在天体物理感兴趣的表面上O原子在非常低的温度下的有效扩散机制
Efficient diffusive mechanisms of O atoms at very low temperatures on surfaces of astrophysical interest
摘要：
在星际尘埃颗粒的低温下，表面化学通过弱结合（物理吸附）到表面的H原子的扩散机制进行。然而，直到最近，人们还不知道比氢重的原子是否能在星际颗粒上扩散得足够快，从而与其他吸积物质发生反应。此外，模型仍然需要简单的还原和氧化反应发生在晶粒上，以解释各种分子的丰度。在本文中，我们研究O-原子的扩散和反应性的各种天体物理学相关的表面（水冰的三种不同的形态，硅酸盐，和石墨）在6.5 - 25 K的温度范围内。实验值被用来推导扩散定律，强调O原子扩散的量子力学隧道在温度低至6.5 K。在每个表面上的扩散速率，根据模拟结果，进行了计算，并给出了作为表面温度的函数的经验定律。相对扩散速率为k_H2Oice > k_sil > k_graph >> k_expected。一个有效的O-原子扩散的天体物理相关的时间尺度的影响进行了讨论。我们的研究结果表明，O原子可以扫描任何可用的反应伙伴（例如，或者另一个H原子，如果有的话，或者像O或OH这样的表面自由基）以比吸积更快的速率。此外，随着致密云成熟，H2变得比H丰富得多，O/H比增加，晶粒上O原子的反应性使得O与H一起成为主要的反应伙伴之一。
英文摘要：
At the low temperatures of interstellar dust grains, it is well established that surface chemistry proceeds via diffusive mechanisms of H atoms weakly bound (physisorbed) to the surface. Until recently, however, it was unknown whether atoms heavier than hydrogen could diffuse rapidly enough on interstellar grains to react with other accreted species. In addition, models still require simple reduction as well as oxidation reactions to occur on grains to explain the abundances of various molecules. In this paper we investigate O-atom diffusion and reactivity on a variety of astrophysically relevant surfaces (water ice of three different morphologies, silicate, and graphite) in the 6.5 - 25 K temperature range. Experimental values were used to derive a diffusion law that emphasizes that O atoms diffuse by quantum mechanical tunnelling at temperatures as low as 6.5 K. The rate of diffusion on each surface, based on modelling results, were calculated and an empirical law is given as a function of the surface temperature. Relative diffusion rates are k_H2Oice > k_sil > k_graph >> k_expected. The implications of an efficient O-atom diffusion over astrophysically relevant time-scales are discussed. Our findings show that O atoms can scan any available reaction partners (e.g., either another H atom, if available, or a surface radical like O or OH) at a faster rate than that of accretion. Also, as dense clouds mature H2 becomes far more abundant than H and the O/H ratio grows, the reactivity of O atoms on grains is such that O becomes one of the dominant reactive partners together with H.
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