Negative Ions in Space

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    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 H2 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 C6H 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.


    • Negative Ions in Space

      Rights statement: Copyright © 2017 American Chemical Society. This is an open access article published under a Creative Commons Attribution License (, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.

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    Scopus record Negative Ions in Space
    Original languageEnglish
    Number of pages31
    Pages (from-to)1765-1795
    JournalChemical Reviews
    Journal publication date08 Feb 2017
    Issue number3
    Early online date23 Jan 2017
    Publication statusPublished - 08 Feb 2017

    ID: 123765399