New Rate Constants of Hydrogenation of CO on H<sub>2</sub>O‐CO Ice Surfaces

Awad, Zainab; Chigai, Takeshi; Kimura, Yuka; Shalabiea, Osama M.; Yamamoto, Tetsuo

doi:10.1086/429856

Cited by 33 publications

(38 citation statements)

References 33 publications

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“…The reaction barriers for H + CO and H + H 2 CO are temperature dependent and increase with temperature. Our values are in good absolute agreement with the barriers found by Awad et al (2005), who also found a similar temperature behaviour. Their values were obtained using a rate equation analysis for T = 10, 15, and 20 K using the data from Watanabe et al (2006).…”

Section: Comparison To the Experimentssupporting

confidence: 91%

Hydrogenation reactions in interstellar CO ice analogues

Fuchs¹,

Cuppen²,

Ioppolo³

et al. 2009

A&A

426

517

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Context. Hydrogenation reactions of CO in inter-and circumstellar ices are regarded as an important starting point in the formation of more complex species. Previous laboratory measurements by two groups of the hydrogenation of CO ices provided controversial results about the formation rate of methanol. Aims. Our aim is to resolve this controversy by an independent investigation of the reaction scheme for a range of H-atom fluxes and different ice temperatures and thicknesses. To fully understand the laboratory data, the results are interpreted theoretically by means of continuous-time, random-walk Monte Carlo simulations. Methods. Reaction rates are determined by using a state-of-the-art ultra high vacuum experimental setup to bombard an interstellar CO ice analog with H atoms at room temperature. The reaction of CO + H into H 2 CO and subsequently CH 3 OH is monitored by a Fourier transform infrared spectrometer in a reflection absorption mode. In addition, after each completed measurement, a temperature programmed desorption experiment is performed to identify the produced species according to their mass spectra and to determine their abundance. Different H-atom fluxes, morphologies, and ice thicknesses are tested. The experimental results are interpreted using Monte Carlo simulations. This technique takes into account the layered structure of CO ice. Results. The formation of both formaldehyde and methanol via CO hydrogenation is confirmed at low temperature (T = 12−20 K). We confirm that the discrepancy between the two Japanese studies is caused mainly by a difference in the applied hydrogen atom flux, as proposed by Hidaka and coworkers. The production rate of formaldehyde is found to decrease and the penetration column to increase with temperature. Temperature-dependent reaction barriers and diffusion rates are inferred using a Monte Carlo physical chemical model. The model is extended to interstellar conditions to compare with observational H 2 CO/CH 3 OH data.

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Section: Comparison To the Experimentssupporting

confidence: 91%

Hydrogenation reactions in interstellar CO ice analogues

Fuchs¹,

Cuppen²,

Ioppolo³

et al. 2009

A&A

426

517

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“…L is the length of the diffusion path (we take it 1 × 10 −6 cm), approximately half the thickness of the mantle. We adopt D for H atoms 2.50 × 10 −21 cm 2 s −1 estimated by Awad et al (2005). For the H 2 molecules we use the coefficient given by Strauss & Chen (1994) for diffusion in hexagonal ice extrapolated to 10 K (D H 2 = 5.90 × 10 −8 cm 2 s −1 ).…”

Section: Hydrogen Diffusion Through Mantlementioning

confidence: 99%

Subsurface chemistry of mantles of interstellar dust grains in dark molecular cores

