Absolute cross sections for dissociative electron attachment to acetylene and diacetylene

Absolute cross sections for the production of the two astronomy-relevant negative ions HCC⁻ and HCCCC⁻ by dissociative electron attachment to acetylene C₂H₂ and diacetylene C₄H₂ were measured (with a ±25% error bar). Acetylene yielded the C₂H⁻ ion at an electron energy of 2.95 eV with a cross section of 3.6±0.9 pm² and also the C₂⁻ ion at 8.1 eV with a cross section of 4.1±1 pm². Diacetylene yielded the C₄H⁻ ion at 2.5 eV with a cross section of 3.0±0.8 pm² and at 5.25 eVwith a cross section of 73±17 pm². Weaker C₄⁻, C₂H⁻, and C₂⁻ signals were also observed from diacetylene. The identity of the negative ion resonances mediating the dissociation and the consequences for the production of these ions in discharges are discussed. An alternate path for C₄H⁻ formation, from the O⁻-C₄H₂ ion-molecule reaction, was also observed.