Isotope effect in dissociative electron attachment to acetylene

Abstract

Absolute partial cross sections for dissociative electron attachment to $\text{H}--\text{C}\ensuremath{\equiv}\text{C}--\text{H}$ and $\text{D}--\text{C}\ensuremath{\equiv}\text{C}--\text{D}$ have been measured using a time-of-flight ion spectrometer combined with a trochoidal electron monochromator. For ${\text{C}}_{2}{\text{H}}_{2}$ the present data are in good agreement with the total cross sections measured previously in our laboratory using the total ion-collection method. The present measurements on deuterated acetylene revealed a pronounced isotope effect at the first dissociative electron attachment band at 3 eV, with the cross section for ${\text{C}}_{2}{\text{D}}^{\ensuremath{-}}/{\text{C}}_{2}{\text{D}}_{2}$ being 14.4 times smaller than that for ${\text{C}}_{2}{\text{H}}^{\ensuremath{-}}/{\text{C}}_{2}{\text{H}}_{2}$. The dominant fragments at the second dissociative attachment band around 8 eV are the light fragments ${\text{H}}^{\ensuremath{-}}$ or ${\text{D}}^{\ensuremath{-}}$. These bands exhibit weaker isotope effects, in agreement with their assignment to Feshbach resonances with narrow autodetachment widths.