Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case

Martinazzo, Rocco; Tantardini, Gian Franco

doi:10.1063/1.2177655

Cited by 32 publications

(30 citation statements)

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“…Hellmann-Feynman forces were computed on-the-fly with DFT while the Newton equations of motion were integrated using a Verlet algorithm, as implemented in VASP, with a 0.40-fs time step. Carbon and target H atoms were initially kept at rest, implying T s = 0 K, and left free to move during the whole simulation; ER reaction is expected to be little influenced by the surface temperature [even when vibrational excitation of the target occurs (22,42)], and thus this T s value is expected to be appropriate for ISM conditions (T s = 5-100 K). Only the height of one of the carbon atoms furthest away from the target was kept fixed to prevent block translation of the whole system.…”

Section: Methodsmentioning

confidence: 99%

Insights into H ₂ formation in space from ab initio molecular dynamics

Casolo

Tantardini

Martinazzo

2013

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

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Hydrogen formation is a key process for the physics and the chemistry of interstellar clouds. Molecular hydrogen is believed to form on the carbonaceous surface of dust grains, and several mechanisms have been invoked to explain its abundance in different regions of space, from cold interstellar clouds to warm photondominated regions. Here, we investigate direct (Eley-Rideal) recombination including lattice dynamics, surface corrugation, and competing H-dimers formation by means of ab initio molecular dynamics. We find that Eley-Rideal reaction dominates at energies relevant for the interstellar medium and alone may explain observations if the possibility of facile sticking at special sites (edges, point defects, etc.) on the surface of the dust grains is taken into account.graphite-graphene | interstellar chemistry | hydrogen recombination | density functional theory T he complicated interaction between hydrogen atoms and graphite has been widely studied in the last decade because of its relevance in various fields, from hydrogen storage (1) to nuclear fusion (2), from graphene technology (3-6) to interstellar chemistry (7).In the chemistry of the interstellar medium (ISM), H 2 is a precursor for more complex molecules through its protonated form H þ 3 , shields the clouds from the radiation field, and acts as primary cooling agent during the gravitational collapse, which ultimately leads to star formation (7). Its appearance in the early universe steered the development of the first stars and triggered galaxy formation (8).In the harsh ISM conditions, molecular hydrogen is continuously dissociated by stellar UV radiation and cosmic rays; hence efficient synthetic pathways are needed to explain the observed abundances (7). These are known to involve the carbonaceous surface of interstellar dust grains (9), but the detailed mechanisms behind H 2 formation on graphitic surfaces have yet to be fully uncovered.Physisorbed H atoms can only be invoked in cold molecular clouds, where the surface temperature is low enough to prevent desorption (T s ≤ 40 K) from the shallow physisorption well (E ∼ 4 kJ·mol −1 ) (10, 11). In this case, hydrogen molecules may follow from either an Eley-Rideal (ER)/hot-atom (HA) or a LangmuirHinshelwood (LH) pathway, because H atoms are very mobile even at extremely low temperatures (11). Stable (chemisorbed) species on the basal graphitic plane, however, require energetic projectile atoms to overcome an activation barrier to sticking (E ∼ 20 kJ·mol −1 ) (12, 13), which is due to the carbon sp 2 -sp 3 rehybridization needed by the bond formation process, although tunneling might help in this context (14). Chemically bound H atoms are immobile on the surface [they desorb rather than diffusing (15)]; hence LH pathway is ruled out in sufficiently warm environments where only chemisorbed species can be found. No matter how a first chemisorbed species is formed, secondary H atoms undergo facile sticking and preferentially form ortho and para (16, 17) dimers because of the aromatic nature of the...

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Section: Methodsmentioning

confidence: 99%

Insights into H ₂ formation in space from ab initio molecular dynamics

Casolo

Tantardini

Martinazzo

2013

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

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“…This assumption of Eley-Rideal-like reaction for physisorbed atoms is similarly made in the model of Cuppen et al (2006) with the difference that they do not consider desorption at the formation of the new molecule. This direct Eley-Rideal process is expected to have a large cross section at temperatures relevant to the interstellar medium (Martinazzo & Tantardini 2006), and it is thus a reasonable assumption to assume that it dominates over rejection. To estimate the influence of this assumption on our results, we computed the contribution of this direct ER-like reaction process to the total mean formation rate through physisorption and found it to be a negligible fraction (always less than 1% of the total average formation rate).…”

