The coupling of free carriers and lattice vibrational modes is studied in CdS and InSb by room temperature far infrared reflectance measurements. When the free carrier concentration is such that the plasma frequency is at or near the longitudinal optical mode frequency it is shown that while the transverse modes remain fixed, the longitudinal modes mix with the plasma mode and are shifted in a manner similar to that described by Varga in 1965. Using the dielectric function for the coupled system permits the determination of electron effective masses by reflectance measurements with lattice effects excluded. This method allows one to investigate the frequency shifts and effective mass values on crystals in which the mobility varies by more than an order of magnitude as demonstrated by the analysis of both CdS and InSb. In the case of CdS where the polaron coupling constant is large enough to have noticeable effects, account is taken of the polaron mass and of the additional absorption in the plasma mode at frequencies above the longitudinal optical phonon frequency --Abstract, page i
