OPTICAL ABSORPTION SPECTRA OF LIQUID ALKALI HALIDES-F-CENTER GENERATION AT VERY HIGH TEMPERATURES

Abstract
We have measured optical absorption coefficients of liquid alkali halides such as NaCl, KCl and CsCl up to very high temperatures and found that there appears a new absorption band originated from F-center even in "pure" alkali halides by raising temperature, which is known to be generated usually by the addition of small amount of alkali metals to liquid alkali halides. The measurements have been performed using sapphire cell, the use of which is necessary to avoid a chemical reaction between the hot sample and the optical cell. The absorption band becomes large with increasing temperature and disappears again with decreasing temperature. The logarithmic plots of the area of the absorption bands versus reciprocal temperature for these liquids lie on the straight lines, which suggests that the F-center are generated by the thermal activation.