Far infrared studies of diatomic cubic crystals by dispersive Fourier transform spectroscopy

The far infrared optical properties and complex dielectric response functions of two alkali halide crystals (CsT and KI) and two III-V compound semiconductor crystals (InAs and InP) have been studied using the technique of dispersive Fourier transform spectroscopy. The values of the transverse optic and longitudinal optic phonon frequencies at wave vector q=0 determined from these results agree quite well with published values, and the dielectric functions were used to calculate the imaginary parts of the an harmonic self-energies (oj,V) of the q=0 transverse optic modes. These are the first reported measurements on Csl by dispersive Fourier transform spectroscopy, and reasonable agreement is obtained between the frequencies of measured features in (oj,V ) and published two-phonon frequencies. In the case of KI the present measurements have been made at a higher resolution than any reported previously, and they provide a sensitive test of the validity of various theoretical calculations available in publications. Also, in the case of KI some two-phonon difference bands have been observed and assigned in the, spectrum of (oj,V) for the first time. No measurements of the optical constants of InP in the far infrared region using this technique have been reported before, and, for both In Lambdas and InP, these are the first reported experimental determinations of the imaginary part of the frequency dependent damping function. A number of features in the spectra of (oj 'V ) for In Lambdas and InP are revealed and attributed to two-phonon decay processes with the aid of a critical point analysis based on published data for these materials. Since the performance of the dispersive reflection interferometer was limited by the specimen size a prototype focused beam interferometer was constructed, and its performance has been demonstrated by recording a water vapour absorption spectrum at a resolution of 2 cm using end mirrors 5mm in diameter.