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 ± 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.


Introduction
It is commonly thought that COMs are produced by recombination reactions of heavy radical species on interstellar dust grains as the temperature rises with star formation [Garrod & Herbst 2006;Taquet et al. 2012].In recent years, however, COMs have been detected even in cold molecular clouds where radicals other than H atoms are considered thermally immobile [Öberg et al. 2010;Bacmann et al. 2012;Cernicharo et al. 2012;Vastel et al. 2014;Jiménez-Serra et al. 2016;Soma et al. 2018].Therefore, several reaction models have been proposed to explain COMs formation in cold regions [Chang & Herbst 2016;Shingledecker et al. 2018;Jin & Garrod 2020].In either case, the radical species formed on the dust are essential for COMs formation.
The abundance ratio of CH3O (methoxy) to CH2OH (hydroxymethyl) radicals on dust is of great interest because it would affect the relative yields of many COMs, such as HCOOCH3 (methyl formate), CH3OCH3 (dimethyl ether), CH3OCH2OH (methoxymethanol), HCOCH2OH (glycolaldehyde), and (CH2OH)2 (ethylene glycol), which have been found in astronomical observations.Therefore, quantitative determination of the branching ratio is crucial for constructing better chemical models of COMs formation.Various processes have been proposed for the formation of CH3O and/or CH2OH on dust under a low-temperature environment, such as I) hydrogenation of H2CO [e.g., Watanabe & Kouchi 2002], II) reactions in which the hydrogen atoms of methanol (CH3OH) are abstracted by H atoms, and III) photodecomposition of solid CH3OH by ultraviolet (UV) photons.Although the branching ratio in case I) has not been determined experimentally, theoretical calculations, including the effect of the water ice surface, have reported that CH3O formation is dominant over CH2OH formation [Song & Kästner 2017].In case II), from deuterium-exposure experiments of CH3OH on an ice surface, it was suggested that CH2OH is predominantly formed [Nagaoka et al. 2005[Nagaoka et al. , 2007;;Hidaka et al. 2009].The formation of COMs through co-deposition experiments involving H, CO, and CO-hydrogenated species (H2CO, and/or CH3OH), including the reaction processes in cases I) and II), has been extensively demonstrated in previous research [e.g., Fedoseev et al. (2015); Chuang et al. (2016); He et al. (2022)].These studies reported the formation of many CH2OH-bearing COMs such as HCOCH2OH and (CH2OH)2, while CH3O-bearing COMs were less abundant.For case III), the branching ratio was deduced from the types of COMs generated in photolysis experiments of CH3OH solids [Öberg et al. 2009;Paardekooper et al. 2016;Yocum et al. 2021;Tenelanda-Osorio et al. 2022].Chuang et al. (2017) performed experiments co-exposing mixed solids (CO, H2CO, and/or CH3OH) to UV and H atoms under various UV/H atom flux ratios, in which the above three radical formation processes can occur.However, their experiments showed that the relative yields of CH3O-bearing COMs (i.e., HCOOCH3) to HCOCH2OH and (CH2OH)2) are significantly low in contradiction to astronomical observations [Chuang et al. 2017].This suggests that reaction processes other than the abovementioned ones are decisively lacking.
Recently, the direct observation of an ice dust analog using transmission electron microscopy indicated that CO solid and the products of successive CO hydrogenation reactions would not fully cover the water ice mantles of dust even in the CO freezing phase [Kouchi et al. 2021a, b].Under such a surface structure, CH3OH can exist next to water molecules; thus, chemical processes with H2O would play an important role.Our previous photolysis experiments of CH3OH adsorbed on amorphous solid water (ASW) surface showed the abundant formation of CH3OCH2OH and HCOOCH3 derived from CH3O compared to HCOCH2OH and (CH2OH)2 derived from CH2OH [Ishibashi et al. 2021], which is qualitatively consistent with their astronomical observations [Jørgensen et al. 2020;Mininni et al. 2020;Manigand et al. 2020;El-Abd at al. 2019;Tercero et al. 2018;Rivilla et al. 2017;Coutens et al. 2015;Taquet et al. 2015].Ishibashi et al. suggested that the reaction of CH3OH and OH formed from photodecomposition of water ice effectively provides CH3O radical, leading to the predominant formation of CH3O-bearing molecules.That is, reaction (1) is more prevailing than reaction (2).
In this study, we determined the branching ratio (CH3O/CH2OH) in the reaction of CH3OH and OH radicals on the ASW surface at 10 K by employing the Cs + ion pickup method, a high-sensitivity surface analysis method [Kang 2011;Ishibashi et al. 2021].This innovative method provides 2-3 orders k2 On ASW On ASW k1 of magnitude higher sensitivity for detecting adsorbates than conventional methods such as Fourier transform infrared spectroscopy, enabling in situ observation of even radical species with low abundance on the surface.

