Impurity‐Induced Far‐Infrared Absorption in the Phonon Gap Regions of KI and KBr

Measurements are made on doped KI and KBr using a far‐infrared grating spectrometer, which is briefly described. The absorption of KI:OD − at 4.3 °K exhibits localized vibrational and translational gap modes. An attempt to assign these modes is made by proposing a term scheme. Measurements of doped KBr between 2.5 and 80 °K confirm the boundaries of the acoustic‐optical gap as 96 cm −1 and 100 cm −1 , respectively.