Kalvāns¹,

Shmeld²

2010

A&A

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Context. The abundances of many observed compounds in interstellar molecular clouds still lack an explanation, despite extensive research that includes both gas and solid (dust-grain surface) phase reactions. Aims. We aim to qualitatively prove the idea that a hydrogen-poor subsurface chemistry on interstellar grains is responsible for at least some of these chemical "anomalies". This chemistry develops in the icy mantles when photodissociation reactions in the mantle release free hydrogen, which escapes the mantle via diffusion. This results in serious alterations of the chemical composition of the mantle because pores in the mantle provide surfaces for reactions in the new, hydrogen-poor environment.Methods. We present a simple kinetic model, using existing astrochemical reaction databases. Gas phase, surface and subsurface pore reactions are included, as are physical transformations of molecules. Results. Our model produces significantly higher abundances for various oxidized species than most other models. We also obtain quite good results for some individual species that have adequate reaction network. Thus, we consider that the hydrogen-poor mantle chemistry may indeed play a role in the chemical evolution of molecular clouds. Conclusions. The significance of outward hydrogen diffusion has to be proved by further research. A huge number of solid phase reactions between many oxidized species is essential to obtain good, quantitative modeling results for a comparison with observations. We speculate that a variety of unobservable hydrogen-poor sulfur oxoacid derivatives may be responsible for the "disappearance" of sulfur in dark cloud cores.

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“…The available activation energy for CO + H͑D͒ has been reported with a large variation ͑1000-7630 K͒ in literature. [13][14][15][16][17]38 Therefore, we attempted to estimate the activation barrier height by the analysis of experimental results with a model calculation of potential tunneling. The specific tunneling rate constant for a particle with energy E is described as k͑E͒ = P͑E͒, where P͑E͒ is the barrier permeability of the particle with a kinetic energy of E and is the frequency factor.…”

Section: -5mentioning

confidence: 99%

“…The parameter of the barrier width d would have the same value for each reaction system because the shape of barrier is governed by the electron configuration. Thus, the ratio of reaction rate constants is represented as a function of the activation barrier height E a and the parameter of the barrier width d. The barrier height of the H + CO reaction was derived theoretically by Werner et al, 15 Woon,13,14 Andersson and Grüning, 16 and Awad et al, 38 and experimentally determined by Wang et al 17 The calculated values are concentrated around 2000 K except for 7630 K as indicated by Awad et al 38 The barrier height determined by the gas phase experiment is 1000 K, 17 which is significantly smaller than the calculated values. Consequently, using Eqs.…”

Section: -5mentioning

confidence: 99%

Temperature, composition, and hydrogen isotope effect in the hydrogenation of CO on amorphous ice surface at 10–20K

Hidaka

Kouchi

Watanabe

2007

The Journal of Chemical Physics

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An experiment on the addition reaction of a D atom ͑deuteration͒ to CO on a cold ice surface is performed by deuterium atom exposure of three types of samples ͑pure solid CO, CO-capped H 2 O ice, and CO -H 2 O mixed ice͒ at 10-20 K. The variation of IR absorption spectra for the samples was measured by a Fourier transform infrared spectrometer during exposure to deuterium atoms. Reactions on pure solid CO were observed only at 10 K, while reactions on CO-capped H 2 O ice and CO-H 2 O mixed ice were observed to proceed even at 20 K. This indicates that the coexistence of H 2 O at the surface raises the reactive temperature. In addition, the experiment on H atom exposure was also carried out at 15 K to compare the reaction rate constant between the H and D atoms. The ratio of reaction rate constant k D / k H obtained is about 0.08 at 15 K. The authors provide information on the potential energy for the H + CO reaction at the surface by using the ratio k D / k H and by a model calculation of the potential tunneling with the asymmetric Eckart potential.

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New Rate Constants of Hydrogenation of CO on H₂O‐CO Ice Surfaces

Cited by 33 publications

References 33 publications

Hydrogenation reactions in interstellar CO ice analogues

Hydrogenation reactions in interstellar CO ice analogues

Subsurface chemistry of mantles of interstellar dust grains in dark molecular cores

Temperature, composition, and hydrogen isotope effect in the hydrogenation of CO on amorphous ice surface at 10–20K

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New Rate Constants of Hydrogenation of CO on H2O‐CO Ice Surfaces

Cited by 33 publications

References 33 publications

Hydrogenation reactions in interstellar CO ice analogues

Hydrogenation reactions in interstellar CO ice analogues

Subsurface chemistry of mantles of interstellar dust grains in dark molecular cores

Temperature, composition, and hydrogen isotope effect in the hydrogenation of CO on amorphous ice surface at 10–20K

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New Rate Constants of Hydrogenation of CO on H₂O‐CO Ice Surfaces