LERMA-LAMAp, UMR8112 du CNRS, Observatoire de Paris et Université de Cergy-Pontoise, 5 Mail Gay-Lussac, 95000 Cergy Pontoise Cedex, France.
Phys Chem Chem Phys. 2011 May 7;13(17):8037-45. doi: 10.1039/c0cp01462a. Epub 2011 Mar 29.
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 ϕ(0) = 2 × 10(16) D-atoms cm(-2) per layer of H(2)O. 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.
星际介质中水冰的形态仍然是一个悬而未决的问题。尽管气态水的吸积不可能是覆盖冷分子云中尘埃颗粒的观测到的冰壳的唯一可能起源，但众所周知，从气相中吸积到保持在 10 K 的表面上的水形成冰膜，其表现出非常高的孔隙率。也知道在暗云中，H(2) 在尘埃颗粒的冰状表面上形成，并且当吸附原子反应形成 H(2) 时释放的部分能量（4.48 eV）沉积在冰中。本工作中描述的实验研究重点研究了原子氢暴露和随后的复合如何导致冰形态的相关变化。使用程序升温脱附（TPD）技术和 TPD 光谱的反演分析方法，我们表明，在 D 原子辐照后，无定形水冰样品的孔隙率呈指数下降。这种减少与冰的厚度成反比，并且每一层 H(2)O 的值为 ϕ(0) = 2 × 10(16) D-原子 cm(-2)。我们还使用了一个模型，该模型证实了多孔冰上的结合位无论其能量深度如何都被破坏，并且孔隙率的降低实际上对应于有效面积的降低。这种减少似乎与转移到多孔冰网络的 D(2)形成能的分数相匹配。在星际条件下，这种效应可能是有效的，并且与其他压实过程一起，为相信星际冰是非晶态和无孔的提供了很好的论据。
