Pauli Repulsion Effects in Scattering from and Catalysis by Surfaces

The physical origin of the repulsion between adsorbates and a substrate crystal at short distances is discussed. This steric repulsion [3] or frozen orbital repulsion [4] is shown to consist of a generally attractive classical Coulomb interaction of the superimposed charge distributions and a repulsive contribution (Pauli repulsion) due to a kinetic energy increase caused by the antisymmetry requirement The non-pairwise additivity of the Pauli repulsion is shown to be important for scattering of ions at hyperthermal energies (in the order of 50 eV) from a surface. At a metallic surface relief of Pauli repulsion with the conduction electrons may easily occur because of the finite density of states at the Fermi energy. The finite-cluster representation of this effect by configuration changes (i.e. cluster excitations) is discussed for CO on Cu adsorption. The enhancement of the selectivity of the ethylene epoxidation by the presence of subsurface O below the catalytic Ag surface provides another example of the importance of the relief of Pauli repulsion.