Low-Temperature Far-Infrared Spectra of Germanium and Silicon

New high-resolution low-temperature (7.5\ifmmode^\circ\else\textdegree\fi{}K) far-infrared data on intrinsic germanium and silicon are presented. The findings further substantiate the naturally anticipated similarities in band patterns of the vibrational spectra of diamond-type crystals. The low-temperature spectra of germanium and silicon show clearly resolved splits in their principal absorption bands analogous to the intriguing doublet in the band at about 2000 ${\mathrm{cm}}^{\ensuremath{-}1}$ in diamond. A possible explanantion is indicated. The room-temperature far-infrared refractive index of both germanium and silicon and the 7.5\ifmmode^\circ\else\textdegree\fi{}K refractive index of germanium have been measured by means of interference fringes.