Image-Potential Surface States

Figure 5.15 Squared wave functions for the Shockley surface state (n = 0) and the n = 1 image-potential surface state. The Shockley surface state is depicted for both a step and an image potential.

There remains the estimated value of °(H+/H2)vs. UHV based on binding energies for image potential-induced surface states,49 which is,... 

Similar to the failures of the free-electron model of metals (Ashcroft and Mermin, 1985, Chapter 3), the fundamental deficiency of the jellium model consists in its total neglect of the atomic structure of the solids. Furthermore, because the jellium model does not have band structure, it does not support the concept of surface states. Regarding STM, the jellium model predicts the correct surface potential (the image force), and is useful for interpreting the distance dependence of tunneling current. However, it is inapplicable for describing STM images with atomic resolution. 

First of all, the existence of the surface establishes a certain correlation between different electronic states, particularly between incident and reflected electrons. In addition there are electron density variations both within the surface and along the surface normal. Since the screening radius increases with decreasing electron density, it is obviously unrealistic to third of a spherical potential hole accompanying its electron in the immediate surface region, where the electron density drops very rapidly to zero. At the very lowest densities, in particular, the potential must somehow go over in continuous fashion into the classical image potential applying far away from the metal surface. Finally, from the dynamic nature of these interactions it follows that the problem must be dealt with self-consistently each electron contributes to the holes of all the other electrons in a manner dependent on the detailed features of its own potential hole. 

Authors do not really agree on the charge redistribution which takes place at the MgO(lOO) surface, because the ionicity of the oxide appears to be nearly total in HF approaches [61,63], while in DFT and semi-empirical methods, the Mg-0 bonds present a small but non-negligible covalent character [60,66]. The surface projected Density of States (DOS) displays surface states just at the top of the VB and bottom of the conduction band (CB), originating from a reduction of the Madelung potential on the surface atoms. These states also exist on NiO(lOO) [74] and CoO(100)[75] and were shown to play a role in the formation of STM images (Fig. 2). A small reduction of the HOMO-LUMO gap results in MgO(lOO) as well as CaO(lOO) [60,64,67,68], and NiO(lOO) [74], which is qualitatively consistent with experimental observation [76-79]. 

Marchf et al. have applied image potential arguments to predict the orientation of a diatomic molecule of H2 type adsorbed in a precursor slate with respect to a metal surface. Their arguments are illustrated in Fig.(2.60). They use the valence bond picture of chemical bonding in H2 that we discussed in section 2.2. Bonding in H2 is considered to be the interaction of a covalent neutral state 0 nd the two... 

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