Chemisorption adatom site

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. 7.4. Chemisorption of adatom of site (bond) energy a(Pa) onto electrified chain of length m. Substrate has site (bond) energy on(P), where an = a + nT(n = 1,..., m), T being the potential gradient.

Summarizing, it is clear that the indirect interaction between adatoms has a significant effect on the chemisorption properties of the system. Most noticeably, the chemisorption energy has a damped, oscillatory dependence on the adatom separation, as first quantified in (8.1) by Grimley. Thus, multi-atom adsorption occurs preferentially with the atoms at certain sites relative to one another. 

The adsorption site, i.e. the chemisorption position of the adatoms on (within, below) the substrate surface, thanks to the polarisation dependence of SEXAFS. Often a unique assignment can be derived from the analysis of both polarisation dependent bond lengths and relative coordination numbers. The relative, polarisation dependent, amplitudes of the EXAFS oscillations indicate without ambiguity the chemisorption position if such position is the same for all adsorbed atoms. More than one chemisorption site could be present at a time (surface defect sites or just several of the ideal surface sites). If the relative population of the chemisorption sites is of the same order of magnitude, then the analysis of the data becomes difficult, or just impossible. 

In the case of the simplest mechanism of repeated adsorption-desorption events of the unaltered molecules, the retention time and the peak shape are insensitive to the composition of the carrier gas — now we would add provided that it constantly modifies the column surface. Some of the more complex mechanisms of migration are indistinguishable from the outside because they are analogously insensitive. It is, for instance, simple chemisorption, when the initial electronic structure of the adsorptive substantially changes upon adsorption, but is restored at the desorption stage. Such is also the microscopic history of metallic adatoms in metal columns. Another case is the chromatography of molecular halides in columns loaded with alkali halides the adsorbed state is a surface complex between the two halides see Sect. 1.5.1. In both examples the structure of the original adsorption sites is not necessarily restored. It is, of course, unimportant in the experiments with tracers. 

Prior to the early 1990s, all structural studies of alkali-metal chemisorption found the adatom located at high coordination sites at which the alkali-metal atom is bound in three- or four-fold hollow sites. A comprehensive survey of alkali-metal adsorption studies prior to 1988 may be found in the book edited by Bonzel (Bonzel et al., 1989). Several more recent LEED, SEXAFS and X-ray studies have implicated low coordination (top) sites, as in the case of Cu(lll)p(2x2)-Cs, or substitutional behavior. These results may signal that the current understanding of the alkali-metal bonding at surfaces is incomplete. 

Silicon chemisorption on metals has been studied in only one case an early LEED study of a (lxl)-Si overlayer on Mo 100) (see table 7). The Si occupies 4-fold hollow sites in which the shortest Si-Mo distance, 2.51 A, is between the adatom and the Mo atoms in the top layer. This agrees well with the sum of the covalent radii 2.49 A. It is interesting to compare Si adsorption on Mo(100) to C adsorption on the same substrate. Since the C adatom is significantly smaller than Si (the covalent radii are 0.77 A and 1.17 A respectively), the C adatom is able to sit much deeper in the hollow site, actually forming a bond to the second layer Mo atom. This is apparent from the relative adsorption heights of the two species which are 0.12 A and 1.61 A for C and Si, respectively. 

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