Section: Surface Chemistry Via Langmuir-hinshelwood Mechanismmentioning

confidence: 99%

Surface chemistry in the interstellar medium

Bron

Bourlot

Petit

2014

A&A

118

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Context. The H 2 formation on grains is known to be sensitive to dust temperature, which is also known to fluctuate for small grain sizes due to photon absorption. Aims. We aim at exploring the consequences of simultaneous fluctuations of the dust temperature and the adsorbed H-atom population on the H 2 formation rate under the full range of astrophysically relevant UV intensities and gas conditions. Methods. The master equation approach is generalized to coupled fluctuations in both the grain's temperature and its surface population and solved numerically. The resolution can be simplified in the case of the Eley-Rideal mechanism, allowing a fast computation. For the Langmuir-Hinshelwood mechanism, it remains computationally expensive, and accurate approximations are constructed. Results. We find the Langmuir-Hinshelwood mechanism to become an efficient formation mechanism in unshielded photon dominated region edge conditions when taking those fluctuations into account, despite hot average dust temperatures. It reaches an importance comparable to the Eley-Rideal mechanism. However, we show that a simpler rate equation treatment gives qualitatively correct observable results in full cloud simulations under the most astrophysically relevant conditions. Typical differences are a factor of 2−3 on the intensities of the H 2 v = 0 lines. We also find that rare fluctuations in cloud cores are sufficient to significantly reduce the formation efficiency. Conclusions. Our detailed analysis confirms that the usual approximations used in numerical models are adequate when interpreting observations, but a more sophisticated statistical analysis is required if one is interested in the details of surface processes.

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“…The formation of H 2 that involves chemisorbed atoms through the ER mechanism has been studied by DFT (Jeloaica & Sidis 1999;Ferro & Allouche 2003) and dynamics calculations (Rutigliano et al 2001;Ree et al 2002;Morisset et al 2003;2004b;Martinazzo & Tantardini 2006). Based on these calculations, different mechanisms have been proposed to contribute to the H 2 formation through the ER mechanisms: the direct ER that involves isolated H atoms (monomers, Morisset et al 2003;2004b;Martinazzo & Tantardini 2006), barrier-less formation of H 2 involving one H atom in a para-dimer configuration (Bachellerie et al 2007) and formation by diffusing H atoms in physisorbed states (Bonfanti et al 2007). In the LH mechanism, the two H atoms are adsorbed (physisorbed) on the graphite surface, diffuse on the surface and collide to desorb in a hydrogen molecule.…”

Section: Introductionmentioning

confidence: 99%

“…In the LH mechanism, the two H atoms are adsorbed (physisorbed) on the graphite surface, diffuse on the surface and collide to desorb in a hydrogen molecule. This mechanism has also been studied (Morisset et al 2004a;Martinazzo & Tantardini 2006). The different mechanisms to form H 2 on graphite surfaces are the following: M1) LH mechanism: two physisorbed H atoms encounter each other on the surface; M2) direct Eley-Rideal mechanism involving H atom chemisorbed in a monomer (only one H atom chemisorbed on the cycle, Fig.…”

Section: Introductionmentioning

confidence: 99%

When sticking influences H₂formation

Cazaux¹,

Morisset

Spaans³

et al. 2011

A&A

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Aims. Because of their catalytic properties, interstellar dust grains are crucial to the formation of H 2 , the most abundant molecule in the Universe. The formation of molecular hydrogen strongly depends on the ability of H atoms to stick on dust grains. In this study we determine the sticking coefficient of H atoms chemisorbed on graphitic surfaces, and estimate its impact on the formation of H 2 . Methods. The sticking probability of H atoms chemisorbed onto graphitic surfaces is obtained using a mixed classical-quantum dynamics method. In this, the H atom is treated quantum-mechanically and the vibrational modes of the surface are treated classically. The implications of sticking for the formation of H 2 are addressed by using kinetic Monte Carlo simulations that follow how atoms stick, move and associate with each other on dust surfaces of different temperature.Results. In our model, molecular hydrogen forms very efficiently for dust temperatures lower than 15 K through the involvement of physisorbed H atoms. At dust temperatures higher than 15 K and gas temperatures lower than 2000 K, H 2 formation differs strongly if the H atoms coming from the gas phase have to cross a square barrier (usually considered in previous studies) or a barrier obtained by density functional theory (DFT) calculations to become chemisorbed. The product of the sticking times efficiency can be increased by many orders of magnitude when realistic barriers are considered. If graphite phonons are taken into account in the dynamics calculations, then H atoms stick better on the surface at high energies, but the overall H 2 formation efficiency is only slightly affected. Our results suggest that H 2 formation can proceed efficiently in photon-dominated regions, X-ray dominated regions, hot cores and in the early Universe when the first dust is available.

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Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case

Cited by 32 publications

References 36 publications

Insights into H ₂ formation in space from ab initio molecular dynamics

Insights into H ₂ formation in space from ab initio molecular dynamics

Surface chemistry in the interstellar medium

When sticking influences H₂formation

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Quantum study of Eley-Rideal reaction and collision induced desorption of hydrogen atoms on a graphite surface. II. H-physisorbed case

Cited by 32 publications

References 36 publications

Insights into H 2 formation in space from ab initio molecular dynamics

Insights into H 2 formation in space from ab initio molecular dynamics

Surface chemistry in the interstellar medium

When sticking influences H2formation

Contact Info

Product

Resources

About

Insights into H ₂ formation in space from ab initio molecular dynamics

Insights into H ₂ formation in space from ab initio molecular dynamics

When sticking influences H₂formation