Electrostatic Effects in Atomic Adsorbates on Jellium

It is worth considering what sort of charge transfer adsorption may cause, since this may strongly influence the work function. 

Consider an atom approaching the surface in Fig. 6.23. If the upper level of the atom originally contained an electron, then upon adsorption it will transfer part of this electron density to the metal and become positively charged. This is the case with alkali atoms. The atom forms a dipole with the positive end towards the outside, which counteracts the double layer that constitutes the surface contribution to the work function of the metal (Fig. 6.13). Thus alkali atoms reduce the work function of a metal surface simply because they all have a high-lying s electron state that tends to donate charge to the metal surface. 

Conversely, an atom in Fig. 6.23 with an affinity level that initially is empty becomes partly occupied upon adsorption. Hence, charge is transferred from the metal to the atom. This sets up a dipole that increases the surface contribution to the work function. This is the case for adsorbed halides, which will be negatively charged at the surface. We will later see that such dipole fields can explain promotion and inhibition effects caused by various adsorbates in catalysis. 

More detailed information can be obtained by calculating the charge contours by a procedure called the density-functional method. We will not explain how it works 

A map of the electron density distribution around these atoms provides important information. It tells us to what distance from the adatom the surface is perturbed or, in catalytic terms, how many adsorption sites are promoted or poisoned by the adatom. The charge density contours in Fig. 6.27 are lines of constant electron density. 

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