We measured infrared absorption spectra of crystalline II of CH4 and succeeded in detecting a prominent Q(2) peak in the ν3 vibrational region by rapid cooling after annealing as well as previously reported rovibrational and librational-vibrational peaks. The integral intensities of the R(0), R(1), and Q(2) peaks were found to show biexponential dependence on time. This clearly demonstrates the interconversion among the three nuclear-spin isomers occupying low-lying rotational levels. The two relaxation rates obtained by biexponential fitting were (0.48, 2.3), (1.1, 4.1), (2.3, 5.1), and (3.4, 15.3) in units of inverse hour (h−1) at 5.2, 6.0, 6.5, and 7.0 K, respectively.

We have established an apparatus for terahertz and mid-infrared spectroscopy in an ultrahigh vacuum and have measured absorption spectra of D2O clusters trapped in solid Ar. To assign terahertz absorption peaks due to the D2O dimer, trimer, and tetramer, the dependence of the spectrum on the annealing temperature and D2O dilution was analyzed. The assignment was also examined by ab initio calculations with the use of the “our own N-layered integrated molecular orbital and molecular mechanics” method, where the flexibility of surrounding Ar atoms was systematically incorporated. We identified all the intermolecular fundamentals of the dimer and those with significant intensities of the trimer and tetramer, whose structural symmetries were revealed to be broken down. After isolating the D2O clusters in solid Ar, we sublimated only Ar atoms to leave behind matrix-sublimation ice, which was found to be amorphous- or crystal-like depending on the formation conditions: the dilution and sublimation temperature. The crystallinity of matrix-sublimation ice was determined by decomposing its terahertz spectrum into the spectra of amorphous and crystalline ices. Since the crystallinity got higher by raising the dilution and sublimation temperature, the diffusion of the D2O monomer on the surface of sublimating solid Ar was found to be crucial to the crystallization of the sublimation ice.

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.