Experimental
Experiments were conducted using an apparatus designed for the surface reaction experiments, which was equipped with a highly sensitive detection system based on the Cs + ion pickup method as described elsewhere [Ishibashi et al. 2021].The ASW (~10 ML) was prepared by background vapor deposition of H2O onto an aluminum substrate at 30 K, followed by generating OH radicals through exposing ASW for 2-15 min to UV photons (~6×10 12 photons cm −2 s −1 ) from a conventional deuterium lamp (115-400 nm).The photons from the lamp photodissociate H2O mainly into H + OH with minor channels, H2 + O and 2H + O [Slanger & Black 1982].However, the most of these volatile products, especially H atom, on the surface immediately desorb upon photodissociation at 30 K. In the present experiment, to avoid possible effects of such volatiles, the deposition and exposure temperature was set to 30 K. The coverage of OH on the ASW was estimated to be an order of magnitude of 0.01 from a signal intensity of OH radical (see Section 3.1).
After cooling ASW to 10 K, the CH3OH gas (Kanto chemical, purity 99.8 %) was then deposited through a microcapillary plate with a different gas line for H2O at a rate of ~1.3 × 10 11 molecules cm −2 s −1 onto the UVirradiated ASW.The total amount of deposited methanol was estimated to be approximately 2.0 × 10 14 molecules cm −2 over 20 minutes using a reflectiontype FTIR measurement.The present experiments also used methanol isotopologues, i.e., CD3OH (Acros organics, 99.5 atom % D) and CH3OD (Sigma-Aldrich, 99.5 atom % D).According to the coverage of OH and the amount of CH3OH, the kinetic energy of gaseous CH3OH was expected to be sufficiently lost to the thermal energy level before colliding with OH.
The Cs + ion injection energy for monitoring the surface composition is about ~17 eV, which enable us to nondestructively monitor the reactant and product species (Mass X) on the surface.The mass numbers of the picked-up species are identified by mass analysis with a quadrupole mass spectrometer without an ionization cell as a mass number of 133 + X.In the present experiments, the Cs + ion bombardment did not affect the reaction system (e.g., radical destruction, diffusion induction), as demonstrated in Appendix A1.
To clarify the details of reaction paths, we also used deuterated isotopologues of methanol.It is important to prevent undesired alterations in the degree of deuteration of the sample gases which can be caused by the deuterium-hydrogen (D-H) substitution reaction with H2O adsorbed on the metal inner wall of the gas line systems.In our setup, the gas line with a microcapillary plate used for methanol deposition was isolated from the main chamber by a gate valve which was closed during ASW preparation.This arrangement minimized the H2O contamination in the gas line for isotopologues.A few weeks before changing the sample isotopologue for the deposition, the gas line system was baked out at 60℃ for over one week, and the isotopologues were circulated via a pre-gas flow.These approaches enabled the isotopologues, even CH3OD, to be deposited with a purity of >90%.
To ensure a constant deposition rate, the opening of the variable leak valve was fixed during all experiments, while the gas flow was controlled by opening and closing the gate valve.The back pressures of all the isotopologues were set to the same value, which was measured by an absolute pressure transducer mounted at the gas cell.165)) deposited on nonirradiated and UVirradiated ASW, respectively.Carbon-bearing species, such as CH3O and/or CH2OH (Mass 31(164)), and H2CO (Mass 30(163)) were observed only for the UV-irradiated ASW, indicating that they were produced by the interactions of the methanol with the photoproducts of ASW such as OH.Since CH3O and CH2OH cannot be separated by mass analysis, we also performed isotopelabeling experiments using methanol isotopologues CD3OH (Mass 35(168)) and CH3OD (Mass 33( 166)) (hereafter, these experiments are denoted as "X experiment," where X represents CH3OH, CD3OH, or CH3OD).Figs.1(c) and (d) show the pickup spectra obtained in CD3OH and CH3OD experiments on nonirradiated and UV-irradiated ASW, respectively.On the nonirradiated ASW, different isotopologues from deposited methanol (i.e., CHD2OH in Fig. 1(c) and CH3OH in Fig. 1(d)) and HDO were detected, which may have originated from the impurity in the chemical reagent and/or might have been formed by the deuterium-hydrogen (D-H) exchange with water at the gas line before the methanol deposition.Thus, those isotopologues contaminants will also exist on UV-irradiated ASW.On the UV-irradiated ASW, mass peaks corresponding to the reaction products of deuterated counterparts of CH3O, CH2OH, and H2CO were observed.
Those peak patterns indicate the formation of methoxy and hydroxymethyl radicals occur.However, unfortunately, those reaction products sometimes overlap with the contaminants and some photoproducts of ASW (i.e., O2, HO2, and H2O2).Therefore, we carefully corrected the signal intensities of reaction products to eliminate the contributions of contaminants and photo products according to the methods shown in Appendix A2.The reaction pathways in the experiments using each methanol isotopologue were identified by monitoring the time variations of reactants and products, as shown in the following section.CH3OD experiment also increased with a similar trend as H2CO in the CH3OH experiment (Fig. 2(e)).Note that although the signal intensity of each adsorbate is proportional to its number density on the surface, the proportional constants are not identical due to difference in pickup efficiency.

