The indirect interaction between atoms or molecules adsorbed on metals

Abstract

If the chemisorption of an atom (or molecule) on a metal involves a sharing of the conduction electrons with the adsorbed atom it follows that, because two chemisorbed atoms share the same electrons, there will be an interaction between them. This interaction, which is an indirect one, can arise because a discrete atomic level which lies in the conduction band of the metal is broadened and becomes a virtual level when the atom is chemisorbed. The two virtual levels going with two chemisorbed atoms only coincide when the atoms are infinitely far apart, otherwise the virtual levels are split. If the Fermi level falls near the original virtual level, the occupancy of the two virtual levels will be sensitive to this splitting, and because of this there will be a significant interaction between the chemisorbed atoms. This is likely to occur with the alkali metal atoms on tungsten and nickel, and with hydrogen, but not for many molecules. An investigation using a simple model shows that the indirect interaction is long range (inverse square law) and oscillatory.