Terahertz and mid-infrared spectroscopy of matrix-isolated clusters and matrix-sublimation ice of D2O

Abstract

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 Isotope-labeling Study of CO2Formation Pathways in Molecular Clouds: Ultraviolet Photolysis of CO–H2O Ice

“…According to previous studies (Hama et al 2017;Kawauchi et al 2018), codeposited inert molecules act as surfactants and support the diffusion of water molecules, with the result that the partial crystallization of water ice takes place. It has also been known that the mid- (Backus et al 2004) and far-infrared (Yamakawa et al 2020) absorption bands of partially crystallized ice can be decomposed into crystalline and amorphous components. Thus, in sample 4, the H 2 18 O ice was likely to be partially crystallized by the existence of CO. Additionally, the dangling 18 OH band in the spectrum of sample 4 was significantly redshifted by CO, compared with sample 5.…”