The Time Evolution of Products during Methanol Deposition
The experiments using methanol isotopologues revealed the occurrence of the following reactions: These results indicate that the Mass 31 signal in the CH3OH experiment Thus, the formation of formaldehyde from methanol requires two OH radicals.Because the surface coverage of OH is estimated to be ~0.01, the occurrence of reactions ( 7)-( 11) on the ice surface suggests that methoxy and/or hydroxymethyl encounter OH radicals at 10 K via diffusion.It  1)-( 6), the methoxy and hydroxymethyl radicals are produced from reactions with one OH.In contrast, from reactions ( 7)-( 11), two OH radicals are required for formaldehyde (D2CO and H2CO) production.Therefore, the yields of products from former and latter reactions should have a different dependence of [OH]0.The evolution of number densities for the reaction products with the CH3OH deposition time assumed from the above reaction pathways (CH3OH experiment as an example) was approximated as follows (the derivation of the equation is found in Appendix A4).
where k1+2 = k1 + k2.Similar equations can be written for the isotopologues by choosing an appropriate k.When formaldehyde is produced in Eq. ( 14), the amount produced at a specific time should quadratically correlate with [OH]0.Thus, the correlations shown in Fig. 4 are strong evidence that formaldehyde is produced in reactions represented by Eq. ( 14).According to Eqs. ( 12) and ( 13), the variation in the number densities of methoxy and hydroxymethyl yields include the square terms of   the radicals were approximately the same between the isotopologues, we can conclude that the branching ratio obtained in each isotopologue experiment should not be affected by the isotopic effect on the H (D) abstraction reactions.This assumption seems reasonable because the pickup efficiencies between methanol isotopologues did not vary substantially (See Section 3.3 for a discussion of this cause).The discrepancy in the pickup intensities above ~500 s between CD3O in Fig. 2(c) and CH3O in Fig. 2(d) is likely due to isotopic effects on the consumption rate of methoxy radicals through the sequential reactions ( 8) and ( 7) (see Section 3.4 for detail).Therefore, the branching ratio (CH3O/CH2OH) was estimated under the reasonable assumption that the same branching ratios would be obtained in each isotopologue experiment.
In a simple analysis, a branching ratio of ~4 was obtained using the pickup signal intensity ratio of CD3O and CD2OH produced in reactions (3) and (4), respectively, or that of CH3O and CH2OD produced in reactions ( 5) and ( 6), respectively, from Fig. 5.However, this idea cannot be adopted because (i) methoxy and hydroxymethyl radicals may have different pickup efficiencies, and (ii) these radicals are further consumed in reactions ( 7), ( 8), (10), and (11).Instead, to derive the branching ratio of these radicals, we used the HDO signals generated as by-products of CH3O from Therefore, the use of the HDO pickup signal as a probe is appropriate for estimating the reaction branching ratio.HDO is also produced in sequential reactions ( 8) and ( 11) in which formaldehyde is produced.
However, the contribution of the HDO yields from reactions ( 8) and ( 11) can be separated from those from reactions (4) and ( 5) because the HDO yields from former and latter reactions should have the different dependence of [OH]0, as shown in Fig. 4. From Eq. ( 12)-( 14), the number density of HDO at a specific time "ta" in each isotopologue experiment were described as following equations, respectively, under the above reasonable assumption of k1 = k3 = k5 and k2 = k4 = k6.
In these equations, the first term refers to the former reaction, and the second term refers to the latter reaction.Note that the second terms in Eqs. ( 12) and ( 13) are not necessary to be considered in HDO productions, because HDO is not consumed by sequential reactions unlike radicals, as mentioned above.Therefore, by fitting the pickup signal intensity of HDO at a specific time relative to [OH]0 with a quadratic function, the contribution of HDO from the latter reactions ( 8) and ( 11) can be evaluated.
We branching ratio was calculated as 4.3 ± 0.6.Even when the data points are fitted with the quadratic function, the derived branching ratio of 3.8 ± 0.8 is consistent to the above value within the error.In addition, the values for the chi-square degrees freedom in these fittings are almost equivalent.
In addition, the first term of Eq. ( 15) (i.e., A × [OH]0) refers to the total amount of CH3O produced by reaction (5), which is equal to the total amount of CD3O produced by reaction (3), and the second term of Eq. ( 16) (i.e., C × [OH]0 2 ) refers to the amount of CD3O consumed by reaction (8).
Therefore, the relation of consumption to production "C/A × [OH]0" indicated that ~14% of the produced CD3O radicals were consumed by the sequential reaction (8) at 240 s under the condition of ~1% OH abundance (i.e., [OH]0 ~ 130 in the 15 min UV-exposed ASW experiment).This is also consistent with the fact that in Fig. 4, the contribution of consumption by sequential reactions of methoxy radicals is small and the yields of methoxy radicals can be approximately linear with respect to [OH]0 within the error range.

