Untangling the Cosmic Dust Catalyzed Synthesis of Complex Organic Molecules in the Interstellar Medium

由于地面和空间射电望远镜的不断改进，到目前为止，已经在星际介质中探测到了200多个分子。由于低密度和低温度的苛刻条件，这些分子的数量和复杂性不能仅用气相反应来解释。根据目前的研究状况，分子云中的尘埃颗粒对检测到的分子的形成起着至关重要的作用。分子云内部的低温--低至5K--导致冻结气体在尘埃颗粒上积聚。这些冰主要由水、一氧化碳、二氧化碳、甲醇、甲烷、甲醛和氨组成。冰中较高的密度增加了不同分子之间反应的可能性，也有助于耗散放热反应释放的能量。在过去的几十年里，许多基于实验室的研究表明，将这些冰置于电离辐射下会导致复杂分子的形成，其中部分分子具有生物相关性。然而，这些研究大多忽略了冰层中心的尘埃颗粒可能起到的作用。相反，大多数实验都是在生长在金属衬底或红外透过性晶体上的冰中进行的，以便于光谱表征。最近，一些研究表明，冰生长的表面可以通过促进某些分子的形成，或者部分或完全阻止它的形成，在反应中发挥关键作用。因此，该项目的主要目标是系统地研究宇宙尘埃对冰层中几类关键分子形成的影响。为了实现这一点，不同的气体混合物模拟在星际介质的不同区域发现的冰，被凝聚到银镜的表面上，在超高压室中保持在5K的温度下。具有与宇宙尘埃相似的结构和组成的微粒被包裹在气体中，并并入冰中。然后，冰受到VUV辐射，其光谱组成模拟了分子云中的内部辐射场。利用不同的互补光谱方案，可以选择性地检测在冰中合成的分子。最后，将探测到的分子与在成分相同但没有微粒的冰中合成的分子进行定量比较，就可以评估宇宙尘埃类似物对不同分子合成的影响。

Owing to the constant improvement of earth- and space-based radio telescopes, more than 200 molecules have been detected in the interstellar medium by now. Due to the harsh conditions with low densities and temperatures, the amount and complexity of these molecules cannot be accounted for by gas-phase reactions alone. According to the current state of research, dust particles in molecular clouds play a crucial role in the formation of the detected molecules. The low temperatures inside molecular clouds – down to 5 K – lead to an accretion of frozen gases on the dust particles. These ices consist mainly of water, carbon monoxide, carbon dioxide, methanol, methane, formaldehyde, and ammonia. The higher density in the ices increases the probability of reactions between different molecules and also helps dissipate the energy released by exothermal reactions. In the last decades numerous laboratory based studies have shown that subjecting such ices to ionizing radiation leads to the formation of complex molecules, parts of which are biologically relevant. However, most of these studies neglected a possible role of the dust particle in the center of the ice. Instead, most experiments are conducted in ices grown on either metal substrates or IR transmissive crystals to facilitate spectroscopic characterization. Recently some studies revealed that the surface on which the ice is grown can play a crucial role in the reactions by either favoring the formation of some molecules or by partly or completely preventing it. The primary goal of this project is therefore a systematic study of the influence of cosmic dust on the formation of several key classes of molecules in the ices. To achieve this, different gas mixtures mimicking ices found in different regions of the interstellar medium are condensed onto the surface of a silver mirror held at temperatures of 5 K in a UHV chamber. Microparticles with a structure and composition similar to that of cosmic dust are entrailed in the gas to be incorporated into the ice. Afterwards the ices are subjected to VUV radiation with a spectral composition that simulates that of the internal radiation field in the molecular clouds. Exploiting different complementary spectroscopy schemes, molecules synthesized in the ice are detected isomer-selectively. Lastly a quantitative comparison of the molecules detected with those synthesized in ices of the same compositions but without microparticles will then allow to assess the influence of the cosmic dust analogues on the synthesis of different molecules.