Cosmic anions in the laboratory

The molecular diversity of cold interstellar space has been recently enriched with the detection of anions, C4H−, C4H−, C8H−, C3N−, C5N−, and CN−, all linear carbon chains (1-7). The circumstellar envelope of the evolved carbon star IRC +10216 is the only object so far in which all of them have been identified. The anion to neutral abundance ratio derived from astronomical observations of this source ranges from 10-4 to 0.5 depending on the species (8). Electron attachment onto neutral counterparts is considered as the main formation mechanism of medium-size and large anions whereas the small CN− anion could be the product of Cx− + N reaction. However, only a few chemical pathways leading to the formation or to the destruction of anions have been investigated so far. Experimental studies of the kinetics, products and branching ratios of reactions involving these species are required to assess precisely the production and destruction chemical routes. In our laboratory, we have explored the reaction of CN− and C3N− with cyanoacetylene HC3N over the 50-300 K temperature range in uniform supersonic flows using the CRESU (French acronym standing for Reaction Kinetics in Uniform Supersonic Flow) technique. Cyanopolyynes such as HC3N are abundant in circumstellar envelopes of carbon rich stars, peaking at 1 ppm. The results show that the CN− + HC3N reaction contributes directly to the growth of larger anions (9) whereas C3N- + HC3N does not. The investigation is currently extended in the laboratory to other anions through the synthesis of adapted molecular precursors. The development of a versatile selected anion source, which will be combined with the CRESU apparatus, is also presented. Acknowledgments This work is supported by the French ANR (project ANION COS CHEM), the CNRS-INSU Programme de Chimie du Milieu Interstellaire and the CNRS-INSU Programme National de Planetologie. References 1.M. C. McCarthy, C. A. Gottlieb, H. Gupta, P. Thaddeus, Laboratory and astronomical identification of the negative molecular ion C6H-. Astrophys. J. 652, L141 (Dec 1, 2006). 2.J. Cernicharo, M. Guelin, M. Agundez, K. Kawaguchi, M. McCarthy, P. Thaddeus, Astronomical detection of C4H-, the second interstellar anion. Astron. Astrophys. 467, L37 (May, 2007). 3.S. Brunken, H. Gupta, C. A. Gottlieb, M. C. McCarthy, P. Thaddeus, Detection of the carbon chain negative ion C8H- in TMC-1. Astrophys. J. 664, L43 (Jul, 2007). 4.P. Thaddeus, C. A. Gottlieb, H. Gupta, S. Bruenken, M. C. McCarthy, M. Agundez, M. Guelin, J. Cernicharo, Laboratory and astronomical detection of the negative molecular ion C3N-. Astrophys. J. 677, 1132 (Apr 20, 2008). 5.J. Cernicharo, M. Guelin, M. Agundez, M. C. McCarthy, P. Thaddeus, Detection of C5N− and Vibrationally Excited C6H in IRC +10216. Astrophys. J. Lett. 688, L83 (2008). 6.M. Agundez, J. Cernicharo, M. Guelin, C. Kahane, E. Roueff, J. Klos, F. J. Aoiz, F. Lique, N. Marcelino, J. R. Goicoechea, M. Gonzalez Garcia, C. A. Gottlieb, M. C. McCarthy, P. Thaddeus, Astronomical identification of CN-, the smallest observed molecular anion. Astron. Astrophys. 517, L2 (2010). 7.A. J. Remijan, J. M. Hollis, F. J. Lovas, M. A. Cordiner, T. J. Millar, A. J. Markwick-Kemper, P. R. Jewell, Detection of C8H- and comparison with C8H toward IRC+10 216. Astrophys. J. 664, L47 (Jul 20, 2007). 8.M. Agundez, J. Cernicharo, M. Guelin, The chemistry of molecular anions in circumstellar sources. AIP Conference Proceedings 1642, 362 (2015). 9.L. Biennier, S. Carles, D. Cordier, J.-C. Guillemin, S. D. Le Picard, A. Faure, Low temperature reaction kinetics of CN- + HC3N and implications for the growth of anions in Titan’s atmosphere. Icarus 227, 123 (2014).