The Mechanism of Surface Diffusion of H and D Atoms on Amorphous Solid Water: Existence of Various Potential Sites

Hama, Tetsuya; Kuwahata, Kazuaki; Watanabe, Naoki; Kouchi, Akira; Kimura, Yuki; Chigai, Takeshi; Pirronello, V.

doi:10.1088/0004-637x/757/2/185

Cited by 89 publications

(149 citation statements)

References 62 publications

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“…theoretically showed that the tunneling diffusion of physisorbed H atoms is strongly suppressed on an amorphous surface because of the nonperiodic adsorption potential sites with different energy depths (25). We previously observed a small KIE during the surface diffusion of H and D atoms on amorphous solid water (25). Vidali, Pirronello, and coworkers also reported the thermally activated diffusion of H and D atoms on silicates and amorphous carbon (26)(27)(28).…”

Section: Discussionmentioning

confidence: 95%

“…Adsorption and desorption of H and D atoms interacting with the surface through van der Waals forces only have semiclassical KIEs owing to the lower zero-point energy of a D atom than an H atom. theoretically showed that the tunneling diffusion of physisorbed H atoms is strongly suppressed on an amorphous surface because of the nonperiodic adsorption potential sites with different energy depths (25). We previously observed a small KIE during the surface diffusion of H and D atoms on amorphous solid water (25).…”

Section: Discussionmentioning

confidence: 98%

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Quantum tunneling observed without its characteristic large kinetic isotope effects

Hama

Ueta

Kouchi

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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Classical transition-state theory is fundamental to describing chemical kinetics; however, quantum tunneling is also important in explaining the unexpectedly large reaction efficiencies observed in many chemical systems. Tunneling is often indicated by anomalously large kinetic isotope effects (KIEs), because a particle's ability to tunnel decreases significantly with its increasing mass. Here we experimentally demonstrate that cold hydrogen (H) and deuterium (D) atoms can add to solid benzene by tunneling; however, the observed H/D KIE was very small (1-1.5) despite the large intrinsic H/D KIE of tunneling (≳100). This strong reduction is due to the chemical kinetics being controlled not by tunneling but by the surface diffusion of the H/D atoms, a process not greatly affected by the isotope type. Because tunneling need not be accompanied by a large KIE in surface and interfacial chemical systems, it might be overlooked in other systems such as aerosols or enzymes. Our results suggest that surface tunneling reactions on interstellar dust may contribute to the deuteration of interstellar aromatic and aliphatic hydrocarbons, which could represent a major source of the deuterium enrichment observed in carbonaceous meteorites and interplanetary dust particles. These findings could improve our understanding of interstellar physicochemical processes, including those during the formation of the solar system. quantum tunneling | kinetic isotope effect | heterogeneous reactions | reaction dynamics | astrochemistry

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Section: Discussionmentioning

confidence: 95%

Section: Discussionmentioning

confidence: 98%

Quantum tunneling observed without its characteristic large kinetic isotope effects

Hama

Ueta

Kouchi

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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“…Recent experiments by Hama et al (2012) have shown that the majority of H-atom binding sites on ASW are shallow, with diffusion barriers 22 meV. A small fraction of the sites were found to have higher diffusion barriers ( 30 meV), which might correspond to nanopore sites.…”

Section: Astrophysical Significancementioning

confidence: 96%

Dynamics of Co in Amorphous Water-Ice Environments

Karssemeijer

Ioppolo

Hemert

et al. 2013

ApJ

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The long-timescale behavior of adsorbed carbon monoxide on the surface of amorphous water ice is studied under dense cloud conditions by means of off-lattice, on-the-fly, kinetic Monte Carlo simulations. It is found that the CO mobility is strongly influenced by the morphology of the ice substrate. Nanopores on the surface provide strong binding sites, which can effectively immobilize the adsorbates at low coverage. As the coverage increases, these strong binding sites are gradually occupied leaving a number of admolecules with the ability to diffuse over the surface. Binding energies and the energy barrier for diffusion are extracted for various coverages. Additionally, the mobility of CO is determined from isothermal desorption experiments. Reasonable agreement on the diffusivity of CO is found with the simulations. Analysis of the 2152 cm −1 polar CO band supports the computational findings that the pores in the water ice provide the strongest binding sites and dominate diffusion at low temperatures.

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“…In this case, formed species can be quantified at the desired sample temperature and kinetic curves are obtained [082,083]. Different initial exposures and temperatures are probed to obtain information on, for instance, the reaction order.…”

Section: P a G E |mentioning

confidence: 99%

Atom addition reactions in interstellar ice analogues

Linnartz

Ioppolo

Fedoseev

2015

International Reviews in Physical Chemistry

160

142

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It was in 'The Magellanic Cloud' (1955) -a science fiction novel by Stanislaw Lem -that engineers traveling to another star noticed that their spacecraft for unknown reasons overheated. The cause had to be outside the spaceship, but obviously there was only emptiness, at least compared to terrestrial conditions. The space between the stars, the interstellar medium, however, is not completely empty and at the high speed of the spacecraft the cross-section with impacting particles, even from such a dilute environment, was found to be sufficient to cause an overheating. Today, 60 years later, the interstellar medium has been studied in detail by astronomical observations, reproduced in dedicated laboratory experiments and simulated by complex astrochemical models. The space between the stars is, indeed, far from empty; it comprises gas, dust and ice and the molecules detected so far are both small (diatomics) and large (long carbon chains, PAHs and fullerenes), stable and reactive (radicals, ions, and excited molecules) evidencing an exotic and fascinating chemistry, taking place at low densities, low temperatures and experiencing intense radiation fields.Astrochemists explain the observed chemical complexity in space -so far 185 different molecules (not including isotopologues) have been identified -as the cumulative outcome of P a g e | 2 reactions in the gas phase and on icy dust grains. Gas phase models explain the observed abundances of a substantial part of the observed species, but fail to explain the number densities for stable molecules, as simple as water, methanol or acetonitrile -one of the most promising precursor species for the simplest amino acid glycine -as well as larger compounds such as glycolaldehyde, dimethylether and ethylene glycol. Evidence has been found that these and other complex species, including organic ones, form on icy dust grains that act as catalytic sites for molecule formation. It is here where particles 'accrete, meet, and greet' (i.e., freeze out, diffuse and react) upon energetic and non-energetic processing, such as irradiation by vacuum UV light, interaction with impacting particles (atoms, electrons and cosmic rays) or heating.This review paper summarizes the state-of-the-art in laboratory based interstellar ice chemistry. The focus is on atom addition reactions, illustrating how water, carbon dioxide and methanol can form in the solid state at astronomically relevant temperatures, and also the formation of more complex species such as hydroxylamine, an important prebiotic molecule, and glycolaldehyde, the smallest sugar, is discussed. These reactions are particularly relevant during the 'dark' ages of star and planet formation, i.e., when the role of UV light is restricted.A quantitative characterization of such processes is only possible through dedicated laboratory studies, i.e., under full control of a large set of parameters such as temperature, atom-flux, and ice morphology. The resulting numbers, physical and chemical constants, e.g., barrier heights, react...

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The Mechanism of Surface Diffusion of H and D Atoms on Amorphous Solid Water: Existence of Various Potential Sites

Cited by 89 publications

References 62 publications

Quantum tunneling observed without its characteristic large kinetic isotope effects

Quantum tunneling observed without its characteristic large kinetic isotope effects

Dynamics of Co in Amorphous Water-Ice Environments

Atom addition reactions in interstellar ice analogues

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