Comparison with Gas Phase Reaction
In the gas phase, the CH3OH + OH reaction has been extensively studied both experimentally and theoretically [Xu & Lin 2007;Shannon et  that the branching ratio of the CH3OH + OH reaction under the high-pressure limit (HPL) conditions was almost 100% CH3O at temperatures <80 K.In the HPL calculations, since the formed pre-reactive complex (PRC) rapidly relaxes energy (fully equilibrated and thermalized) with third body collisions before the abstraction reaction, the reaction proceeds by tunneling in the ground state of the PRC at very low temperatures.
Therefore, the branching ratio is determined by the difference between the rate constants of tunneling reactions toward CH3O and CH2OH formations, which depend on the shape of the potential energy barriers of each reaction [Gao et al. 2018].Intuitively, reactions on the ASW surface might be similarly to those in HPL conditions because the PRC formed on ASW can rapidly dissipate energy at the surface.
However, the reaction branching ratio on the ASW surface obtained in this study was not extremely biased toward CH3O, unlike the HPL gas-phase results.The difference in the branching ratio is likely due to the We fitted HDO from the CH3OD experiments with y = Ax (solid line) because of the negligible contribution of reaction (11), and HDO from the CD3OH experiments with y = Bx + Cx 2 (dashed line).The obtained fitting values are A = 0.379 ± 0.008, B = 0.089 ± 0.010, and C = 0.00042 ± 0.00009.From the ratio of the linear components (A/B) obtained from the fitting, we estimated the CH3O/CH2OH branching ratio to be 4.3 ± 0.6.The UV irradiation times are 2, 3, 5, 7, 10, and 15 min.Each plot and error bar was obtained from the results of three UV experiments, with two blank experiments performed before and after the UV experiments.In each of the CH3OD and CD3OH experiments, there are three plots at UV 15 min, two plots at UV 10, 5, and 2 min, and one plot at UV 7 and 3 min.The fact that multiple plots at the same [OH]0 show similar values indicates that contamination is correctly removed by the blank experiments no matter when the experiment is performed.
barrier shape of the CH3OH + OH reaction on the ASW surface.According to previous quantum chemical calculations, the involvement of one or two H2O molecules changes the barrier shape of the reaction between CH3OH and OH and forms multiple PRC structural isomers with corresponding multiple transition states [Jara-Toro et al. 2017;Chao et al. 2019;Wu et al. 2020].At the cold surface, multiple PRC structural isomers involving H2O would rapidly dissipate energy to the surface after their formation; therefore, they would be unable to overcome their isomerization barrier to obtain the most stable structure and should be able to exist stably with different structures.In addition, on the ASW surface, more H2O molecules will be involved in the PRC composition; hence, it can be inferred that more multiple PRC structural isomers will form depending on the adsorption sites and orientations of CH3OH and OH as shown in left side of Fig. 7.In fact, CH3OH and OH radicals are known to exist in various adsorption states on the ASW surface [Miyazaki et al. 2020;Ferrero et al. 2020].In addition, most recently, various reaction barriers depending on the adsorption structure of CH3OH and OH on ASW were calculated for their reaction [Sameera et al. 2023].In that case, the rate constant ratio (i.e., k1/k2) has a unique value depending on each PRC structure on ASW surface.Therefore, the branching ratio obtained in the present experiments should be average of the branching ratios obtained from each of the multiple PRC structural isomers on ASW (just denoted by PRC α, PRC β, and PRC γ in right side of Fig. 7).The predominant formation of CH3O is qualitatively understandable, considering that bond formation between CH3OH and H2O molecules occurs preferentially on the hydroxyl group side of CH3OH, such as PRC β and γ, under H2O abundant conditions at low temperatures [Dawes et al. 2016].That is, the OH radical adsorbed on ASW tends to interact with the hydroxyl side of CH3OH.In this context, the adsorption orientations and structures of reactants may be important factors influencing the reaction branching on the ice surface.
