FAR-INFRARED DISPERSIVE-REFLECTION MEASUREMENTS ON NACL, COMPARED WITH CALCULATIONS BASED ON CUBIC AND QUARTIC ANHARMONICITY .2. LOW-TEMPERATURE

Accurate far-infrared dispersive-reflection measurements have been performed on a large single crystal of NaCl to yield the reflectance amplitude and phase between 15 and 500 cm** minus **1 at 48 K. These have been augmented with power-transmission measurements in those regions where the phase is small. Some of the difficulties encountered while extending this technique to low temperatures are discussed, including the unfortunate tendency of the evaporated aluminum reference surface to break away from the crystal. The measurements are compared with calculations of the damping of the transverse-optic resonance in NaCl, resulting from one-phonon, two-phonon, and three-phonon relaxation processes, which result from isotopic impurities, cubic anharmonicity, and quartic anharmonicity, respectively. The overall agreement is good but the experimental accuracy is less than that obtained at 290 K.