Changes in the morphology of interstellar ice analogues after hydrogen atom exposure
Changes in the morphology of interstellar ice analogues after hydrogen atom exposure
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DOI：
10.1039/c0cp01462a
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发表时间：
2011-01-01
影响因子：
3.3
通讯作者：
Pirronello, Valerio
中科院分区：
文献类型：
--
作者：
Accolla, Mario;Congiu, Emanuele;Pirronello, Valerio
水冰在星际介质中的形态仍然是一个悬而未决的问题。虽然气态水的吸积不是冷分子云中覆盖尘埃颗粒的冰盖的唯一可能来源，但众所周知，在保持在10K的表面上，从气相中吸积的水形成了表现出非常高孔隙率的冰膜。我们还知道，在暗云中，H-2的形成发生在尘埃颗粒的冰面上，当被吸附的原子反应形成H-2时，释放出的部分能量(4.48 eV)被沉积在冰中。本工作中描述的实验研究重点是原子氢暴露和随后的复合如何导致冰形态的相关变化。利用程序升温脱附(TPD)技术和程序升温脱附(TPD)谱的反转分析方法，我们发现无定形水冰样品的孔隙率在D原子辐照后呈指数下降。这种减少与冰的厚度成反比，其值为Phi(0)=2×10(16)D原子厘米(-2)每层水。我们还使用了一个模型，该模型证实了无论其能量深度如何，多孔冰上的结合位置都被破坏了，并且孔隙率的减少实际上对应于有效面积的减少。这种减少似乎与转移到多孔冰网络的D-2形成能的比例相一致。在星际条件下，这种效应很可能是有效的，与其他压实过程一起，提供了一个很好的论据，证明星际冰是无定形和无孔的。
The morphology of water ice in the interstellar medium is still an open question. Although accretion of gaseous water could not be the only possible origin of the observed icy mantles covering dust grains in cold molecular clouds, it is well known that water accreted from the gas phase on surfaces kept at 10 K forms ice films that exhibit a very high porosity. It is also known that in the dark clouds H-2 formation occurs on the icy surface of dust grains and that part of the energy (4.48 eV) released when adsorbed atoms react to form H-2 is deposited in the ice. The experimental study described in the present work focuses on how relevant changes of the ice morphology result from atomic hydrogen exposure and subsequent recombination. Using the temperature-programmed desorption (TPD) technique and a method of inversion analysis of TPD spectra, we show that there is an exponential decrease in the porosity of the amorphous water ice sample following D-atom irradiation. This decrease is inversely proportional to the thickness of the ice and has a value of phi(0) = 2 x 10(16) D-atoms cm(-2) per layer of H2O. We also use a model which confirms that the binding sites on the porous ice are destroyed regardless of their energy depth, and that the reduction of the porosity corresponds in fact to a reduction of the effective area. This reduction appears to be compatible with the fraction of D-2 formation energy transferred to the porous ice network. Under interstellar conditions, this effect is likely to be efficient and, together with other compaction processes, provides a good argument to believe that interstellar ice is amorphous and non-porous.