Monomolecular Step Migrations

Elementary steps and their dynamic behavior have been observed on surfaces of a wide variety of crystals. However, no one has so far succeeded in directly visualizing growing elementary steps of ice crystals, besides our recent works [16, 19]. The difficulty in visualization is mainly due to (i) the small height of elementary steps of ice crystals, (ii) high equilibrium vapor pressure of ice, and (iii) QLLs that appear on ice crystal surfaces at temperatures near the melting point 

Many researchers have performed various optical microscopic observations of the surface morphology of ice crystals, such as steps [10, 20-23], height topography [24-31], and QLLs [32, 33], by ordinary bright field microscopy [10, 23, 24], differential interference contrast microscopy [20-22, 25, 29-31, 33], two-beam interferometry [26-28], and laser reflection microscopy [32], However, no one has yet succeeded in proving, by their optical observations, that they could visuahze individual elementary steps, mainly due to the small height of elementary steps. 

Transmission electron microscopy (TEM) has a sufficiently high resolution to visualize solid surfaces at the atomic level. However, we cannot apply TEM for observation of ice crystal surfaces because of the high equilibrium vapor pressure of ice. Although scanning electron microscopy (SEM) can be used under equilibrium water vapor pressure, the resolution limit of SEM in the vertical direction cannot reach the molecular level. 

Atomic force microscopy (AEM) is one of the most widely used techniques for observing solid surfaces at the molecular level. Nevertheless, it is generally 

Ih ice single crystals were grown heteroepitaxially on a cleaved 0001 face of an Agl crystal from supersaturated water vapor in a nitrogen environment The temperature of the ice single crystals (Tg pi ) was set at —15.0°C, at which temperature basal ( 0001 ) faces of ice crystals grow most widely [8]. To supply water vapor to the sample ice crystals, other ice crystals were prepared (as a source of water vapor) on a copper plate that was set 16 mm from the Agl crystal, and then the temperature of the source ice crystals was separately set at—13.0 °C. By 

---

The same process takes place on an oxide surface. The NiOg octahedra shown with the wireframe of Fig. 1, for example, ultimately detaches as aNi(II) monomer [shown in Fig. 5 as Ni(H20)6 monomolecular step migrates. The coordination numbers of oxygens co-ordinated to adjacent Ni(II) atoms are all decreased by one as a monomer detaches. These metals re-establish their inner coordination spheres by movement and association with water molecules or adsorbates.To maintain a... 

The coordination of these oxygens at the monomolecular step will, of course, change as the mineral reacts with the aqueous solution because the MnOe octahedra shown with the wireframe (Figure 2) must detach as a surface complex as the step migrates. This detachment causes newly uncovered metals to reestablish their inner-coordination spheres by movement and deprotonation of water molecules. If the reaction proceeds at steady state, these water molecules dissociate to maintain a fixed charge density and a fixed numbers of different metal-ligand coordination numbers (7, 8). 

Migration of the hydride locks the configuration of the enantiomeric alkene-carbon centre in general this step is also reversible but probably not in this instance. In the examples studied neither the dihydride intermediates nor the alkyl intermediates have been observed and so it seems reasonable to assume that addition of H2 is also the rate-determining step. Since the latter is a bimolecular reaction and the other ones are monomolecular rearrangement reactions, one cannot in absolute terms say that oxidative addition is rate-determining. 

---