Polar surfaces, electronic structure

Studies in recent years on the surface properties of transition metal oxides have demonstrated that the surface structural stability, the surface electronic structure, and the surface chemical reactivity depend on the crystallographic orientation of the exposed surface and the presence of surface imperfection, such as steps and point defects (1 ). ZnO is one recent example. The natural surfaces of ZnO, which can be prepared in a relatively well-ordered state, include he Zn-polar (0001), the 0-polar (OOOT), and the nonpolar (IOIO) surfaces. (See Figure 1 for a schematic representation of these surfaces). These surfaces have been shown to possess different chemisorptive properties and reactivities. It was shown that CO2 was desorbed from a nonpolar surface at about 120 0, but from a Zn-polar surface at (2 ). 

From both recent experimental and computational studies the Surface electronic structure of non-polar compound semiconductors comprises a filled valence band separated from an empty conduction band by a surface bandgap. The surface... 

The anisotropy in reflectance of a surface is a result of anisotropy in the surface electronic structure (and hence a difference in the response of a surface to Hght polarized in two different directions). The anisotropy in the surface electronic structure may be the result of (i) a surface having an anisotropic geometry or (ii) intrinsically isotropic material being arranged into structures that are aligned in a preferential direction on an isotropic surface. To date, the appHcations of RAS have concentrated on the first category. The RAS response from clean... 

In order to describe the second-order nonlinear response from the interface of two centrosynnnetric media, the material system may be divided into tlnee regions the interface and the two bulk media. The interface is defined to be the transitional zone where the material properties—such as the electronic structure or molecular orientation of adsorbates—or the electromagnetic fields differ appreciably from the two bulk media. For most systems, this region occurs over a length scale of only a few Angstroms. With respect to the optical radiation, we can thus treat the nonlinearity of the interface as localized to a sheet of polarization. Fonnally, we can describe this sheet by a nonlinear dipole moment per unit area, -P ", which is related to a second-order bulk polarization by hy P - lx, y,r) = y. Flere z is the surface nonnal direction, and the... 

The surfaces lying normal to the ends of the polar axes differ in their electronic structures because they have differing chemical species exposed. 

Orientation Determination. While polarized edge studies, together with a known sample orientation, can provide information about the electronic structure of the absorber, one can also use polarized edges to probe ordered systems of unknown orientation. This sort of approach was used in a study of B adsorbed on graphite (26,27). In this case, the orientational dependence of an edge transition was used to calculate the degree of orientational purity of the graphite surface. 

In words, s describes the interaction of the solute charge distribution component p, with the arbitrary solvent orientational polarization mediated by the cavity surface. The arbitrary weights p,, previously defined by (2.11), enter accordingly the definition of the solvent coordinates, and reduce, in the equilibrium solvation regime, to the weights tv,, such that the solvent coordinates are no longer arbitrary, but instead depend on the solute nuclear geometry and assume the form se<> = lor. weq. In equilibrium, the solvent coordinates are correlated to the actual electronic structure of the solute, while out of equilibrium they are not. 

For interfacial systems, potential functions should ideally be transferrable from the gas-phase to the condensed phase. Aqueous-mineral interfaces are not in the gas phase (although they may be close, see (7)), but both the water molecules and the atoms/ions in the substrate are in contact with an environment that is very different from their bulk environment. The easiest different environment to test, especially when comparing with electronic structure calculations, is a vacuum, so there is likely to be a great deal of information available on either the surface of the solid or the gas-phase polynuclear ion or the gas-phase aquo complex (i.e., Fe(H20)63+, C03(H20)62-). The gas-phase transfer-ability requirements on potential functions are challenging, but it is difficult to imagine constructing effective potential functions for such systems without using gas-phase systems in the construction process. This means that any water molecules used on these complexes must also transfer from the gas phase to the condensed phase. A fundamental aspect of this transferability is polarization. 

To verify the anisotropy observed on the silver surface and to attempt to understand the effect of the electrochemical solution on the surface electronic and structural properties, Bradley et al. [124] have examined the SH response from a Ag(111) surface in UHV. The experiments on this crystal were then repeated after an inert transfer to the electrochemical cell. The SH experiments performed in the electrochemical cell were again conducted at the PZC to minimize the effect of the dc electric field on the surface properties. Fig. 5.3 a and b show the results for the crystal examined in UHV for p- and s-polarized output at 532 nm. The solution data is consistent with the previous in-situ results of Koos et al. [122] shown in Fig. 5.1. More importantly, when the fits to the UHV data are compared to the subsequent results performed in solution, nearly identical values for the relative magnitudes of the a and c(3) coefficients are found (see Fig. 5.5 for comparison). Bradley et al. [124]... 

---