Development of an Apparatus for Terahertz Spectroscopy in an Ultrahigh Vacuum : Temperature Dependence of the Spectrum of Vapor-Deposited D₂O Ice

Abstract

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 lowdensity and high-density amorphous ice as well as that between hexagonal crystalline ice Ih and cubic one.