Adsorption and recombination of hydrogen atoms on a model graphite surface

Aronowitz, S.; Chang, Soon Woong

doi:10.1086/163230

Cited by 42 publications

(20 citation statements)

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“…The dust provides a surface for hydrogen atoms to meet and react and also removes enough of the reaction exothermicity so as to stabilize H 2 formation. In the last few decades, the formation of H 2 in the ISM has been studied in extensive detail, both theoretically (Gould & Salpeter 1963;Hollenbach & Salpeter 1970Smoluchowski 1981Smoluchowski , 1983Aronowitz & Chang 1985;Duley & Williams 1986;Farebrother et al 1999;Biham et al 2001;Green et al 2001;Chang et al 2005Chang et al , 2006Acharyya et al 2005;Chakrabarti et al 2006) and experimentally (Brackmann & Fite 1961;Schutte et al 1976;Pirronello & Averna 1988;Pirronello et al 1997aPirronello et al , 1997bPirronello et al , 1999Manicò et al 2001;Hornekaer et al 2003;Roser et al 2002Roser et al , 2003.…”

Section: Introductionmentioning

confidence: 99%

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Iqbal

Acharyya

Herbst

2012

ApJ

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We have used the continuous-time random-walk Monte Carlo technique to study the formation of H 2 from two hydrogen atoms on the surface of interstellar dust grains with both physisorption and chemisorption sites on olivine and carbonaceous material. In our standard approach, atoms must first enter the physisorption site before chemisorption can occur. We have considered hydrogen atom mobility due to both thermal hopping and quantum mechanical tunneling. The temperature range between 5 K and 825 K has been explored for different incoming H fluxes representative of interstellar environments with atomic hydrogen number density ranging between 0.1 cm −3 and 100 cm −3 and dust grain sizes ranging from 100 sites to 10 6 sites, the latter corresponding roughly to olivine grains of radius 0.2 μm. In addition, we have also considered rough surfaces with multiple binding sites. Tunneling is found to dominate the surface chemistry at low temperature, but as the temperature increases, the scenario changes. The inclusion of chemisorption sites can provide a meaningful efficiency for H 2 production up to temperatures as high as 700 K depending upon the depth of the chemisorption well. We found that over virtually the entire temperature range studied, the use of rate equations overestimates the H 2 formation rate to some extent. This overestimate is large at high temperatures, due to very low surface residence times. We have also considered models in which chemisorption sites are entered directly and diffusion proceeds only to other chemisorption sites.

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Section: Introductionmentioning

confidence: 99%

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Iqbal

Acharyya

Herbst

2012

ApJ

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“…It was suggested that these enhanced binding sites are chemical adsorption (chemisorption) sites. Their binding energies were calculated, mainly for carbonaceous dust materials (Bennet, McCaroll & Messmer 1971; Dovesi et al 1976; Dovesi, Pisani & Roetti 1981; Aronowitz & Chang 1985). However, recent theoretical and experimental results indicate that chemisorption sites play an important role only at much higher surface temperatures.…”

Section: The Abundance Of H2 In Photon‐dominated Regionsmentioning

confidence: 99%

Molecular hydrogen formation on porous dust grains

Perets

Biham

2006

Monthly Notices of the Royal Astronomical Society

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Recent laboratory experiments on interstellar dust analogues have shown that H 2 formation on dust-grain surfaces is efficient in a range of grain temperatures below 20 K. These results indicate that surface processes may account for the observed H 2 abundance in cold diffuse and dense clouds. However, high abundances of H 2 have also been observed in warmer clouds, including photon-dominated regions (PDRs), where grain temperatures may reach 50 K, making the surface processes extremely inefficient. It was suggested that this apparent discrepancy can be resolved by chemisorption sites. However, recent experiments indicate that chemisorption processes may not be efficient at PDR temperatures. Here we consider the effect of grain porosity on H 2 formation, and analyse it using a rate-equation model. It is found that porosity extends the efficiency of the recombination process to higher temperatures. This is because H atoms that desorb from the internal surfaces of the pores may re-adsorb many times and thus stay longer on the surface. However, this porosity-driven extension may enable efficient H 2 formation in PDRs only if porosity also contributes to significant cooling of the grains, compared to non-porous grains.

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“…In the Langmuir-Hinshelwood mechanism, the adsorbed H atoms quickly equilibrate and diffuse on the surface of the grain either by thermal activation or by tunneling. When two adsorbed H atoms encounter each other, an H 2 molecule may form (Williams 1968;Hollenbach & Salpeter 1970;Smoluchowski 1981;Aronowitz & Chang 1985;Duley & Williams 1986;Pirronello & Averna 1988;Sandford & Allamandolla 1993;Takahashi et al 1999;Farebrother et al 2000). The steady-state production rate of molecular hydrogen, R H2 (cm −3 s −1 ) can be expressed by…”

Section: Introductionmentioning

confidence: 99%

The Formation of H 2 and HD with the Master Equation Approach

Biham

Lipshtat

Perets

2006

IAU

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Abstract. The formation of H2 and HD molecules on interstellar dust grains is studied using rate equation and master equation models. Rate equations are used in the analysis of laboratory experiments, which examine the formation of molecular hydrogen on astrophysically relevant surfaces. However, under interstellar conditions, rate equations are not suitable for the calculation of reaction rates on dust-grain surfaces. Due to the low flux and the sub-micron size of the grains, the populations of H and D atoms on a single grain are likely to be small. In this case the reaction rates are dominated by fluctuations and should be calculated using stochastic methods. The rate of molecular hydrogen formation in interstellar clouds is evaluated using the master equation, taking into account the distribution of grain sizes.

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Adsorption and recombination of hydrogen atoms on a model graphite surface

Cited by 42 publications

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KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Molecular hydrogen formation on porous dust grains

The Formation of H 2 and HD with the Master Equation Approach

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Adsorption and recombination of hydrogen atoms on a model graphite surface

Cited by 42 publications

References 0 publications

KINETIC MONTE CARLO STUDIES OF H2FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

KINETIC MONTE CARLO STUDIES OF H2FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Molecular hydrogen formation on porous dust grains

The Formation of H 2 and HD with the Master Equation Approach

Contact Info

Product

Resources

About

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE