Theory of the Beta Band in Alkali Halide Crystals

It is shown that the $\beta$ band in alkali halides with the NaCl lattice consists of six components. Their energies are estimated in NaCl. It is suggested that the observed $\beta$ band is the component with the lowest energy and that the other components are not easily observed because they lie in the fundamental band. The calculated oscillator strength is 0.10, which is probably too small by a factor of 5. The final state for the $\beta$-band transition undergoes autoionization with the rate 1.8×${10}^{13}$ ${\sec }^{-1\mathrm{}}$. The width of the $\beta$ band is increased by only 0.012 ev or about 10% by this process.