Excitons in Alkali Halides

Exciton states in alkali halides are analyzed on the basis of the energy band picture. First the excitons at the point \(\varGamma\) are discussed. A simple treatment in which the spin-orbit and the electron-hole exchange interactions are taken into account indicates that the intensity ratio of the halogen doublet is fairly sensitive to the exchange energy, while the doublet splitting is not very sensitive. It is shown that a pure triplet exciton state exists below the first allowed exciton state. Although such a state has not yet been detected, lifetime broadening of the first peak due to the existence of such a hidden state is conceived to make the magnitude of the width of the first peak comparable with that of the second peak. The pronounced structure observed in K and Rb halides in the spectral region just above the step is ascribed to d ε exciton formed at the point X . Multiplicities of the observed absorption peaks are explained in terms of this model. Semiquantitative analysis of the triplet structure in KBr and RbBr is made.