星际条件下D原子在多孔非晶水冰表面上的迁移率
Mobility of D atoms on porous amorphous water ice surfaces under interstellar conditions
摘要：
目标。氢原子在星际尘埃颗粒表面低温下的迁移率仍然是一个有争议的问题。在稠密的云层中，吸附物质的氢化（即，CO），以及随后的氘化的吸积分子取决于氢原子的流动性的水冰。天体化学模型普遍认为H原子在尘埃颗粒表面是移动的，即使仍然存在争议。本文给出了10 K时H原子在多孔水冰表面上迁移的直接实验证据。方法.在超高真空室中，O2沉积在多孔无定形水冰基底上。然后在10 K下将D原子沉积到表面上。使用程序升温脱附（TPD）并同时监测O2和D2的脱附。结果我们发现，如果增加D原子的沉积时间，则TPD过程中脱附的O2量会减少。因此，即使这些分子先前已经扩散到厚水冰的孔隙中，O2也会被D原子破坏。我们的结果可以很容易地解释，如果D是移动在10 K的水冰面上。一个简单的速率方程模型拟合我们的实验数据和最佳拟合曲线，获得了D原子的扩散势垒为22（+-）2 meV。因此，氢化作用可以有效地发生在星际尘埃颗粒上。这些实验结果与大多数计算结果一致，并验证了几个模型中使用的假设。
英文摘要：
Aims. The mobility of H atoms on the surface of interstellar dust grains at low temperature is still a matter of debate. In dense clouds, the hydrogenation of adsorbed species (i.e., CO), as well as the subsequent deuteration of the accreted molecules depend on the mobility of H atoms on water ice. Astrochemical models widely assume that H atoms are mobile on the surface of dust grains even if controversy still exists. We present here direct experimental evidence of the mobility of H atoms on porous water ice surfaces at 10 K. Methods. In a UHV chamber, O2 is deposited on a porous amorphous water ice substrate. Then D atoms are deposited onto the surface held at 10 K. Temperature-Programmed Desorption (TPD) is used and desorptions of O2 and D2 are simultaneously monitored. Results. We find that the amount of O2 that desorb during the TPD diminishes if we increase the deposition time of D atoms. O2 is thus destroyed by D atoms even though these molecules have previously diffused inside the pores of thick water ice. Our results can be easily interpreted if D is mobile at 10 K on the water ice surface. A simple rate equation model fits our experimental data and best fit curves were obtained for a D atoms diffusion barrier of 22(+-)2 meV. Therefore hydrogenation can take place efficiently on interstellar dust grains. These experimental results are in line with most calculations and validate the hypothesis used in several models.
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