Subsurface atoms

The effects of adsorbate coverage (film thickness) on the Pd 3d5/2 XPS peak positions of the Pd/W(l 1 0), Pd/ Re(0001), and Pd/Mo(l 10) systems were systematically investigated [63]. The peak positions reported for Pd coverage in excess of 1 ML represent a product of electrons emitted from surface and subsurface atoms. For the case of Pd(lOO), theoretical calculation suggest that the Pd 3d5/2 XPS BE of the surface atoms is 0.4 eV lower than that of bulk Pd. A similar difference has been observed experimentally for Ni and Pt surfaces. These shifts in BE are a consequence of variations in the coordination number of the surface atoms compared to bulk atom. If we reference the combined peak of bulk and surface atoms in 40 ML of Pd on W(1 1 0) to that of Pd(l 00) a difference of —0.8 eV is obtained between the Pd 3ds/2 BE of a pseudomorphic monolayer of Pd on W(110) and that of the surface atoms of Pd(l 00). The corresponding shifts... 

Another very important technique for fundamental consideration of multicomponent systems is low energy ion scattering (LEIS) [Taglauer and Heiland, 1980 Brongersma et al., 2007]. This is a unique tool in surface analysis, since it provides the ability to define the atomic composition of the topmost surface layer under UHV conditions. The signal does not interfere with the subsurface atomic layers, and therefore the results of LEIS analysis represent exclusively the response from the outer surface. In LEIS, a surface is used as a target that scatters a noble gas ion beam (He, Ne, ... 

Using the dendrimer route, it is possible to prepare supported catalysts not available via traditional routes. Dendrimer derived Pt-Au catalysts having compositions within the bulk miscibility gap can be prepared on several oxide supports. For all the supports studied, the bimetallic catalysts exhibited synergism with respect to mono- and cometallic catalysts for the CO oxidation and hydrocarbon NOx SCR reactions. The bimetallic Pt-Au catalysts also showed evidence of exchanging surface and subsurface atoms in response to strongly binding ligands such as CO. 

Fig. 3. Resolution of subsurface atomic positions in STM scans on Al(lll) and comparison with experiments. The difference in apparent height between the fee hollow site and the hep hollow site of about 5 pm in experiments (a) and (b) is fully accounted for in the simulations (c).

Recently Koutecky (23) has shown that localized subsurface states may also occur if the perturbing potential due to the existence of the surface is large enough. These states would have wave functions localized about subsurface atoms and their energy in the forbidden zone would be between the surface states and the band out of which the states had been perturbed. These treatments have all assumed that the potential depends only on the direction into the crystal a one-dimensional potential. The discussion above of the cleaned germanium surface has shown that a three-dimensional potential may be necessary. A second failing of these treatments is their lack of consistency the effect on the potential of the filling of the surface states with... 

In this model the N atom of the NV -center belongs to the second subsurface layer. Partial and complete optimization of the geometry was carried out for the C33H3o[NV] cluster. For the first case, the relaxation of only N and C atoms, being the first and the second neighbors to N atom forming the first and the second subsurface atomic layers was taken into account. All the other atoms of the cluster were fixed. The starting and relaxed distances for the C-C and C-N pairs are presented in Fig. 2. 

In this respect, magnetic measurements give valuable information. They can show whether poisons react not only with surface metal atoms but also with subsurface atoms as for the case of H2S chemisorption over Nt/Si02 catalysts (refs. 11. 12) the loss of magnetization caused by H2S chemisorption at saturation at room temperature is twice that produced by H2 chemisorption at atmospheric pressure and room temperature in a separate experiment. Assuming that hydrogen chemisorption is restricted to surface metal atoms, then, it can be deduced that H2S is able to attack the nickel particle in depth (corrosive chemisorption). 

Three ideas give a coherence to recent work on the first two of these topics CO breakdown, the importance of M-C Hy species, and subsurface atoms (which are also noted in Section 7). 

Fig. 9.12. Sequence of phases associated with Au(OOl). At high temperatures, distorted hexagonal phases are superseded by disordered phase with 1x1 symmetry. RDH refers to rotated-distorted hexagonal phase, DH refers to distorted hexagonal phase and DIS refers to disordered phase. Note that the schematic of the rotated hexagon associated with RDH exaggerates the rotation. In addition, the orientation relationship between the hexagonal structures and the subsurface atoms is not represented.

Fig. 1. The dimerization of the Si(lOO) face as obtained by Roberts [6], The displacement of the subsurface atoms is important.

However, LEIS (ISS) offers one advantage an extreme surface sensitivity can be obtained with He ions because ions scattered at subsurface atoms will neutralize at the exit path. If only scattered ions are detected, the signal will therefore originate exclusively from the surface monolayer of atoms. This property is extremely useful in, for example, studies of surface segregation. 

Ey (q), Ey (te), when the vacancy is still in the nanoshell at the subsurface atomic layer such a site in the plane 111 has three vacancies and in the plane 100 it has four (Figure 7.9a) ... 

Figure 11.11. Top and side views of the surface bilayer of Si(l 11) the larger shaded circles are the surface atoms the smaller shaded circles are the subsurface atoms which are bonded to the next layer below. The large open circles represent the adatoms in the two possible configurations, the Hs and T4 positions.

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