arXiv subjects

Naoki Watanabe

Publications and source records attributed to Naoki Watanabe.

At least 19 recordsLinked to original sources

arXiv2026

Auger Spectroscopy via Generative Quantum Eigensolver: A Quantum Approach to Molecular Excitations

Auger electron spectroscopy, a way of characterizing electronic structure through core-level decay processes, is widely used in materials characterization; however direct calculation from molecular geometry requires accurate treatment of many excited states, posing a challenge for classical methods. We present a hybrid quantum-classical workflow for calculating Auger spectra that combines the generative quantum eigensolver (GQE) for ground-state preparation, the quantum self-consistent equation-of-motion method for excited-state calculations, and the one-centre approximation for Auger transition rates. GQE uses a GPT-2 model to generate quantum circuits for ground-state optimization, allowing our workflow to benefit from HPC parallelization and GPU-acceleration for favourable scaling with system size. We demonstrate the validity of our workflow by calculating the Auger spectrum of water with the STO-3G basis set and demonstrating qualitative and quantitative agreement with spectra obtained using completely classical full configuration interaction calculations, from the computational literature, and from the experimental literature. We also find that for water, substituting the variational quantum eigensolver (VQE) for GQE results in near-identical spectra, but that the ground state estimator generated by GQE contains about half the total gate count as that generated by VQE.

quant-ph↗

arXiv2026

Experimental and computational studies of the hydrogenation of carbon disulfide (CS2) on ice analogues

Carbon disulfide (CS$_2$) is one of the sulfur-bearing species expected to be present in the interstellar medium (ISM). In this study, we investigated the surface reactions of solid CS$_2$ with hydrogen (H) atoms on amorphous solid water (ASW) using laboratory experiments supported by computational calculations. Our results show that CS$_2$ reacts with H atoms through quantum tunneling in the initial step, followed by successive H addition reactions, with or without activation barriers, on icy surfaces. These processes lead to the formation of several sulfur-bearing species, including hydrogen sulfide (H$_2$S), methyl mercaptan (CH$_3$SH), and small amounts of dithioformic acid (HC(S)SH) and methanedithiol (CH$_2$(SH)$_2$). The observed reactivity of CS$_2$ with H atoms provides a plausible explanation for the non-detection of CS$_2$ in interstellar ices. Furthermore, the efficient hydrogenation of the complex molecules derived from CS$_2$, namely HC(S)SH and CH$_2$(SH)$_2$, suggests that these species could be easily undergone with H atoms to produce other S-bearing species under ISM conditions.

astro-ph.GA↗

arXiv2026

H$_2$ Ortho-Para Spin Conversion on Inhomogeneous Grain Surfaces. II. impact of the rotational energy difference between adsorbed ortho-H$_2$ and para-H$_2$ and implication to deuterium fractionation chemistry

We investigate how the H$_2$ ortho-to-para ratio (OPR) and dueterium fractionation in star-forming regions are affected by nuclear spin conversion (NSC) on dust grains. Particular focus is placed on the rotational energy difference between ortho-H$_2$ (o-H$_2$) and para-H$_2$ (p-H$_2$) on grain surfaces. While the ground state of o-H$_2$ has a higher rotational energy than that of p-H$_2$ by 170.5 K in the gas phase, this energy difference is expected to become smaller on solid surfaces, where interactions between the surface and adsorbed H$_2$ molecules affect their rotational motion. A previous study by Furuya et al. (2019) developed a rigorous formulation of the rate for the temporal variation of the H$_2$ OPR via the NSC on grains, assuming that adsorbed o-H$_2$ has higher rotational energy than adsorbed p-H$_2$ by 170.5 K, as in the gas phase. In this work, we relax the assumption and re-evaluate the rate, varying the rotational energy difference between their ground states. The re-evaluated rate is incorporated into a gas-ice astrochemical model to study the evolution of the H$_2$ OPR and the deuterium fractionation in prestellar cores and the outer, cold regions of protostellar envelopes. The inclusion of the NSC on grains reduces the timescale of the H2 OPR evolution and thus the deuterium fractionation, at densities of >10$^4$ cm$^{-3}$ and temperatures of <14-16 K (depending on the rotational energy difference), when the ionization rate of H$_2$ is 10$^{-17}$ s$^{-1}$.

