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 pm(2) and also the C-2(-) ion at 8.1 eV with a cross section of 4.1 +/- 1 pm(2). Diacetylene yielded the C4H- ion at 2.5 eV with a cross section of 3.0 +/- 0.8 pm(2) and at 5.25 eVwith a cross section of 73 +/- 17 pm(2). Weaker C-4(-), C2H-, and C-2(-) 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.