The Energetics and Efficiency of H<sub>2</sub> Formation on the Surface of Simulated Interstellar Grains

Williams, D. A.; Williams, Dudley; Clary, DC; Farebrother, A.; Fisher, AJ; Gingell, JM; Jackman, Richard B.; Mason, Nigel; Meijer, Anthony J. H. M.; Perry, Jennifer C.; Price, S. D.; Rawlings, J. M. C.

doi:10.1017/cbo9780511564635.015

Cited by 4 publications

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“…But due to our limited understanding of the relevant properties of interstellar grains (composition, structure and hydrogen coverage) and hence of grain surface reactions, the H 2 formation mechanism is not yet understood. Numerous theoretical and experimental studies have thus been dedicated to a better understanding of the H 2 formation process (Gould and Salpeter, 1963;Hollenbach and Salpeter, 1971;Sandford and Allamandola, 1993;Duley, 1996;Parneix and Brechignac, 1998;Pirronello et al, 1997;Pirronello et al, 1999;Takahashi et al, 1999;Katz et al, 1999;Williams et al, 2000;Sidis et al, 2000;Biham et al, 2001;Joblin et al, 2001;Cazaux and Tielens, 2002;Cazaux and Tielens, 2003). Another approach to this issue is to use observations to provide estimates of the H 2 formation rate in different regions of the ISM in order to see how it depends on the local physical parameters.…”

Section: Excitation and Formation Of Hmentioning

confidence: 99%

Molecular Hydrogen

et al. 2005

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Observations of H 2 line emission in galactic and extragalactic environments obtained with the Infrared Space Observatory (ISO) are reviewed. The diagnostic capability of H 2 observations is illustrated. We discuss what one has learned about such diverse astrophysical sources as photondominated regions, shocks, young stellar objects, planetary nebulae and starburst galaxies from ISO observations of H 2 emission. In this context, we emphasise use of measured H 2 line intensities to infer important physical quantities such as the gas temperature, gas density and radiation field and we discuss the different possible excitation mechanisms of H 2 . We also briefly consider future prospects for observation of H 2 from space and from the ground.

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Section: Excitation and Formation Of Hmentioning

confidence: 99%

Molecular Hydrogen

et al. 2005

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“…Numerous theoretical and experimental studies have thus been dedicated to the study of the H 2 formation process (Sandford & Allamandola 1993;Duley 1996;Parneix & Brechignac 1998;Pirronello et al 1997Pirronello et al , 1999Takahashi et al 1999;Katz et al 1999;Williams et al 2000;Sidis et al 2000; Send offprint requests to: E. Habart, e-mail: habart@arcetri.astro.it Biham et al 2001;Joblin et al 2001;Cazaux & Tielens 2002. Another approach to this issue is to examine the H 2 formation rate in different regions of the ISM in order to see how it depends upon the local physical parameters.…”

Section: Introductionmentioning

confidence: 99%

Some empirical estimates of the H₂formation rate in photon-dominated regions

Habart¹,

Boulanger²,

Verstraete³

et al. 2004

A&A

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Abstract. We combine recent ISO observations of the vibrational ground state lines of H 2 towards Photon-Dominated Regions (PDRs) with observations of vibrationally excited states made with ground-based telescopes in order to constrain the formation rate of H 2 on grain surfaces under the physical conditions in the layers responsible for H 2 emission. We briefly review the data available for five nearby PDRs. We use steady state PDR models in order to examine the sensitivity of different H 2 line ratios to the H 2 formation rate R f . We show that the ratio of the 0-0 S(3) to the 1-0 S(1) line increases with R f but that one requires independent estimates of the radiation field incident upon the PDR and the density in order to infer R f from the H 2 line data. We confirm earlier work by Habart et al. (2003) on the Oph W PDR which showed that an H 2 formation rate higher than the standard value of 3 × 10 −17 cm 3 s −1 inferred from UV observations of diffuse clouds is needed to explain the observed H 2 excitation. From comparison of the ISO and ground-based data, we find that moderately excited PDRs such as Oph W, S140 and IC 63 require an H 2 formation rate of about five times the standard value whereas the data for PDRs with a higher incident radiation field such as NGC 2023 and the Orion Bar can be explained with the standard value of R f . We compare also the H 2 1-0 S(1) line intensities with the emission in PAH features and find a rough scaling of the ratio of these quantities with the ratio of local density to radiation field. This suggests but does not prove that formation of H 2 on PAHs is important in PDRs. We also consider some empirical models of the H 2 formation process with the aim of explaining these results. Here we consider both formation on classical grains of size roughly 0.1 µm and on very small (∼10 Å) grains by either direct recombination from the gas phase (Eley-Rideal mechanism) or recombination of physisorbed H atoms with atoms in a chemisorbed site. We conclude that indirect chemisorption where a physisorbed H-atom scans the grain surface before recombining with a chemisorbed H-atom is most promising in PDRs. Moreover small grains which dominate the total grain surface and spend most of their time at relatively low (below 30 K for χ ≤ 3000) temperatures may be the most promising surface for forming H 2 in PDRs.

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“…Hydrogen adsorption on carbon based materials such as graphite and graphene is relevant to hydrogen storage, 9 band gap engineering, [10][11][12][13] and potentially as the first step in H 2 formation in the interstellar medium. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Although there is enormous interest in H adsorption on carbonaceous surfaces, with graphene, graphite and polycyclic aromatic hydrocarbons (PAHs) being the most widely studied model systems, we still don't fully understand the seemingly simple process of how a single H atom adsorbs on the surface.…”

mentioning

confidence: 99%

“…The classic low temperature adsorption process where QNEs can be expected to play a role is H adsorption on the surfaces of carbonaceous materials. Hydrogen adsorption on carbon based materials such as graphite and graphene is relevant to hydrogen storage, band gap engineering, − and potentially as the first step in H 2 formation in the interstellar medium. − Although there is enormous interest in H adsorption on carbonaceous surfaces, with graphene, graphite and polycyclic aromatic hydrocarbons (PAHs) being the most widely studied model systems, we still do not fully understand the seemingly simple process of how a single H atom adsorbs on the surface. First, there are question marks over the energy barrier a H atom encounters upon adsorbing on the surface.…”

mentioning

confidence: 99%

Cooperative Interplay of van der Waals Forces and Quantum Nuclear Effects on Adsorption: H at Graphene and at Coronene

et al. 2014

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The energetic barriers that atoms and molecules often experience when binding to surfaces are incredibly important to a myriad of chemical and physical processes. However these barriers are difficult to describe accurately with current computer simulation approaches. Two prominent contemporary challenges faced by simulation are the role of van der Waals forces and nuclear quantum effects. Here we examine the widely studied model systems of hydrogen

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The Energetics and Efficiency of H₂ Formation on the Surface of Simulated Interstellar Grains

Cited by 4 publications

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Molecular Hydrogen

Molecular Hydrogen

Some empirical estimates of the H₂formation rate in photon-dominated regions

Cooperative Interplay of van der Waals Forces and Quantum Nuclear Effects on Adsorption: H at Graphene and at Coronene

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The Energetics and Efficiency of H2 Formation on the Surface of Simulated Interstellar Grains

Cited by 4 publications

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Molecular Hydrogen

Molecular Hydrogen

Some empirical estimates of the H2formation rate in photon-dominated regions

Cooperative Interplay of van der Waals Forces and Quantum Nuclear Effects on Adsorption: H at Graphene and at Coronene

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The Energetics and Efficiency of H₂ Formation on the Surface of Simulated Interstellar Grains

Some empirical estimates of the H₂formation rate in photon-dominated regions