We have measured absolute dissociative electron attachment cross sections for H and D (=Cl and Br) using a trochoidal electron spectrometer equipped with a total ion collection collision chamber, operated both in a field-free passive ion collection mode, and an active ion collection mode, with an extracting electric field. Our results for Br⁻ from HBr are in an excellent quantitative agreement with recent predictions of the nonlocal resonance model, but the cross sections for Cl⁻ from HCl and DCl are, respectively, 2.2 times and 2.8 times smaller than the theoretical predictions. Cross sections for the production of H⁻ and D⁻ in the 6–10 eV range are much smaller than those for ⁻ and the isotope effect is much weaker, consistent with the assignment to core-excited Feshbach resonances. There is one exception, however, the 9 eV band in HCl has a large isotope effect.
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