Impurity induced far infrared absorption in doped NaCl and KCl

Far infrared absorption measurements with NaCl single crystals doped with AgCl and CuCl respectively show resonant band mode lines at 4.2 ?K at 52.5 cm^?1 and 23.6 cm^?1 respectively. Their measured halfwidths are 10 cm^?1 and 0.7 cm^?1 respectively. The halfwidth of the NaCl: Cu⁺ line increases strongly with increasing temperature. In NaCl:Mg⁺⁺, NaCl:Ca⁺⁺ and NaCl:Ag⁺ a temperature independent broad absorption is observed above 50 cm^?1, which behaves roughly as the density of phonon states in the unperturbed crystal. In KCl: Cu⁺ and KCl: Br^? a temperature independent absorption with a maximum at 108 cm^?1 is obtained corresponding to an expected region of high density.