The Solar System D/H Ratio: Observations and Theories. In: Benz W, Kallenbach R, Lugmair GW, editors. From Dust to Terrestrial Planets. Dordrecht: Springer Netherlands; 2000. pp. 201-24. [DOI: 10.1007/978-94-011-4146-8_14]

Radical reactions on interstellar icy dust grains: Experimental investigations of elementary processes

Molecular clouds (MCs) in space are the birthplace of various molecular species. Chemical reactions occurring on the cryogenic surfaces of cosmic icy dust grains have been considered to play important roles in the formation of these species. Radical reactions are crucial because they often have low barriers and thus proceed even at low temperatures such as ∼10 K. Since the 2000s, laboratory experiments conducted under low-temperature, high-vacuum conditions that mimic MC environments have revealed the elementary physicochemical processes on icy dust grains. In this review, experiments conducted by our group in this context are explored, with a focus on radical reactions on the surface of icy dust analogues, leading to the formation of astronomically abundant molecules such as H₂, H₂O, H₂CO, and CH₃OH and deuterium fractionation processes. The development of highly sensitive, non-destructive methods for detecting adsorbates and their utilization for clarifying the behavior of free radicals on ice, which contribute to the formation of complex organic molecules, are also described.

Gas-Phase Isotopic Fractionation Study of Singly and Doubly Deuterated Isotopologues of Water in the H-D Exchange Reaction by Cavity Ring-Down Spectroscopy

The underlying mechanisms of the triple-oxygen (¹⁶O, ¹⁷O, and ¹⁸O) isotopic content of deuterated (D) isotopologues of water in H-D exchange reactions in the gas phase remain elusive. Herein, we have demonstrated a high-resolution gas-phase spectral analysis of doubly (D₂O) and singly (HDO) deuterated isotopologues of water in the region around 7.8 μm using quantum cascade laser-based cavity ring-down spectroscopy. Isotopic fractionations among doubly and singly deuterated species of water, D₂¹⁶O, HD¹⁶O, HD¹⁷O, and HD¹⁸O, in the gas phase were carried out by probing the fundamental and hot band transitions in the ν₂ (bending) mode of D₂O and the fundamental ν₂ transitions for the other water isotopes. We subsequently investigated the fractionations of different D-enriched water isotopologues for the H-D exchange reaction using various mixtures of D₂O in H₂O. We explored the potential role of triple-oxygen isotopic contents through enrichments and depletions of HD¹⁶O, HD¹⁷O, and HD¹⁸O, involved in the H-D reaction. Our first clear, direct, and quantitative experimental evidence reveals a new picture of gas-phase isotopic fractionation chemistry in a mixture of light and heavy water (H₂O-D₂O).