Absolute cross sections for dissociative electron attachment to acetylene and diacetylene

Abstract

Absolute cross sections for the production of the two astronomy-relevant negative ions $\mathrm{H}\mathrm{C}{\mathrm{C}}^{\ensuremath{-}}$ and $\mathrm{H}\mathrm{C}\mathrm{C}\mathrm{C}{\mathrm{C}}^{\ensuremath{-}}$ by dissociative electron attachment to acetylene ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ and diacetylene ${\mathrm{C}}_{4}{\mathrm{H}}_{2}$ were measured (with a $\ifmmode\pm\else\textpm\fi{}25%$ error bar). Acetylene yielded the ${\mathrm{C}}_{2}{\mathrm{H}}^{\ensuremath{-}}$ ion at an electron energy of $2.95\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with a cross section of $3.6\ifmmode\pm\else\textpm\fi{}0.9\phantom{\rule{0.3em}{0ex}}{\mathrm{pm}}^{2}$ and also the ${\mathrm{C}}_{2}^{\ensuremath{-}}$ ion at $8.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with a cross section of $4.1\ifmmode\pm\else\textpm\fi{}1\phantom{\rule{0.3em}{0ex}}{\mathrm{pm}}^{2}$. Diacetylene yielded the ${\mathrm{C}}_{4}{\mathrm{H}}^{\ensuremath{-}}$ ion at $2.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with a cross section of $3.0\ifmmode\pm\else\textpm\fi{}0.8\phantom{\rule{0.3em}{0ex}}{\mathrm{pm}}^{2}$ and at $5.25\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$with a cross section of $73\ifmmode\pm\else\textpm\fi{}17\phantom{\rule{0.3em}{0ex}}{\mathrm{pm}}^{2}$. Weaker ${\mathrm{C}}_{4}^{\ensuremath{-}}$, ${\mathrm{C}}_{2}{\mathrm{H}}^{\ensuremath{-}}$, and ${\mathrm{C}}_{2}^{\ensuremath{-}}$ 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 ${\mathrm{C}}_{4}{\mathrm{H}}^{\ensuremath{-}}$ formation, from the ${\mathrm{O}}^{\ensuremath{-}}\text{\ensuremath{-}}{\mathrm{C}}_{4}{\mathrm{H}}_{2}$ ion-molecule reaction, was also observed.