Dipole-Dipole Resonance Forces

London has shown that at fairly large interatomic distances $r$, the polarization energy of a molecule composed of two identical atoms varies not as the customary $r^{-6\mathrm{}}$, but rather as $r^{-3\mathrm{}}$ in case one atom is in the ground state, and the other is in an excited state which can combine by dipole transitions with the ground level. The present paper is concerned with the size and sign of this dipole-dipole resonance energy obeying the inverse cube law. The sign has no immediate relation to the sign of the exchange and penetration forces which predominate at closer approach. This possibility of a change from repulsion to attraction results in one of the stable levels of the group of first excited states (derived from separate atom configuration $S+P$ for example) having a lip of activation energy before decomposition into atoms. There will also be a "repulsive" state having a shallow minimum at large separations. Molecules of the heavy atoms have an activation lip so large that the whole stable region is above the dissociation energy. Potential curves of this type have previously been derived empirically from spectroscopic analysis in the case of ${\mathrm{Hg}}_2$ and ${\mathrm{Cd}}_2$. The lip is less pronounced in lighter atoms but is none the less real, and has experimental confirmation in the ${\mathrm{Na}}_2$ spectrum.