astro-ph.GA↗

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

Formation of unsaturated carbon chains through carbon chemisorption on solid CO

The interaction of carbon atoms with solid carbon monoxide (CO) is a fundamental process in astrochemistry, influencing the formation of complex organic molecules in interstellar environments. This study investigates the adsorption and reaction mechanisms of carbon atoms on solid CO under cryogenic conditions, employing a combination of experimental techniques, including photostimulated desorption and resonance-enhanced multiphoton ionization (PSD-REMPI) and infrared spectroscopy, alongside quantum chemical calculations. The results reveal the formation of oxygenated carbon chains, such as CCO, C$_3$O$_2$, and C$_5$O$_2$, as well as CO$_2$. The findings highlight the role of chemisorption and subsequent reactions in driving molecular complexity on solid CO, with implications for the chemical evolution of interstellar ices and the potential formation of prebiotic molecules.

astro-ph.GA↗

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

Hydrogenation of acetaldehyde on interstellar ice analogs reveals limited destruction

We sought to determine which are the main hydrogenation paths of acetaldehyde (CH3CHO). As a partially unsaturated molecule, CH3CHO can have links with more hydrogenated species, like ethanol (C2H5OH) or with more unsaturated ones, like ketene (H2CCO). We used highly accurate quantum chemical calculations to determine the reaction rate constants for the CH3CHO + H/D reaction. Our theoretical results are confronted against our experiments on the hydrogenation and deuteration of CH3CHO ice. We find that acetaldehyde resists hydrogenation, with only a 10\% of conversion to products different than CH3CHO. This is due to a predominance of H-abstraction at the HCO moiety, with reaction rate constants up to four orders of magnitude higher than the next possible reaction channel, that is hydrogenation at the aldehydic carbon. The formed CH3CO radical experiences barrierless or nearly barrierless reactions in all possible reaction positions, reforming CH3CHO and creating a closed loop that protects the molecule against hydrogenation. We constrain the branching ratios for the second reaction from experiments. Our experiments agree with the calculations and from the combination of both we can explain the presence of H2CCO, CO, CH4, C2H5OH, H2CO or CH3OH as minor products at the end of the reaction. We provide recommendations for future modeling efforts. Our results show limited destruction of acetaldehyde, reinforcing the vision of this molecule as an abundant and resilient COM. From the experiments, we are not able to observe the reactive desorption of this molecule. Our results align with other modeling works, showing that the link between CH3CHO and C2H5OH is not direct. Finally, our results can explain the excess of CH3CDO found in prestellar cores.

astro-ph.GA↗

arXiv2024

Chemical pathways of SO2 with hydrogen atoms on interstellar ice analogues

Sulfur dioxide (SO2) is a sulfur-containing molecule expected to exist as a solid in the interstellar medium (ISM). In this study, we performed laboratory experiments and computational analyses on the surface reactions of solid SO2 with hydrogen atoms on amorphous solid water (ASW) at low temperatures. After 40 min of exposure of SO2 deposited on ASW to H atoms, approximately 80% of the solid SO2 was lost from the substrate at 10-40 K, and approximately 50% even at 60 K, without any definite detection of reaction products. Quantum chemical calculations suggest that H atoms preferentially add to the S atom of solid SO2, forming the HSO2 radical. Further reactions of the HSO2 radical with H atoms result in the formation of several S-bearing species, including HS(O)OH, the S(O)OH radical, HO-S-OH, HS-OH, and H2S. In codeposition experiments involving H and SO2, we confirmed the formation of H2S, HS(O)OH, and/or HO-S-OH. However, the yields of these S-bearing species were insufficient to account for the complete loss of the initial SO2 reactant. These findings suggest that some products desorbed into the gas phase upon formation. This study indicates that a portion of SO2 in ice mantles may remain unreacted, avoiding hydrogenation, while the remainder is converted into other species, some of which may be subject to chemical desorption.

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↗

arXiv2024

Methane Formation Efficiency on Icy Grains: Role of Adsorption States

Methane (CH4) is one of the major components of the icy mantle of cosmic dust prevalent in cold, dense regions of interstellar media, playing an important role in the synthesis of complex organic molecules and prebiotic molecules. Solid CH4 is considered to be formed via the successive hydrogenation of C atoms accreting onto dust: C + 4H -> CH4. However, most astrochemical models assume this reaction on the ice mantles of dust to be barrierless and efficient, without considering the states of adsorption. Recently, we found that C atoms exist in either the physisorbed or chemisorbed state on compact amorphous solid water, which is analogous to an interstellar ice mantle. These distinct adsorption states considerably affect the hydrogenation reactivity of the C atom. Herein, we elucidate the reactivities of physisorbed and chemisorbed C atoms with H atoms via sequential deposition and co-deposition processes. The results indicate that only physisorbed C atoms can produce CH4 on ice. Combining this finding with a previous estimate for the fraction of physisorbed C atoms on ice, we determined the upper limit for the conversion of C atoms into CH4 to be 30%.

