2019ApJ...878..162B

The observed abundances of interstellar carbon chain anions are not yet fully understood in recent models of astrochemical reaction networks. The major formation route is assumed to be the chain growth of neutral carbon chains followed by the formation of anions via radiative electron attachment. Besides this, chemical reactions with ions and, in a single study, the anionic chain growth reaction C_2nH^– + C₂H₂ - C_2n+2H^– + H₂ have been proposed to influence molecular abundances. In the present work, the title reactions have been investigated experimentally by velocity map imaging in a crossed beam setup. All reactions take place at well-defined collision energies in the range from 0.3 to 3.0 eV. The dominant products are C₄H^–, C₆H^–, and C₈H^– in the reactions with n = 1, 2, 3, respectively. Preferentially at low energies, C₄^–, C₆^–, and C₈^– are also formed. The reactions of C₂H^–, C₄H^–, and C₆H^– with acetylene are slower than those with C₂^–, C₄^–, and C₆^–, respectively. Increasing collision energy and reactant ion chain length appears to enhance the reaction rates of the hydrogenated species relative to the latter ions, forming both C_2n+2H^– and C_2n+2H₂^–. These results are discussed with the aim to clarify their possible relevance in interstellar environments.