Exchange Forces Between Neutral Molecules and a Metal Surface

A Heitler-London treatment of the exchange part of the mutual energy of a neutral atom and a metal is developed. The resulting interactions are evaluated on the basis of a simplified model of the metal and lead to a convenient and simple expression for the total exchange energy. In Section III this expression is applied to the interaction of ${\mathrm{H}}_2$ and He with metals where it is found to represent a repulsion. By adding this exchange interaction to the attractive van der Waals interaction between the molecule and the metal, a potential curve of the usual type possessing a minimum is obtained for each system. The depths of these minima are compared with observed heats of van der Waals adsorption. A discussion is given in an appendix of the effects of some of the simplifications and approximations employed.