Regularities between Exciton States andF-Center Excited States of Alkali Halides

The extent to which the intrinsic and the $F$-center absorption for alkali halides have common features has been explored by plotting the respective band photon energies against each other. Of the many possible binary combinations of excitonic and $F$-center energy-level separations for the nine potassium, rubidium, and cesium chlorides, bromides, and iodides, two sets of corresponding bands can be recognized for each halide by the very nearly equal energy separations between the exciton fundamental and the $F$ band, i.e., $\Delta h\nu \left(\mathrm{intrinsic}\right)=\Delta h\nu \left(F \mathrm{center}\right)$. The remaining structure in the excitonic spectra can then be attributed to upper components of halogen doublets. Splittings are very nearly constant for the iodides. The first two bands of the modified Rydberg exciton series, which converges at the conduction-band edge, correlate with the $F$ and $K$ bands. The $L$ bands comprise another series which, with their corresponding excitonic bands, exhibit a linear dependence on the consecutive energy levels of the free alkali atom.