astro-ph.GA↗

arXiv2024

Photodesorption efficiency of OH radical on the ice surface in the wavelength range from ultraviolet to visible

The photodesorption efficiencies of the hydroxyl (OH) radical from the water ice surface were measured in the range of 310-700 nm for the first time. Although isolated H$_2$O molecules and OH radicals do not absorb visible photons, the photodesorption of OH interacting with the ice surface was found to occur by a one-photon process in the entire visible range. The photodesorption efficiency strongly depended on the wavelength, and the analyses of photodesorption cross sections indicated that only strongly bound OH radicals were desorbed at longer wavelengths, whereas both weakly and strongly bound OH radicals were desorbed at shorter wavelengths.

physics.chem-ph↗

arXiv2024

Multiple chemical tracers finally unveil the intricate NGC\,1333 IRAS\,4A outflow system. FAUST XVI

The exploration of outflows in protobinary systems presents a challenging yet crucial endeavour, offering valuable insights into the dynamic interplay between protostars and their evolution. In this study, we examine the morphology and dynamics of jets and outflows within the IRAS\,4A protobinary system. This analysis is based on ALMA observations of SiO(5--4), H$_2$CO(3$_{0,3}$--2$_{0,3}$), and HDCO(4$_{1,4}$--3$_{1,3}$) with a spatial resolution of $\sim$150\,au. Leveraging an astrochemical approach involving the use of diverse tracers beyond traditional ones has enabled the identification of novel features and a comprehensive understanding of the broader outflow dynamics. Our analysis reveals the presence of two jets in the redshifted emission, emanating from IRAS\,4A1 and IRAS\,4A2, respectively. Furthermore, we identify four distinct outflows in the region for the first time, with each protostar, 4A1 and 4A2, contributing to two of them. We characterise the morphology and orientation of each outflow, challenging previous suggestions of bends in their trajectories. The outflow cavities of IRAS\,4A1 exhibit extensions of 10$''$ and 13$''$ with position angles (PA) of 0$^{\circ}$ and -12$^{\circ}$, respectively, while those of IRAS\,4A2 are more extended, spanning 18$''$ and 25$''$ with PAs of 29$^{\circ}$ and 26$^{\circ}$. We propose that the misalignment of the cavities is due to a jet precession in each protostar, a notion supported by the observation that the more extended cavities of the same source exhibit lower velocities, indicating they may stem from older ejection events.

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↗

arXiv2023

Surface Diffusion of Carbon Atoms as a Driver of Interstellar Organic Chemistry

Many interstellar complex organic molecules (COMs) are believed to be produced on the surfaces of icy grains at low temperatures. Atomic carbon is considered responsible for the skeletal evolution processes, such as C-C bond formation, via insertion or addition reactions. Before reactions, C atoms must diffuse on the surface to encounter reaction partners; therefore, information on their diffusion process is critically important for evaluating the role of C atoms in the formation of COMs. In situ detection of C atoms on ice was achieved by a combination of photostimulated desorption and resonance enhanced multiphoton ionization methods. We found that C atoms weakly bound to the ice surface diffused approximately above 30 K and produced C2 molecules. The activation energy for C-atom surface diffusion was experimentally determined to be 88 meV (1,020 K), indicating that the diffusive reaction of C atoms is activated at approximately 22 K on interstellar ice. The facile diffusion of C at T > 22 K atoms on interstellar ice opens a previously overlooked chemical regime where the increase in complexity of COMs as driven by C atoms. Carbon addition chemistry can be an alternative source of chemical complexity in translucent clouds and protoplanetary disks with crucial implications in our current understanding on the origin and evolution of organic chemistry in our Universe.

astro-ph.GA↗

arXiv2022

Diffusion activation energy and desorption activation energy for astrochemically relevant species on water ice show no clear relation

