KINETIC MONTE CARLO STUDIES OF H{sub 2} FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Journal Article · Sun May 20 00:00:00 EDT 2012 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/751/1/58· OSTI ID:22037232

Iqbal, Wasim; Acharyya, Kinsuk ^[1]; Herbst, Eric ^[2]

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1. S. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata 700 098 (India)
2. Departments of Chemistry, Astronomy, and Physics, University of Virginia, Charlottesville, VA 22904 (United States)

We have used the continuous-time random-walk Monte Carlo technique to study the formation of H{sub 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{sup -3} and 100 cm{sup -3} and dust grain sizes ranging from 100 sites to 10{sup 6} sites, the latter corresponding roughly to olivine grains of radius 0.2 {mu}m. 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{sub 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{sub 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.

• OSTI ID: 22037232
• Journal Information: Astrophysical Journal, Vol. 751, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ASTROPHYSICS
ATOMS
CARBONACEOUS MATERIALS
CHEMICAL REACTION KINETICS
CHEMISORPTION
COMPUTERIZED SIMULATION
COSMIC DUST
COSMOCHEMISTRY
DIFFUSION
GRAIN SIZE
GRAPH THEORY
HYDROGEN
MOBILITY
MOLECULES
MONTE CARLO METHOD
OLIVINE
QUANTUM MECHANICS
SURFACES
TUNNEL EFFECT