Although it is generally known that tunneling reactions often bring large isotopic effects, little isotopic effects were detected on the branching ratio on the ASW surface.In the HPL gas-phase calculations at T <70 K, because the formation of the PRC preceding the hydrogen abstraction from methanol by OH was the rate-determining process [Gao et al. 2018], the isotopic effect on the overall abstraction reaction rate constant (i.e., the sum of methoxy and hydroxyethyl formation reactions) was almost negligible.In the present experiment, the sum of the rate constants for the formation of methoxy and hydroxymethyl radicals (i.e., k1+2) shows no substantial isotopic effect, which is presumably because the association process between methanol and OH (i.e., adsorption, diffusion, and PRC formation process) is the rate-determining process of the reaction.In fact, it has been reported that when tunneling reaction rate on the cold surface is limited by surface diffusion, a significant isotope effect does not appear [Hama et al 2015].
However, our result of no substantial isotopic effects for branching ratio (i.e., k1/k2) on ASW could not be explained solely by the above reasons.In the gas-phase, because the branching ratio for the formation of CH3O or CH2OH was determined by the competition between the two tunneling reactions after PRC formation, isotopic effects could affect the branching ratio in the CD3OH and CH3OD cases.This is because different isotopic atom (i.e., H or D) abstraction reactions occur from the methyl and hydroxyl groups, respectively, which could change their competing reaction rate ratios.
According to the HPL calculation, the rate constant of CH3O formation has been reported to be 3-4 orders of magnitude larger than that of CH2OH formation at low temperatures, where the branching to CH3O is almost unity [Gao et al. 2018].In such cases, even if there are large isotopic effects on the reactions, the formation of methoxy radical could remain overwhelmingly dominant in each isotopologue experiment, and the isotopic effect may not appear in the branching ratio detected experimentally.
However, our results would conflict with the above consideration, which is suggested to explain the reason for the lack of isotopic effects because the substantial formation of hydroxymethyl radicals (same order of methoxy formation, methoxy:hydroxymethyl ~4:1) was observed.It is difficult to understand the observed branching ratio on ASW from the analogy in the gas-phase reactions using a single PRC.The speculation that the branching ratio is determined by the sum of the contributions of multiple PRC structures could explain the lack of isotopic effects in the branching ratios measured on ASW.In other words, if methoxy radical production is overwhelmingly dominant in some types of PRCs such as PRC γ in Fig. 7 and hydroxymethyl radical production is overwhelmingly dominant in other types of PRCs such as PRC α in Fig. 7, and the ratio of the presence of these two types of PRCs broadly classified is ~4:1, the branching ratio (methoxy/hydroxymethyl) of ~4 could be obtained without experimentally detectable isotopic effects.should appear significantly at the long deposition time because it is attributed to the difference of consumption terms of CH2OH and CH3O (see Eq. 12 and 13), respectively.In Fig. 8, the discrepancy in the signal intensity between methoxy and hydroxymethyl radicals at each isotopologues experiments significantly appears at above 500 s, and the intensities of methoxy radicals tend to be smaller than that of hydroxymethyl radicals with time.This implies that the contributions of consumption by the sequential reaction with the diffusion of methoxy radicals are more significant than that of hydroxymethyl radicals in both isotopologues experiments.The adsorption energies of CH3O and CH2OH on ASW were calculated to be 0.32 eV [Sameera et al. 2021] and 0.46 eV [Sameera et al. 2023] as the average value for several adsorption sites.The value of CH2OH is significant, considering the value of CH3OH is 0.39 eV [Sameera et al. 2023].Thus, the relation of diffusivity can be concluded to CH3O > CH3OH > CH2OH under the simple assumption that adsorption energy positively correlates to the activation energy of diffusion.This finding supports the results in Fig. 6 that the pickup intensity of HDO can be represented using a linear function, meaning neglect of reaction (11), in CH3OD experiments, although a quadratic function was required to fit that of HDO in CD3OH experiments.

Diffusion of Methoxy Radical
Additionally, we can discuss the difference in the signal intensity between CH3O and CD3O after 500 s in Fig. 2c,d.This difference may be due to isotopic effects appeared in abstraction reaction and/or diffusion of methoxy radicals in the methoxy consumption reactions ( 7) and ( 8).The consumption of CH3O (i.e., reaction (7)) should be larger than that of CD3O (i.e., reaction (8)), because the abstraction of D atoms tends to be slower than that of H atoms, and CD3O should also tend to be slower than CH3O due to the non-identical binding energy on ASW by the zero-point vibrational energy difference (k7 > k8).In the case of hydroxymethyl radicals, the fact that little isotope effect appears after ~500 s may be due to the small contributions of their sequential reactions ( 10) and ( 11) Unfortunately, the present experiments could not reveal the mechanism of methoxy radical diffusion.It is speculated that the radical could diffuse only when trapped in particularly weak adsorption sites following reaction formation, or diffuse transiently using the heat of reaction.

Astrophysical Implications and Conclusions
Astronomical observations showed that the abundances of CH2OH-bearing COMs (i.e., HCOCH2OH and (CH2OH)2) in the gas phase tend to be smaller than those of CH3O-bearing COMs (i.e., HCOOCH3, CH3OCH3, and CH3OCH2OH) [Jørgensen et al. 2020;Mininni et al. 2020;Manigand et al. 2020;El-Abd at al. 2019;Tercero et al. 2018;Rivilla et al. 2017;Coutens et al. 2015;Taquet et al. 2015].Although gas-phase reactions have also been proposed for these formation pathways [e.g., Balucani et al. 2015], the roles of radicals on dust still need to be evaluated.The exposure of CO and CH3OH solid The present reactions should also take place at high temperatures, where methanol and OH can thermally diffuse [Furuya et al. 2022;Miyazaki et al. 2022].In these high-temperature regions, because methanol would tend to form the most stable adsorption structure [Dawes et al. 2016], the formation of CH3O through the H atom abstraction from the hydroxyl group could be enhanced.
The present results may provide information for updating chemical models of COMs formation.The present value of the branching ratio (CH3O/CH2OH = 4.3 ± 0.6) can also be applied to all methanol isotopologues, as the isotopic effects were little observed in our experiments.The effective reaction rate constants on the dust could depend on their diffusion/association mechanism, since this reaction may be ratedetermining in the process of association between methanol and OH (i.e., PRC formation).The present results also suggest that branching due to chemical reactions on the dust should be statistically considered not only for specific adsorption structures but also for various adsorption structures.
In addition, the present experiments suggest that even at 10 K, methoxy radicals produced by the reaction of CH3OH and OH can diffuse to some extent on the ice surface because the sequential reactions to produce formaldehyde require the diffusion of methoxy radicals to encounter another OH under OH-poor conditions.Therefore, when CH3O is produced on interstellar dust, it may cause diffusive reactions with other molecules and radicals even in cold environments, which could lead to efficient formation of CH3O-derived COMs.Unfortunately, details of the diffusion mechanism cannot be clarified in the present experiments.Because CH3O diffusion even at 10 K crucial in chemical models on dust in cold molecular clouds, the diffusion mechanism, including diffusion distance, needs to be further investigated in the future.In the Cs + ion pickup method, it is difficult to maintain collection efficiency unity between experiments because of technical issues.Hence, to remove the influence on the fluctuation of the collection efficiency, the obtained raw data needed to be corrected for further evaluation.(t) between each experiment, Ix(t) was corrected using the following equation:

Acknowledgments
where Imethanol(t) is the raw signal intensity of methanol and IStandard_methanol(t) is the standard signal intensity of methanol.where σ1 is the reaction cross-section (including the association process) of methanol and OH (reaction 1) and fmethanol is the flux of methanol deposition.
Fig.1(a)shows the mass spectra of ASW vapor-deposited at 10 K before and after UV irradiation for 15 min.The formation of OH radicals was confirmed after UV irradiation by the increase in the peak intensity at Mass 151, which was the sum of Masses 133 (Cs + ) and 17 (OH).Assuming the same pickup efficiency for H2O and OH, the surface abundance of OH is estimated to be ~1% relative to H2O.This abundance agrees with the result of previous study that estimated based on the UV irradiation time and photodissociation cross section of H2O[Miyazaki et al. 2022].Similarly, the formation of O2 (Mass 32(165)), HO2 (Mass 33(166)), and H2O2 (Mass

