Atomic roughness

Fig. 28—The cobblestone model of friction, a spherical molecule moving on atomic roughness (reproduced after Ref. [24]).

Let us consider the ab initio task first. At each data point, we require the ab initio energy, gradient and second derivatives. Up to now, PESs have been constructed for about a dozen reactions involving four atoms, two involving five atoms and two involving six atoms. Roughly speaking, several... 

The comparison of continuum and atomistic models by Luan and Robbins demonstrates that the atomic details of this contact can have a significant influence on the calculated friction. However, those calculations did not explore atomically rough surfaces, which are most likely found in real engineering contacts. The effect of roughness has been investigated recently by Qi et al. in a study of the friction at the interface between two Ni(100) surfaces.85 Two models were considered in that work. In the first model, both surfaces were atomically flat i.e., the rms roughness was 0.0 A. In the... 

Spectroscopic studies on the Fe-Mo protein by EPR and Mossbauer spectroscopy have shown six iron atoms each in a distinctive magnetic environment coupled to an overall S=3/2 spin system (6,7,8) and electron nuclear double resonance (ENDOR) studies suggest one molybdenum per spin system (8). The 5 Fe signals (five or six doublets) observed in the ENDOR spectra (8) indicate a rather asymmetric structure for the Fe/Mo/S aggregate in which the iron atoms roughly can be grouped into two sets of trios, each set having very similar hyperfme parameters. 

The monazite structure consists of distorted PO4 tetrahedra with each metal atom roughly equidistant from nine oxygen atoms. Minor amounts of other rare-earth elements may occur. Steady-state liuninescence under X-ray excitation of monazite revealed emission of Gd, Tb, Dy and Sm (Gorobets and Rogojine 2001). Laser-induced time-resolved liuninescence enables us to detect Sm +, Eu and Nd emission centers (Fig. 4.70). 

Steps at metal surfaces were argued to be good candidates for an increased reactivity because of the lower coordination number of the atoms [8]. In a seminal work Davis and Somoijai [9] concluded, e.g. that the ability of Pt to break C-H bonds during hydrocarbon dehydrogenation is enhanced at kinked Pt surfaces pre-dosed with oxygen, while Gland and coworkers demonstrated that CO adsorbs preferentially on the atomically rough steps of Pt(3 21) [10] and that some 02 dissociation [11] may occur at the same surface. 

From the data in Fig. 13 for Ag, Cu, and Pt it appears that the heat of sulfur adsorption increases with increasing atomic roughness of the surface. In the case of Ag, the saturation coverage is reached at the same h2s/ h2 (3-2 x 10-3) on all three low-index planes [(100), (110), and (111)]. However,... 

SiC). In a-SiC, whatever their chemical nature, basal planes are faces of lowest surface energy which, even at high temperatures, present a negligible atomic roughness that is reflected at the macroscopic scale by the formation of very stable facets. 

The active site consists of two iron atoms, roughly 3.25-3.5 A apart, bound to the protein ligand by live histidine side-chain residues. In the reduced deoxy form the core is asymmetric, having one 5-coordinate and one 6-coordinate iron atom. Two protein carboxylate units and one hydroxo group derived from water are bridged across the iron atoms as shown in Fig. 17-E-9. 

In summary, oxide layers on silicon surfaces are chemically dissolved in HF leading to the formation of an atomically rough hydrogen-terminated surface. At (111) surfaces, relatively large domains of ideally terminated Si(lll) H (1x1) can be formed by subsequent immersion in buffered HF of pH 8-9. The smoothening process is controlled by the rate of step recession at the surface. 

Simulations of self-diffusion have been reviewed recently [60]. In addition to molecular motion on flat surfaces (including those with atomic roughness), selfdiffusion constants have been evaluated for atoms adsorbed on surfaces with comers (as in pores with rectangular cross sections or on grooved surfaces) and with steps. In these systems, a deep nearly one-dimensional potential well occurs in the model gas-surface energy at the comers. Atoms adsorbed in this well are essentially localized in one-dimension, which means that self-diffusion hardly occurs in the directions perpendicular to the comer. 

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