arXiv subjects

Atsuki Ishibashi

Publications and source records attributed to Atsuki Ishibashi.

5 recordsLinked to original sources

arXiv2026

Methanol Formation via the Radical-radical Reaction of OH and CH3 Radicals Undergoing Transient Diffusion on Ice at 10 to 60 K

Methanol (CH3OH) is thought to form on interstellar ice dust via successive hydrogenation reactions. The reaction between CH3 and OH radicals could also conceivably generate methanol at temperatures above approximately 20 K, at which temperature hydrogen atoms will not adhere to the ice surface. However, this process has not been verified by well-controlled experiments. Using a newly-developed Cs+ ion pickup technique, the authors investigated the reaction between CH3 and OH radicals on the surface of amorphous solid water, an ice dust analogue, at temperatures from 10 to 60 K. In the present experiments, OH radicals were generated by UV photolysis of water molecules, following which methane (CH4) gas was deposited on the ice substrate. The results show that CH3OH was formed on the ice surface through the sequential reactions CH4 + OH -> CH3 + H2O and CH3 + OH -> CH3OH even at 10 K. Considering the very low surface coverage of reactants in the experimental condition, the second reaction was found to occur as a result of transient diffusion of CH3 due to the heat of the first reaction.

astro-ph.GA↗

arXiv2025

Three-step growth of vapor-deposited ice under mesospheric temperature and water vapor conditions

Polar mesospheric clouds provide clues to physicochemical processes in the mesosphere and lower thermosphere. However, the heterogeneous nucleation and growth processes of water ice under polar mesospheric conditions are poorly understood, especially at the nanoscale. This study used reflection high energy electron diffraction and infrared reflection absorption spectroscopy to analyze the structure of vapor-deposited ice at polar mesospheric temperature (120 K) under vapor pressure conditions. The ice appeared to grow in three steps during vapor deposition, being amorphous water for the first 15 nm, then cubic ice up to 50 nm, and finally hexagonal ice subsequently. This three step growth suggests that the three observed phases can coexist in polar mesospheric clouds, depending on the thickness of water ice. The finding of the three-step growth also shows that the Ostwald rule of stages can hold for vapor deposited ice at low temperature.

astro-ph.EP↗

arXiv2025

Hydrogenation of HOCO and formation of interstellar CO$_2$: A not so straightforward relation

Carbon dioxide (CO$_2$) is one of the most important interstellar molecules. While it is considered that it forms on the surface of interstellar dust grains, the exact contribution of different chemical mechanisms is still poorly constrained. Traditionally it is deemed that the CO + OH reaction occurring on top of ices is the main reaction path for its formation. Recent investigations showed that in reality the reaction presents a more complex mechanism, requiring an additional H-abstraction step. Building on our previous works, we carried out a detailed investigation of such H abstraction reactions with the hydrogen atom as a reactant for the abstraction reaction. We found an unconventional chemistry for this reaction, markedly depending on the isomeric form of the HOCO radical prior to reaction. The favored reactions are t-HOCO + H -> CO + H$_2$O, c-HOCO + H -> CO$_2$ + H$_2$ and t/c-HOCO + H -> c/t-HCOOH. We estimate bounds for the rate constants of the less favored reaction channels, t-HOCO + H -> CO$_2$ + H and c-HOCO + H -> CO + H$_2$O, to be approximately 10$^{4-6}$ s$^{-1}$. However, these estimates should be interpreted cautiously due to the significant role of quantum tunneling in these reactions and the complex electronic structure of the involved molecules, which complicates their study. Our findings underscore the need for detailed investigation into the chemistry of interstellar CO$_2$ and pave the way for a reevaluation of its primary formation mechanisms in the interstellar medium.

astro-ph.GA↗

arXiv2024

Proposed importance of HOCO chemistry: Inefficient formation of CO$_2$ from CO and OH reactions on ice dust

With the advent of JWST ice observations, dedicated studies on the formation reactions of detected molecules are becoming increasingly important. One of the most interesting molecules in interstellar ice is CO$_2$. Despite its simplicity, the main formation reaction considered, CO + OH -> CO$_2$ + H through the energetic HOCO* intermediate on ice dust, is subject to uncertainty because it directly competes with the stabilization of HOCO as a final product which is formed through energy dissipation of HOCO* to the water ice. When energy dissipation to the surface is effective during reaction, HOCO can be a dominant product. In this study, we experimentally demonstrate that the major product of the reaction is indeed not CO$_2$, but rather the highly reactive radical HOCO. The HOCO radical can later evolve into CO$_2$ through H-abstraction reactions, but these reactions compete with addition reactions, leading to the formation of carboxylic acids (R-COOH). Our results highlight the importance of HOCO chemistry and encourage further exploration of the chemistry of this radical.

astro-ph.GA↗

arXiv2023

Determination of the branching ratio of CH$_3$OH + OH reaction on water ice surface at 10 K

The CH$_3$O and CH$_2$OH radicals can be important precursors of complex organic molecules (COMs) in interstellar dust. The COMs presumably originating from these radicals were abundantly found in various astronomical objects. Because each radical leads to different types of COMs, determining the abundance ratio of CH$_3$O to CH$_2$OH is crucial for a better understanding of the chemical evolution to various COMs. Recent work suggested that the reaction between CH$_3$OH and OH on ice dust plays an important role in forming CH$_3$O and CH$_2$OH radicals. However, quantitative details on the abundance of these radicals have not been presented to date. Herein, we experimentally determined the branching ratio (CH$_3$O/CH$_2$OH) resulting from the CH$_3$OH + OH reaction on the water ice surface at 10 K to be 4.3 $\pm$ 0.6. Furthermore, the CH$_3$O product in the reaction would participate in subsequent diffusive reactions even at a temperature as low as 10 K. This fact should provide critical information for COMs formation models in cold molecular clouds.

astro-ph.GA↗