Fig. 2
Fig. 2(a,b) shows the time evolution of the signal intensities for the reactants and products picked up during the deposition of CH3OH onto 15 min UV pre-irradiated ASW.The CH3OH signal increased with the CH3OH deposition time, whereas the preproduced OH signal decreased.Furthermore, the Mass 31 signal corresponding to CH3O and/or CH2OH increased, and then the Mass 30 signal corresponding to H2CO increased sequentially.This result clearly shows that, even at 10 K, the reaction of CH3OH + OH occurred on ASW, followed by H2CO formation.Note that the Y-axis in Fig. 2(b) represents the observed intensities corrected for quantitative analysis (see Appendix A3 for correction details).Hereafter, all experimental data displayed in the figures were corrected in the same manner.Fig. 2(c-e) shows the results of experiments using CH3OH isotopologues (CD3OH and CH3OD) for obtaining isotope-labeling radicals.The methoxy and hydroxymethyl radicals were detected as CD3O (Mass 34) and CD2OH (Mass 33); CH3O (Mass 31) and CH2OD (Mass 32) for the CD3OH (Fig. 2(c)) and CH3OD (Fig. 2(d)) experiments, respectively.In addition, signal intensities of formaldehyde products at Mass 32 (D2CO) in the CD3OH experiment and Mass 30 (H2CO) in the

Fig. 4
Fig. 4 shows the pickup signal intensities of methoxy, hydroxymethyl, and formaldehyde at 240 s for the (a) CD3OH and (b) CH3OD experiments.The variations of D2CO and H2CO intensities were found to be quadratic of [OH]0.

Fig. 5
Fig. 5(a) shows the variations in the signal intensities of CD3O and CH3O in the CD3OH and CH3OD experiments, respectively, with deposition time in the region where the contributions of sequential reactions should be small (0-400 s).In each isotopologue experiment, there was little difference between the peak intensities of CD3O and CH3O (H or D abstraction from OH or OD, respectively).In addition, there was no detectable difference in the pickup signal intensities between CD2OH and CH2OD (D or H abstraction from CD3 or CH3, respectively) (Fig. 5(b)).If the pickup efficiencies of

Figure 4 .
Figure 4. [OH]0 dependence of pickup signal intensities at 240 s after CH3OH vapor deposition begins.(a) CD3O, CD2OH, and D2CO generated in the CD3OH experiments.CD3O and CD2OH have a linear dependence, and D2CO has a square dependence on [OH]0.(b) CH3O, CH2OD and H2CO produced in the CH3OD experiments.CH3O has a linear dependence, and H2CO has a square dependence on [OH]0.CH2OD appears to represent a linearly dependence, although the error is large.Each plot and error bar was obtained from the results of three experiments.The UV irradiation times were 2, 3, 5, 7, 10, and 15 min.

Figure 5 .
Figure 5. (a) Variations in the pickup signal of CD3O (green) and CH3O (red) generated in the CD3OH and CH3OD experiments, respectively.(b) Variations in the pickup signal of CD2OH (blue) and CH2OD (yellow) generated in the CD3OH and CH3OD experiments, respectively.The signal is the mean value of three experiments and the shaded area represents the error.The horizontal dotted line represents zero of the corrected pickup intensity.
measured the [OH]0 dependence of the pickup signal intensities of HDO similar to Fig. 4. Fig. 6(b) shows the [OH]0 dependence of the HDO pickup intensity at the methanol deposition time of 240 s.The contaminations were removed from the plotted HDO intensity (see Appendix A2 for details).The HDO from reactions (4) and (8) obtained in the CD3OH experiments (blue inverted triangles) tended to increase quadratically with respect to [OH]0, whereas the HDO from reactions (5) and (11) in the CH3OD experiments (red squares) increased almost linearly with [OH]0.This is likely due to the negligible contribution of reaction (11) (i.e., the second term in Eq. (15)).Therefore, the [OH]0 dependence of HDO obtained from the CH3OD experiments was fitted with a linear function (y = Ax) by neglecting the contribution of reaction (11) (See Section 3.4 for validity), and that obtained from the CD3OH experiments was fitted with a quadratic function (y = Bx + Cx 2 ).From the obtained linear component ratio (A/B), the CH3O/CH2OH

