Use of Surface States to Explain Activated Adsorption

A theory is advanced connecting activated adsorption with electron surface states in solids. The theory is constructed for ${\mathrm{H}}_2$ but the suggested mechanism would work equally well for other molecules. It is supposed that H atoms interact with surface electron states of the solid when the atom gets close enough to make the latter stable. The stability condition for surface states and its relation to the position of the visiting H atom is investigated in some detail. If the energy of the surface state is low enough, the atom will on reaching the critical distance for stability transfer its electron to the surface state with considerable reduction in total energy. At close distances, exchange sets in. The energy of the various interactions is calculated approximately on the basis of a simplified model of the surface potential field and the surface state wave function. By using a reasonable form for the repulsion between the H nucleus and the positive cores in the metal, total energy curves whose minima lie at depths up to 2.5 volts are obtained. An ${\mathrm{H}}_2$ molecule with sufficient energy may get close enough to the surface to come into the range of interaction of the H atoms with surface states. When this happens the molecule can split into atoms and be bound as such to the surface. The present theory of this interaction seems to be capable of properly accounting for the observed heats of activated adsorption of ${\mathrm{H}}_2$.