Color Centers in Alkali Halides at 5°K

Abstract

This paper reports experimental data on the effect of x-raying NaCl, KCl, and KBr at 5°K (approximately liquid helium temperature). These crystals, grown by Harshaw, were exposed to x-rays (50 kv at 20 and 48 ma) at 5°K, and the absorption spectra were measured from 210 mμ to 1100 mμ. A large concentration of $F$-centers was produced after an exposure of 15 minutes. The $F$-center is narrower than those obtained in additively colored crystals at 20°K. This difference in width also occurs in crystals x-rayed at 78°K. The narrowing of the $F$-band does not seem to be due to the temperature difference, i.e., 5°K and 20°K. No bands were observed to the infrared with the exception of a small $F^{\prime }$-band in NaCl and KBr. Bands which were not known previously were obtained to the ultraviolet. Some bands decrease on warming to 78°K while others grow.