Far-infrared dispersive-reflection measurements on NaCl, compared with calculations based on cubic and quartic anharmonicity. I. Room temperature

Accurate dispersive-reflection measurements have been made on a large single crystal of NaCl at room temperature to yield the reflectance amplitude and phase between 25 and 500 ${\mathrm{cm}}^{-1\mathrm{}}$ in the far infrared. These have been augmented with power-transmission measurements in those regions where the phase is small. From these the remaining optical properties have also been obtained, and specifically the damping spectrum of the zero-wave-vector TO resonance which is primarily responsible for the far-infrared optical properties. This has then been compared with calculations based on both cubic and quartic anharmonicity. The three-phonon damping arising from the quartic term was calculated as accurately as possible, with only one significant approximation involving the neglect of certain terms depending on the third potential derivative. The quartic-coupling coefficient for the rocksalt structure is derived and presented under this approximation. Assignments have been made of the phonons responsible for the major features in both the two- and three-phonon damping spectra, and the predominant role of the flat TO mode is observed in the latter case. The effect of the finite lifetimes of the two or three phonons to which the TO mode relaxes is also clearly seen in the higher-wave-number part of the damping spectrum, and so the calculated spectrum was corrected for this by convoluting with a Lorentzian of experimentally determined width. The resulting overall agreement between experiment and theory is excellent. Some possible causes of the small remaining discrepancies are mentioned. Future low-temperature measurements will be useful in this respect.