Material evolution simulator

本文介绍了一套实验室实验，研究了低温下固态有机物在光子和离子辐照下的演化，目的是模拟星际颗粒上有机难熔地幔的形成和演化。有机残留物是致密云中紫外光分解冰幔的产物，由2-3个含C原子的化合物组成。从实验室实验中，给出了有机残留物形成效率与紫外辐射剂量的函数关系。有机残留物的产率很高，可能是星际介质中固体碳质的主要来源。用10 e真空紫外光子辐照和3 keV He^+离子轰击模拟了有机残留物形成更复杂碳质的过程。后者具有与宇宙射线1 MeV质子相似的阻止本领，其随后的演化过程有两个特征：H、O和N的损失（碳化）和聚合物固体中聚芳簇的形成（聚合）。这两个过程都被证明是发生的，但值得注意的是，宇宙射线离子聚合的效率比吸收单位能量的紫外光子聚合的效率高出约10^3倍。然而，UV光子将主导聚合和碳化，因为在扩散介质中，UV光子存款在有机耐火颗粒地幔中的能量比宇宙射线多10^5倍。在扩散介质中的典型寿命之后，经处理的有机耐火颗粒地幔变成具有约0.6- 1.1eV的相对大的光学带隙的氢化无定形碳，具有少量但显著量的H、O和N，以及尺寸高达30-60环的石墨簇。这使得星际颗粒地幔成为多环芳烃的潜在来源。

A set of laboratory experiments is presented that study the evolution of solid organic matter at low temperature as a result of exposure to photons and ions, with the aim of simulating the formation and evolution of organic refractory mantles on interstellar grains.Organic residue is a product of UV photolysed ice mantles in dense clouds and consists of 2-3 C atom containing compounds. From laboratory experiments, the efficiency of organic residue formation is given as a function of UV radiation dose.The yield of organic residue is so high that probably is the major source of solid carbonaceous matter in the interstellar medium.The processing into more complex carbonaceous matter has been simulated by irradiation with 10e VUV photons and bom-bardment with 3ke V He^+ions. The latter have a stopping power similar to cosmic ray 1 MeVprotons.The subesquent evolution of the residue is characterized by two processes : The loss of H,O.and N (carbonization) and the formation of polyaromatic clusters in the polymeric solid (polymerization) . Both processes are shown to occur, but notably polymerization by cosmic ray ions is found to be abbout 10^3 times more efficient than that by UV photons per unit energy absorbed. UV photons, however, will dominate the polymerization and carbonization, because in the diffuse medium UV photons deposit as much as 10^5 times more energy in the organic refractoy grain mantles than cosmic rays.After a typical lifetime in the diffusc medium, the processed organic refractory grain mantle becomes an hydrogenated amorphous carbon with a relatively large optical gap of about 0.6-1.le V,with small but significant amounts of H,O,and N,and graphitic clusters up to sizes of 30-60 rings. This makes interstellar grain mantles a potential source of PAHs.