4. Electronic properties of surfaces with adsorbates

The actual surfaces of solids are generally covered by atoms, molecules, etc., from the surrounding gaseous environment. This adsorption phenomenon arises from the unique positions of surface atoms in solids, which lack certain bonds compared to those in the bulk. Schematically, adsorbates can fill these missing bonds in two ways.

• If the attractive adsorbate-substrate interaction is weak (i.e., if the interaction forces are of the Van der Waals type), then the energies associated with the adsorbate-substrate bonds are of the same order of magnitude as the heat of liquefaction of the adsorbed gases, which is barely more than 30 kJ/mole (0.3 eV/particle). As a result, the electronic state of the adsorbed gas and that of the initially clean surface are hardly altered by adsorption. This is the phenomenon of physisorption, which we will not study...