Quantum mechanical treatment for the diffusion process of a hydrogen atom on the amorphous water ice surface

Takahashi, Junko; Nagaoka, Masataka; Masuda, Koichi

doi:10.1002/(sici)1097-461x(1998)70:2<379::aid-qua15>3.0.co;2-7

Cited by 6 publications

(4 citation statements)

References 6 publications

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“…The conclusion that can be drawn from these considerations is that ⌬ OH1 does not depend on ⌬E Int but ⌬ OH1 and ⌬E Int and d OH1 presumably depend on a fourth physical variable. From this point of view, Holmes and Sodeau, 32 in their extensive experimental study on the halogenocarbons, have evidenced linear dependencies between ⌬ OH1 and the polarizability of the considered molecules, at least within homogeneous families: CH 3 X, CH 2 X 2 , CHX 3 , CX 4 .…”

Section: G Discussionmentioning

confidence: 93%

“…The study of the interaction of atoms and molecules with water ice surfaces received considerable attention during the last decade due to its importance for the atmospheric chemistry ͑Arctic and Antarctic polar ices, 1 stratospheric clouds, 2 Antarctic ozone hole 3 ͒ and also in the area of astrochemistry ͑icy mantle of interstellar grain particles, 4 planetary ices, ices in the comet tails͒. 5 In both these areas, infrared spectroscopy has proved to be a powerful tool of investigation.…”

Section: Introductionmentioning

confidence: 99%

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Quantum study of the adsorption of small molecules on ice: The infrared frequency of the surface hydroxyl group and the vibrational stark effect

Manca¹,

Allouche²

2001

The Journal of Chemical Physics

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A periodic Hartree-Fock quantum calculation has been developed for the adsorption of small nonpolar model systems ͑C 2 H 2 , C 2 H 4 , O 3 , CO, N 2 , and Ar͒ on ice in order to determine the physical variables responsible for the large infrared frequency shifts of the surface hydroxyls. It is shown that the correlation between these shifts and the corresponding interaction energies is not quite convincing. On the contrary it appears that the frequency shifts are tightly correlated to the variation with adsorption of the local electric field along the OH bond thus demonstrating that the dangling hydroxyl frequency is largely governed by the vibrational stark effect.

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Section: G Discussionmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

Quantum study of the adsorption of small molecules on ice: The infrared frequency of the surface hydroxyl group and the vibrational stark effect

Manca¹,

Allouche²

2001

The Journal of Chemical Physics

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“…Naturally, at such low temperatures as 10 and 70 K, the quantum mechanical effect might be very important in the diffusion process of an H atom, since it could diffuse further via the quantum tunnelling mechanism even after it is trapped thermally. In order to estimate the diffusion constant including the quantum tunnelling effect, a new formalism using the differential diffusion constant was recently developed (Takahashi et al 1998). A rate calculation was performed for an H atom diffusing from one trapped site to another on the amorphous water ice slab.…”

Section: Resultsmentioning

confidence: 99%

“…Our model, in which the hydrogen atom or molecule is physisorbed on the surface of thr icy mantle and has excess thermal energy during most of the processes, could be well treated by this technique (Masuda & Takahashi 1997; Masuda et al 1998). The quantum treatment is necessary only for the diffusion process of the H atom after it loses the excess thermal energy and becomes trapped on the surface of the dust grain (Takahashi, Nagaoka & Masuda 1998).…”

Section: Introductionmentioning

confidence: 99%

The formation mechanism of molecular hydrogen on icy mantles of interstellar dust

Takahashi

Masuda

Nagaoka

1999

Monthly Notices of the Royal Astronomical Society

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The fundamental processes of H2 formation via H + H → H2 on the surfaces of icy mantles of interstellar dust have been investigated consistently within a single model based on a classical molecular dynamics (MD) computational simulation. As a model surface for icy mantles of dust grains, an amorphous water ice slab was generated at 10 and 70 K under periodic boundary conditions. The first and second incident H atoms were then ‘thrown’ on to the model surface. Two MD procedures were employed: (i) the H2)O molecules were treated as rigid (hard ice model); (ii) the intramolecular vibrational modes of H2O were included (soft ice model). The amorphous water ice slabs produced by our MD simulations are found to be good models for the surfaces of icy mantles of dust grains. For the various fundamental processes of H2 formation on the dust surface, the following results emerge. (1) For the sticking of an H atom on to the surface, a sticking probability that depends on the temperature of the incident H atom is obtained. (2) For the diffusion of an H atom on the surface, it is found that the first incident H atom diffuses via the thermal hopping mechanism at the first stage, and then it is always trapped in one of the stable sites on the amorphous ice. The migration length and time have been calculated for the mobility of the incident H atom before it is trapped. The time‐scales of thermal diffusion and desorption of H atoms after trapping have also been estimated. (3) For the reaction of two H atoms on the surface, the following three reaction patterns are observed: (i) H2 is produced via the Langmuir–Hinshelwood mechanism; (ii) H2 is produced via the Eley–Rideal mechanism; (iii) the almost elastic collision of two H atoms occurs without H2 being formed. The effective reactive cross‐section is estimated at about 40 Å2. The reaction probabilities are found to be near unity. (4) For the ejection of H2 from the ice surface, the product H2 is subsequently ejected after the reaction process, using part of the excess energy derived from the H2 formation. The average lifetime of ejection is about 400–600 fs. Most of the ejected H2 molecules are found to be in vibrationally and rotationally excited states.

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Theoretical Simulations of Grain-Surface Processes

Takahashi¹

2000

Symp. - Int. Astron. Union

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Recent advances in theoretical simulations of grain-surface processes are reviewed. Classical molecular dynamics (MD) computer simulations were performed to investigate the whole process of H2 formation on icy mantles of interstellar dust grains within a single model. Amorphous water ice slabs were generated at 10 K and 70 K as a realistic model surface of dust grains, and then two incident H atoms were successively thrown onto the surface to reproduce the H2 formation process via H + H → H2 on the dust surface. The following fundamental processes were studied in detail; 1) the sticking of H atom onto the grain surface, 2) the diffusion of H atom on the surface, 3) the reaction of two H atoms on the surface, 4) the ejection of H2 from the surface. Then, the formation pumping mechanism of H2 and the chemical desorption mechanism of frozen CO molecules in the vicinity of H2 forming sites on dust grains were also studied.

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Quantum mechanical treatment for the diffusion process of a hydrogen atom on the amorphous water ice surface

Cited by 6 publications

References 6 publications

Quantum study of the adsorption of small molecules on ice: The infrared frequency of the surface hydroxyl group and the vibrational stark effect

Quantum study of the adsorption of small molecules on ice: The infrared frequency of the surface hydroxyl group and the vibrational stark effect

The formation mechanism of molecular hydrogen on icy mantles of interstellar dust

Theoretical Simulations of Grain-Surface Processes

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