Negative Ions in Space

Abstract

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.

Formation of Anionic C, N-bearing Chains in the Interstellar Medium via Reactions of H− with HCxN for Odd-valued x from 1 to 7

“…Such finding is in line with the fact that, thus far,the C 7 N − anionic molecule has not been observed although HC 7 N has been observed: Millar et al (2017). on the other hand, both C 3 N − and C 5 N − have been detected by ; Herbst (1981).…”

“…(iv) the C 7 N − formation rates turn out to be smaller than the preciding shorter members of the series, thus suggesting this chemical "inefficiency" as a possible cause for the lack of its observational detection as discussed in Millar et al (2017).…”

Interatomic Coulombic electron capture from first principles

“…Such electron capture processes play a central role in plasma physics [5], astrophysics [6], environmental science [7] and radiation damage [8][9][10]. It was recently predicted that electron attachment to a species can be strongly enhanced in the presence of a chemical environment through the so called Interatomic Coulombic Electron Capture (ICEC) mechanism [11,12].…”

The Fundamental Vibrational Frequencies and Spectroscopic Constants of the Dicyanoamine Anion, NCNCN− (C2N3 −): Quantum Chemical Analysis for Astrophysical and Planetary Environments

“…The chemical reservoir of negative ions in the universe encompasses a broad range of astrophysical and astrochemical environments: from cold interstellar clouds, circumstellar envelopes and carbon-rich Asymptotic Giant Branch (AGB) stars, to cometary comae and planetary atmospheres. The combined theoretical and experimental prospect for their spectroscopic characterization has resulted in unprecedented observations of key cosmic anions in the aforementioned environments (see review by Millar et al (2017) and references therein).…”