2018 Volume 61 Issue 4 Pages 236-243
An apparatus for terahertz spectroscopy in an ultrahigh vacuum has been developed. We used broadband Mylar for the beam splitter in a Fourier transform spectrometer, diamond for optical windows, and a liquid-helium-cooled Si bolometer for the terahertz detector to achieve the spectral range of 50–650 cm−1. For the purpose of keeping the ultrahigh vacuum in a sample chamber, we evacuated the whole optical path by turbo molecular pumps and made its pressure down to 10−4 Pa. Using this apparatus, we measured temperature dependence of the terahertz spectrum of D2O ice vapor-deposited at 10 K. The spectral changes due to the structural transformation from amorphous ice to cubic crystalline ice Ic were successfully observed in the range of 140–160 K. We report the spectral difference between low-density and high-density amorphous ice as well as that between hexagonal crystalline ice Ih and cubic one.