Naturally occurring ices lie on both interstellar dust grains and on celestial objects, such as those in the outer solar system. These ices are continu- ously subjected to irradiation by ions from the solar wind and/or cosmic rays, which modify their surfaces. As a result, new molecular species may form which can be sputtered off into space or planetary atmospheres. We determined the experimental values of sputtering yields for irradiation of oxygen ice at 10 K by singly (He+, C+, N+, O+ and Ar+) and doubly (C2+, N2+ and O2+) charged ions with 4 keV kinetic energy. In these laboratory experiments, oxygen ice was deposited and irradiated by ions in an ultra high vacuum chamber at low temperature to simulate the environment of space. The number of molecules removed by sputtering was observed by measurement of the ice thickness using laser interferometry. Preliminary mass spectra were taken of sputtered species and of molecules formed in the ice by temperature programmed desorption (TPD). We find that the experimental sputtering yields increase approximately linearly with the projectile ion mass (or momentum squared) for all ions studied. No difference was found between the sputtering yield for singly and doubly charged ions of the same atom within the experimental uncertainty, as expected for a process dominated by momentum transfer. The experimental sputter yields are in good agreement with values calculated using a theoretical model except in the case of oxygen ions. Preliminary studies have shown molecular oxygen as the dominant species sputtered and TPD measurements indicate ozone formation.
Functionalization of Benzene Ices by Atomic Oxygen
Harvard-Smithsonian Center for Astrophysics（哈佛-史密森天体物理中心）；Harvard University（哈佛大学）；University of Bern（伯尔尼大学）；The Hebrew University of Jerusalem（耶路撒冷希伯来大学）；UC Berkeley（加州大学伯克利分校）
Different degrees of nitrogen and carbon depletion in the warm molecular layers of protoplanetary disks
观测表明，某些原行星盘温暖分子层中的碳和氧元素丰度与星际介质相比减少了约10-100倍。与此同时，人们对氮知之甚少。为了研究磁盘中氮，碳，氧元素丰度的时间演化，我们开发了一个一维模型，其中包括尘埃沉降，尘埃和冰的湍流扩散，以及气体-冰化学，包括恒星UV/X射线和银河系宇宙射线驱动的化学。我们发现，在温暖的分子层中的气态CO被转化为CO2冰，并通过湍流混合（即，垂直冷指效应）和恒星紫外光子驱动的冰化学。另一方面，气态N2，温暖分子层中的主要氮储存器，较少被冰化学处理，并且以其原样存在。然后，仅通过N2的垂直冷指效应发生氮耗尽。由于N2的结合能低于CO和CO2的结合能，因此氮的贫化程度小于碳和氧的贫化程度，导致氮的元素丰度高于碳和氧的元素丰度。当扩散系数的$α$参数约为0.001时，这种演变发生在100万年内，并进一步进行。因此，N2 H +/CO柱密度比随着时间的推移而增加。简要讨论了垂直输运对中冰面冰组成的影响。
