Correspondence between image and structure

The chemisorption of sulfide ions involves a total charge transfer, as indicated by the perfect correspondence between the ordered structures revealed by the STM images and the flow of charge which accompanies their formation and which was estimated both by cyclic voltammetry and by chronocoulometry [12]. 

For bimetallic PdCu particles crystallized on MgO, an ordered structure (type CsCl) is always obtained after annealing. The particles are epitaxially oriented (001) on the substrate, with perfect coincidence between the lattices. The correspondence between experimental and simulated HRTEM images, and numerical analysis, are indicative of a layer of Pd at the interface between the alloy and MgO, with the atomic columns aligned with those of MgO. 

Since no computer-simulated images of these structures have been published, interpretation of these images has been purely intuitive, based on the 0.7-nm periodicity. Furthermore, the correspondence between the dark and light bands and the underlying structure is not known, although it has been assumed that, at optimum defocus, the dark fringes correspond to the T-0 layers. 

The change in ion hydration between bulk and interface was suspected since a long time [13] to be responsible for the distribution of ions in the vicinity of the interface whereas this effect can be easily understood qualitatively, a consistent quantitative theory has not yet been developed. The reason is that accurate information about the structure of water in the vicinity of an interface and about the corresponding changes in the ion hydration with the distance are yet beyond our reach. In the next section, a simple modality to account in a semi-quantitative manner for the ion hydration effects will be suggested. The effects of the van der Waals interactions and image forces will be compared with those of hydration. 

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