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

Abstract

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.

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

“…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.…”

Computational Modeling of High Temperature Gas-Grain Chemistry in Interstellar Medium (ISM)

“…Considering these new chemisorption processes in this model only decreased the rate of formation roughly an order of magnitude, which is still much higher then what has been found using the Monte Carlo approach by Iqbal et al 25 However, by considering chemisorption processes the abundance of molecular hydrogen decreases by almost two orders of magnitude due to these additional destruction pathways. This should have a significant impact on the gas phase chemistry since water has been known to form through hydrogenation reactions, as follows: 54 …”

“…24 Modelling the chemistry in astrophysical environments is normally done using one of the following techniques: 1) the rate equation approach, 2) the master equation approach, 3) kinetic Monte Carlo method. [20][21][25][26] Each has its own advantages but in this research a rate equation approach will be utilized. Although this approach has the disadvantage of being less accurate then the master equation approach or Monte Carlo method it's computationally less expensive and is easier to couple with a network of gas phase and grain-surface reactions.…”

“…However, when considering diffusion between both a physisorbed site and chemisorbed site there's also a vertical component that should be considered, we're not considering that yet since only have a few of these reactions are in the network. 25 Since we 're not yet considering the interaction between chemisorbed and physisorbed binding sites in any great detail the focus is on temperatures above 200 K, where chemisorption processes should dominate. However, clearly this is problematic since accretion directly into a chemisorption site does have a barrier (.2 eV for H for instance).…”

“…However, clearly this is problematic since accretion directly into a chemisorption site does have a barrier (.2 eV for H for instance). 25 This process is likely to take place at higher temperatures, but at 200 K the physisorption sites are likely to be entered first at which point diffusion to a chemisorption site will occur. This over simplification will be discussed in more detail in the discussion section.…”