Figure 6 .
Figure 6.(a) Reaction scheme for the CH3OD and CD3OH experiments.(b) [OH]0 dependence of HDO intensity at 240 s generated in the CH3OD (red square) and CD3OH (blue inverted triangle) experiments.We fitted HDO from the CH3OD experiments with y = Ax (solid line) because of the negligible contribution of reaction (11), and HDO from the CD3OH experiments with y = Bx + Cx 2 (dashed line).The obtained fitting values are A = 0.379 ± 0.008, B = 0.089 ± 0.010, and C = 0.00042 ± 0.00009.From the ratio of the linear components (A/B) obtained from the fitting, we estimated the CH3O/CH2OH branching ratio to be 4.3 ± 0.6.The UV irradiation times are 2, 3, 5, 7, 10, and 15 min.Each plot and error bar was obtained from the results of three UV experiments, with two blank experiments performed before and after the UV experiments.In each of the CH3OD and CD3OH experiments, there are three plots at UV 15 min, two plots at UV 10, 5, and 2 min, and one plot at UV 7 and 3 min.The fact that multiple plots at the same [OH]0 show similar values indicates that contamination is correctly removed by the blank experiments no matter when the experiment is performed.

Figure 7 .
Figure 7. Schematic diagram of potential energy surfaces for the CH3OH + OH reaction with reference to previous calculations[Xu & Lin 2007, Shannon et al. 2013, Gao et al. 2018, Jara-Toro et al. 2017, Sameera et al. 2023].In the gas phase (HPL), only the lowest energy PRC is considered, and the branching ratio of CH3O is ~100% at low temperature (T < 80 K)[Gao et al. 2018].On the ASW surface, because multiple PRC structural isomers involving many H2O molecules (PRC α, β, γ, and others) can exist stably at low temperature, they have different potential barrier shapes.Therefore, the obtained branching ratio of ~80% CH3O in the present experiment represents the average abundance distribution of the branching ratios obtained from each PRC.Note that each multiple PRC structural isomer has a different absolute energy value.The descriptions of CH3OH…OH, OH…ASW, and CH3OH…OH…ASW indicate formation of a complex (or adsorption).

Fig. 4
Fig. 4 clearly shows that formaldehyde was formed by consuming two OH radicals.It means that the methoxy and/or hydroxymethyl radicals diffuse significantly to encounter secondary OH, because the coverage of OH radicals on UV-irradiated ASW is very small (~ 1%).Therefore, it is expected that the contributions of formaldehyde formation at each reaction pathway (see Fig. 6(a)) strongly depend on the diffusivity of each methoxy and hydroxymethyl radical.Fig. 8 shows a comparison of the time variation of pickup intensities for methoxy and hydroxymethyl radicals obtained (a) CD3OH and (b) CH3OD experiments, respectively.To facilitate the comparison of formaldehyde formation contributions through methoxy and hydroxymethyl radicals, the intensities of hydroxymethyl radicals in that figure are adjusted by multiplying them with the branching ratio (k1/k2 ~ 4.3).The discrepancy between the time variations of k1/k2 × [CH2OH]t and [CH3O]t

Figure 8 .
Figure 8. Comparisons of the trend of time variation of methoxy and hydroxymethyl (multiplied by k1/k2 = 4.3) radicals for (a) CD3OH and (b) CH3OD experiments to examine the degree of the contribution for the consumption reactions.
This work was supported by JSPS Grant-in-Aid for Scientific Research (JP22H00159).The authors thank the staff of Technical Division at Institute of Low Temperature Science for making various experimental devices.Appendix A1.No Effect of Cs + Ion Irradiation on the Surface Reaction in the Present Experimental System We investigated the effect of Cs + ion irradiation on the formation of CH3O, CH2OH, and H2CO.Fig. 9 shows the pickup intensities of the products obtained in the CH3OH experiments, in which Cs + irradiation was delayed by ~300 s.The pickup signal intensities were consistent with those in the experiment without the delay of Cs + irradiation (solid lines), indicating that the effect of Cs + ion irradiation on the surface reaction was negligible.A2.Contributions from Contaminants with the Same Mass as the Reaction Product The pickup signal of the target species (e.g., CD3O in the CD3OH experiments) would have contributions from that of other species at the same mass (

Figs. 10
Figs. 10(a) and (b) show the time variations in the raw pickup signal intensities (Ix(t)) at masses 34 and 35, respectively, after UV irradiation for three different CD3OH experiments.The signal intensity at mass 34 was composed of contributions from three species: CD3O, H2O2, and CHD2OH.The signal intensity appearing before methanol deposition is the contribution of H2O2 formed by pre-UV irradiation to ASW.To correct the variations in Ix Figure 10.The correction procedure (e.g., mass 34 signal in CD3OH experiments): (a) Mass 34 pickup signals: Ix(t), (b) CD3OH (mass 35) pickup signals: Imethanol (t), (c) corrected pickup signal of mass 34: Icorrected_x(t).

Table 1 .
Masses of target species and the contaminants in each experiment.