Ultraviolet Absorption of KI Diluted in KCl Cryst als

The absorption of KI diluted in KCl single crystals was studied for concentrations of ${10}^{-3\mathrm{}}$ to ${10}^{-5\mathrm{}}$ mole fraction KI in the temperature range from 80°K to 700°K. The exact shape of the long-wavelength absorption peak, which is Gaussian around the maximum and exponential at the edge, was measured quantitatively over four orders of magnitude. The total integrated absorption is proportional to the iodide concentration which was determined chemically. In the limit of high temperatures the half-width of the band is proportional to the square root of the temperature and the edge follows Urbach's law. Deviations at low temperatures were shown to arise from zero-point oscillations. The results were compared with models for localized excitons.