Infrared-active localized pair modes in the gap region of KI

New infrared-active lattice gap modes due to ${\mathrm{Na}}^{+}$ and ${\mathrm{Cl}}^{-}$ substitutional impurities in KI have been observed. The line strengths are shown to be proportional to the square of the impurity concentration, indicating the modes are due to associated pairs of ions. The previously reported ${\mathrm{Br}}^{-}$ gap mode at 73.8 ${\mathrm{cm}}^{-1\mathrm{}}$ in KI is also shown to be due to pairs of ${\mathrm{Br}}^{-}$ ions. A new single-ion absorption mode due to ${\mathrm{Br}}^{-}$ has been observed at 88.47 ${\mathrm{cm}}^{-1\mathrm{}}$ (for ${\mathrm{Br}}^{81}$) with an isotopic frequency shift of 0.47 ± 0.05 ${\mathrm{cm}}^{-1\mathrm{}}$.