KINETIC MONTE CARLO STUDIES OF H2 FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE
KINETIC MONTE CARLO STUDIES OF H2 FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE
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DOI：
10.1088/0004-637x/751/1/58
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发表时间：
2012-05-20
影响因子：
4.9
通讯作者：
Herbst, Eric
中科院分区：
文献类型：
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作者：
Iqbal, Wasim;Acharyya, Kinsuk;Herbst, Eric
我们利用连续时间随机漫步蒙特卡罗技术研究了在橄榄石和碳质物质上具有物理吸附和化学吸附位点的星际尘埃颗粒表面两个氢原子生成H-2的过程。在我们的标准方法中，原子必须首先进入物理吸附位点，然后才能发生化学吸附。我们考虑了由于热跳变和量子力学隧穿引起的氢原子迁移。在5 K - 825 K的温度范围内，探索了不同的入射H通量，代表了原子氢数密度在0.1 cm(-3) - 100 cm（-3）之间的星际环境，尘埃颗粒大小在100 - 10(6)个位点之间，后者大致对应半径为0.2 μ m的橄榄石颗粒。此外，我们还考虑了具有多个结合位点的粗糙表面。发现在低温下，隧道作用主导了表面化学，但随着温度的升高，情况发生了变化。根据化学吸附井的深度，化学吸附位点的包含可以在高达700 K的温度下提供有意义的H-2生产效率。我们发现，在研究的几乎整个温度范围内，使用速率方程在一定程度上高估了H-2的形成速率。在高温下，由于表面停留时间非常短，这种高估很大。我们还考虑了直接进入化学吸附位点，扩散只进行到其他化学吸附位点的模型。
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 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-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.