Raman-Active Resonance Modes, Overtones, and Anharmonicity in NaCl:Cu+

The existence of an impurity-activated $E_{\mathrm{g}}$ resonance mode in NaCl:${\mathrm{Cu}}^{+}$ has been suggested by several previous experiments. Raman data presented here reveal this resonance directly and also reveal the three components of the first overtone of the 23.5-${\mathrm{cm}}^{-1\mathrm{}}$ infrared resonance mode. The frequencies of the $E_{\mathrm{g}}$ resonance and the $E_{\mathrm{g}}$ component of the overtone are shifted as a result of a strong anharmonic coupling. Their line shapes and strengths are considerably altered by an interference between the Raman amplitudes. A reasonable fit to the data has been obtained using a simple theory.