The activation energy for desorption (Edes) and that for surface diffusion (Esd) of adsorbed molecules on dust grains are two of the most important parameters for the chemistry in the interstellar medium. Although Edes is often measured by laboratory experiments, the measurement of Esd is sparse. Due to the lack of data, astrochemical models usually assume a simple scaling relation, Esd = fEdes, where f is a constant, irrespective of adsorbed species. Here, we experimentally measure Esd for CH4, H2S, OCS, CH3OH, and CH3CN on water-ice surfaces using an ultra-high-vacuum transmission electron microscope (UHV-TEM). Compiling the measured Esd values and Edes values from the literature, we find that the value of f ranges from ~0.2 to ~0.7, depending on the species. Unless f (or Esd) for the majority of species is available, a natural alternative approach for astrochemical models is running multiple simulations, varying f for each species randomly. In this approach, ranges of molecular abundances predicted by multiple simulations, rather than abundances predicted by each simulation, are important. We here run 10,000 simulations of astrochemical models of molecular clouds and protostellar envelopes, randomly assigning a value of f for each species. In the former case, we identify several key species whose Esd most strongly affects the uncertainties of the model predictions; Esd for those species should be investigated in future laboratory and quantum chemical studies. In the latter case, uncertainties in the Esd of many species contribute to the uncertainties in the model predictions.

astro-ph.GA↗

arXiv2022

Penetration of Non-energetic Hydrogen Atoms into Amorphous Solid Water and their Reaction with Embedded Benzene and Naphthalene

Chemical processes on the surface of icy grains play an important role in the chemical evolution in molecular clouds. In particular, reactions involving non-energetic hydrogen atoms accreted from the gaseous phase have been extensively studied. These reactions are believed to effectively proceed only on the surface of the icy grains; thus, molecules embedded in the ice mantle are not considered to react with hydrogen atoms. Recently, Tsuge et al. (2020) suggested that non-energetic hydrogen atoms can react with CO molecules even in ice mantles via diffusive hydrogenation. This investigation was extended to benzene and naphthalene molecules embedded in amorphous solid water (ASW) in the present study, which revealed that a portion of these molecules could be fully hydrogenated in astrophysical environments. The penetration depths of non-energetic hydrogen atoms into porous and non-porous ASW were determined using benzene molecules to be >50 and ~10 monolayers, respectively (1 monolayer ~ 0.3 nm).

astro-ph.GA↗

arXiv2022

Hydrogen abstraction reactions in formic and thioformic acid isomers by hydrogen and deuterium atoms. Insights on isomerism and deuteration

The isomerism of molecules in the interstellar medium and the mechanisms behind it are essential questions in the chemistry of organic molecules in space. In particular, for the simple formic and thioformic acids, the low temperatures found in molecular clouds indicate that cis-trans isomerization in the gas-phase must be impeded. Reactions happening on top of interstellar dust grains may explain the isomer interconversion at low temperatures. We studied the isomerization processes of formic and thioformic acid susceptible to happen on the surface of interstellar dust grains and initiated by H abstraction reactions. Similarly, deuterium enrichment of the acids can occur by the same mechanism. Our objective is to shed light on both topics to increase our understanding of key precursors of organic molecules in space.

astro-ph.GA↗

arXiv2022

Thermal desorption of interstellar ices. A review on the controlling parameters and their implications fromsnowlines to chemical complexity

The evolution of star-forming regions and their thermal balance are strongly influenced by their chemical composition, that, in turn, is determined by the physico-chemical processes that govern the transition between the gas phase and the solid state, specifically icy dust grains (e.g., particles adsorption and desorption). Gas-grain and grain-gas transitions as well as formation and sublimation of interstellar ices are thus essential elements of understanding astrophysical observations of cold environments (e.g., pre-stellar cores) where unexpected amounts of a large variety of chemical species have been observed in the gas phase. Adsorbed atoms and molecules also undergo chemical reactions which are not efficient in the gas phase. Therefore, the parameterization of the physical properties of atoms and molecules interacting with dust grain particles is clearly a key aspect to interpret astronomical observations and to build realistic and predictive astrochemical models. In this consensus evaluation, we focus on parameters controlling the thermal desorption of ices and how these determine pathways towards molecular complexity and define the location of snowlines, which ultimately influence the planet formation process. We review different crucial aspects of desorption parameters both from a theoretical and experimental point of view. We critically assess the desorption parameters commonly used in the astrochemical community for astrophysical relevant species and provide tables with recommended values. In addition, we show that a non-trivial determination of the pre-exponential factor nu using the Transition State Theory can affect the binding energy value. Finally, we conclude this work by discussing the limitations of theoretical and experimental approaches currently used to determine the desorption properties with suggestions for future improvements.

astro-ph.GA↗