A general formula is developed for the interaction between a neutral molecule and a metal, and its relation to the image force law is exhibited, (Section III). The latter is shown to be valid only for molecules containing slowly moving charges, such as rigid permanent dipoles. A fairly accurate evaluation of the general formula involving empirical polarizabilities, / values, and resonance frequencies is made in Section IV. The numerical values for a number of gases and metals are collected in Tables I and II.
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 H? 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.
A theory is advanced connecting activated adsorption with electron surface states in solids. The theory is constructed for H2 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 H2 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 H2.A TENTATIVE theory of activated adsorption has been proposed by It is assumed that the adsorption of systems with unsaturated valencies is accomplished through a lowering of the surface potential of the solid in the immediate neighborhood of the visiting atom. This would give rise to a localized potential hole in which electrons would be caught and enter into normal exchange binding with the visitor. The forces involved would be large and the binding energy of the same order as in diatomic molecules. For molecules with completed bonds an activation energy is involved. The process in the case of H 2 on a metal is best described in terms of Fig. 1 (which is very similar to Fig. 3 in Lennard-Jones' paper 1 ). The curve (m) represents qualitatively the normal interaction through van der Waals or polarization forces of an H2 molecule with a metal M. The other curve (a) represents the much stronger interaction discussed above between two H atoms and the metal. The separation of the two curves at infinity is the dissociation energy D of the molecule. Consider a molecule approaching the metal with total energy W. If W is sufficient for it to reach the intersection G of the curves (m) and (a) (i.e., W=A), it can dissociate without changing its total energy. The individual H atoms would then follow the atomic curve (a) to their respective minima where they take up vibrational X J. E. Lennard-Jones, Trans. Faraday Soc. 28, 341 (1932). energy W'. The observed heat of adsorption will be E = Q+W-W or simply Q+A when the vibrational energy W is neglected. The combined binding of the two H atoms must be Q+D. In order to give ...
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