Off-lattice Microscopic Monte Carlo Modeling of Molecular Hydrogen Formation on Carbonaceous Dust Grains

Abstract

In this work, we present an off-lattice Monte Carlo model of accretion and migration of hydrogen atoms on a rough surface of carbon dust grain. The migration of physisorbed atoms by means of thermal diffusion and quantum tunnelling through barriers between the surface potential minima is considered. The model is applied to simulations of molecular hydrogen formation in a cold interstellar medium for a temperature range 5-35 K. Eley-Rideal and Langmuir-Hinshelwood mechanisms for the formation of the H${}_2$ molecule were taken into account. We found that the surface potential energy minima that hold the accreted hydrogen atoms (binding energy) has wide dispersion of its values. The minimum energy is three times smaller than the maximum energy for the uneven surface of the model grain. The large dispersion of the binding energies results in an extended range of temperatures where H${}_2$ formation is efficient: 5-25 K. The dispersion of binding energies also reduces efficiency of diffusion due to tunnelling in comparison to that assumed in kinetic equation codes in which constant values of binding energies are employed. Thus, thermal hopping is the main source for the mobility of the hydrogen atoms in the presented off-lattice model. Finally, the model naturally provides the mean values for the ratio of binding-to-desorption energy. This ratio demonstrates weak dependence on temperature and is in the range of 0.5-0.6.

arXiv comment8 pages, 8 figures, accepted to MNRAS

References in corpus (10)

• The evolution of amorphous hydrocarbons in the ISM: dust modelling from a new vantage point
• Molecular Hydrogen Formation on Astrophysically Relevant Surfaces
• Binding energies of interstellar molecules on crystalline and amorphous models of water ice by ab-initio calculations
• Gas-grain chemistry in cold interstellar cloud cores with a microscopic Monte Carlo approach to surface chemistry
• Molecular hydrogen formation on dust grains in the high redshift universe
• H2 formation on interstellar grains and the fate of reaction energy
• Three-dimensional off-lattice Monte Carlo kinetics simulations of interstellar grain chemistry and ice structure
• Comparing gas-phase and grain-catalyzed H2 formation
• Binding Energy Evaluation Platform: A database of quantum chemical binding energy distributions for the astrochemical community
• Robust Binding Energy Distribution Sampling on Amorphous Solid Water Models. Method testing and validation with NH3, CO and CH4