After reviewing briefly the relevant experimental data on exciton luminescence, it is shown by a calculation on KCl that cubic relaxation alone cannot explain the experimental results for the emission energy. A model is set up for the local relaxation of the crystal into a $V_K$-like or diatomic molecule configuration. Claculations with this model show that it can give the correct results for the emission energy in KCl, NaI, KI, and RbI. Furthermore, the model is able to provide an understanding of the symmetry properties of the excition states as deduced from experiments using polarized light. The lifetimes of the exciton states can be understood in terms of the magnitude of the Stokes shift and the degree of localization of the one-particle function that describes the excited electron. Activation energies for the disappearance of the emission bands can be obtained from the model. The concept of "noncrossing of energy levels" which is important in molecular theory plays a prominent role in explaining the exciton data.

• Received 2 May 1966

DOI:https://doi.org/10.1103/PhysRev.151.629