Impurity‐Induced Infrared Absorption in Silver Halides

The impurity‐induced infrared absorption bands are measured at low temperatures for AgBr: Li, AgBr: Na, and AgCl: Li. The absorption bands consist of those due to the localized mode, including the fundamental band, the higher frequency sidebands and the third harmonic bands, and those due to the band modes. The force constant change at the impurity site is discussed using a diatomic three dimensional model. The sideband spectra of the lithium and sodium localized mode in silver bromide are described by the theory based on two‐phonon absorption processes. There are apparent differences in the phonon coupling mechanisms to the host lattice between the lithium localized mode and the sodium one in silver bromide. It is found that for AgBr: Li, the phonon coupling mechanism due to the cubic anharmonicity is dominant and the impurity is cupled to its surrounding ions more softly. On the contrary, for AgBr: Na, the contribution due to the second‐order electric‐dipole moment mechanism is nearly equal to that due to the anharmonicity in high frequency limit of the sideband. Some absorption bands due to the impurity‐induced band mode are observed at 30, 48, and 61 cm −1 in AgBr: 7 Li and AgBr: Na at 2 K.