Leiden Observatory, Leiden University , P.O. Box 9513, 2300 RA Leiden, The Netherlands.
School of Physics and Astronomy, University of Leeds , Leeds LS2 9JT, U.K.
Chem Rev. 2017 Feb 8;117(3):1765-1795. doi: 10.1021/acs.chemrev.6b00480. Epub 2017 Jan 23.
Until a decade ago, the only anion observed to play a prominent role in astrophysics was H. The bound-free transitions in H dominate the visible opacity in stars with photospheric temperatures less than 7000 K, including the Sun. The H anion is also believed to have been critical to the formation of molecular hydrogen in the very early evolution of the Universe. Once H formed, about 500 000 years after the Big Bang, the expanding gas was able to lose internal gravitational energy and collapse to form stellar objects and "protogalaxies", allowing the creation of heavier elements such as C, N, and O through nucleosynthesis. Although astronomers had considered some processes through which anions might form in interstellar clouds and circumstellar envelopes, including the important role that polycyclic aromatic hydrocarbons might play in this, it was the detection in 2006 of rotational line emission from CH that galvanized a systematic study of the abundance, distribution, and chemistry of anions in the interstellar medium. In 2007, the Cassini mission reported the unexpected detection of anions with mass-to-charge ratios of up to ∼10 000 in the upper atmosphere of Titan; this observation likewise instigated the study of fundamental chemical processes involving negative ions among planetary scientists. In this article, we review the observations of anions in interstellar clouds, circumstellar envelopes, Titan, and cometary comae. We then discuss a number of processes by which anions can be created and destroyed in these environments. The derivation of accurate rate coefficients for these processes is an essential input for the chemical kinetic modeling that is necessary to fully extract physics from the observational data. We discuss such models, along with their successes and failings, and finish with an outlook on the future.
直到十年前，唯一被观察到在天体物理学中起重要作用的阴离子是 H。H 的束缚-自由跃迁主导着表面温度低于 7000 K 的恒星的可见光不透明度，包括太阳。H 阴离子也被认为对宇宙早期形成分子氢至关重要。一旦 H 形成，大约在大爆炸后 500000 年，膨胀气体就能够失去内部引力能并坍缩形成恒星物体和“原星系”，从而通过核合成创造出更重的元素，如 C、N 和 O。尽管天文学家已经考虑了一些阴离子可能在星际云中形成的过程，以及多环芳烃可能在其中起的重要作用，但正是 2006 年对 CH 的旋转谱线发射的检测激发了对星际介质中阴离子的丰度、分布和化学的系统研究。2007 年，卡西尼号任务报告了在土卫六高层大气中意外检测到质量电荷比高达约 10000 的阴离子；这一观察同样促使行星科学家研究涉及负离子的基本化学过程。在本文中，我们回顾了星际云、星周包层、土卫六和彗星彗发中阴离子的观测结果。然后，我们讨论了一些可以在这些环境中形成和破坏阴离子的过程。这些过程的准确速率系数的推导对于化学动力学模型的建立是必不可少的，而这些模型对于从观测数据中提取物理信息是必要的。我们讨论了这些模型及其成功和失败，并展望了未来。
