Color Centers in Alkali Halides at 5°K

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 m{μ} to 1100 m{μ}. 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^'-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.