Long-Time Scale Simulations of Tunneling-Assisted Diffusion of Hydrogen on Ice Surfaces at Low Temperature
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- 作者：V. Ásgeirsson ; H. JónssonK. T. Wikfeldt
- 刊名：Journal of Physical Chemistry C
- 出版年：2017
- 出版时间：January 26, 2017
- 年：2017
- 卷：121
- 期：3
- 页码：1648-1657
- 全文大小：551K
- ISSN：1932-7455
文摘
Hydrogen (H) atom diffusion on dust grain surfaces is the rate-limiting step in many hydrogenation reactions taking place in interstellar clouds. In cold (10–30 K) molecular clouds, the dust grains are coated by amorphous water ice. Therefore, H adatom mobility on ice surfaces is of fundamental importance in this context. We have calculated H atom adsorption and diffusion on both crystalline and amorphous ice surfaces using an analytic interaction potential for H2O–H. Tunneling rates for H atom hops between adsorption sites are explicitly calculated, the kinetic Monte Carlo method is used to simulate long time scale evolution and the diffusion coefficient, D, is evaluated for the temperature range 5–120 K. For ice Ih, we find D = 1.6 × 10–7 cm2/s at 10 K and below that temperature tunneling becomes the dominant diffusion mechanism. On the amorphous ice surface, the mobility of H is much slower than for ice Ih, D = 5.8 × 10–11 cm2/s at 25 K. Below 25 K, the H adatom becomes trapped in the deepest adsorption site which is located in a small surface pore. Furthermore, by blocking this site the diffusion increases by several orders of magnitude. H2 formation is thus likely to take place in deep adsorption sites at the low coverage and low temperature characteristic of molecular clouds. Deep adsorption sites can also explain experimental observations indicating that tunneling does not significantly contribute to the diffusivity, since even though H adatoms can tunnel between shallow adsorption sites, the rate-determining transitions out of deep sites require thermal activation. Furthermore, in the context of coarse-grained astrochemical models, we find the ratio of the activation energy of diffusion and the adsorption energy of H to be 0.64 on amorphous water ice.