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

Abstract

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 ${\mathrm{cm}}^{\ensuremath{-}1}$, we have observed two even-parity resonant modes of ${A}_{1g}$ and ${E}_{g}$ symmetries at 25 and 16.35 ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively, and a ${T}_{1u}$ symmetry gap mode at 86.2 ${\mathrm{cm}}^{\ensuremath{-}1}$. 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 ${\mathrm{cm}}^{\ensuremath{-}1}$ 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.