOl) surface. The upper picture shows a top view of the surface, labeling the two threefold coordinated hollow sites. The lower row illustrates the local geometries of the three high-symmetry interstitial sites between the first and second substrate layers. Metal atoms are depicted as light, large spheres, and oxygen atoms as dark, small spheres...

Next, the adsorptions should be paired, involving an adatom and a restatom. When the adsorptions are paired, H chooses the adatom site. HO goes on the restatoms (and corner). The successive introduction of the fragments is sketched on the Figure 8. 

Figure 29.12 CMsBr moLecuLe physisorbed on an adatom site on Si (7 x 7). The dark arrow indicates the reaction site for the downward-propelled bromine atom the lighter arrow indicates the upward ejection of CH3. Both processes occur after capture of a photoexcited substrate electron see text for details. Adapted from Osgood et al, Surface Science, 2004, 573 147, with permission of Elsevier...

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