Dispersive--reflection spectroscopy in the far infrared

A dispersive reflection spectrometer has been built in which the fixed mirror of a commercial far-infrared Michelson interferometer is replaced by a polished sample. This allows one to measure simultaneously and accurately both the amplitude reflectance of the sample and the phase change upon reflection. From these quantities, the complete optical properties can be easily obtained without the need for restrictive and sometimes inaccurate Kramers-Kronig analyses. An optical system has been designed and built in which an eight-section switching mask is placed in front of a partly-aluminized sample, which ratios out many sample surface imperfections and eliminates most asymmetries due to the spectrometer's optics. Furthermore, by alternately switching between the sample and the reference surfaces at each step of the moving mirror, simultaneous interferograms are obtained, thereby removing most of the remaining causes of phase errors which have previously limited the accuracy of this powerful technique. Although extension of these techniques to liquid-helium temperatures is difficult, a dewar is described which allows dispersive measurements on samples at temperatures at least as low as 25 K. High-resolution results are presented for NaCl from 25 to 500 cm⁻¹. These are compared with ab-initio calculations of the damping of the transverse, optic resonance due to cubic and quartic anharmonicity. Shell-model lattice-dynamical data are used as input for the calculations and corrections are also made for the damping of the "final-state" phonons. The overall agreement is excellent at room temperature and good at 48 K.Science, Faculty ofPhysics and Astronomy, Department ofGraduat