Even-Parity Lattice Resonances and Anharmonicity in KI:Ag+

We report here the observation of four impurity-induced lattice resonances in KI:${\mathrm{Ag}}^{+}$. In addition to the well-known $T_{1u}$ symmetry resonant mode at 17.3 ${\mathrm{cm}}^{-1\mathrm{}}$, we have observed two even-parity resonant modes of $A_{1g}$ and $E_g$ symmetries at 25 and 16.35 ${\mathrm{cm}}^{-1\mathrm{}}$, respectively, and a $T_{1u}$ symmetry gap mode at 86.2 ${\mathrm{cm}}^{-1\mathrm{}}$. The two even-parity modes were detected using far-infrared spectroscopic techniques with the sample in an external electric field. The field induces a strong mixing of the modes of opposite parity and results in large field-induced shifts. The $E_g$ symmetry resonance has also been seen in the Raman spectrum of KI:${\mathrm{Ag}}^{+}$. Two weak and broad absorption lines at 29.8 and 44.4 ${\mathrm{cm}}^{-1\mathrm{}}$ are attributed to the $E_g+T_{1u}$ and $A_{1g}+T_{1u}$ combination bands, respectively. These results are interpreted in terms of a model in which the $T_{1u}$ symmetry resonant mode is anharmonically coupled to the even-parity resonances.