Isotope effect in dissociative electron attachment to acetylene

Absolute partial cross sections for dissociative electron attachment to H - C C - H and D - C C - D have been measured using a time-of-flight ion spectrometer combined with a trochoidal electron monochromator. For C2H2 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 C2D-/C2D2 being 14.4 times smaller than that for C2H-/C2H2. The dominant fragments at the second dissociative attachment band around 8 eV are the light fragments H- or D-. These bands exhibit weaker isotope effects, in agreement with their assignment to Feshbach resonances with narrow autodetachment widths.