Absolute cross sections for dissociative electron attachment to HCl, HBr, and their deuterated analogs

We have measured absolute dissociative electron attachment cross sections for HX and DX (X=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--100.3em0exeV range are much smaller than those for X^- and the isotope effect is much weaker, consistent with the assignment to core-excited Feshbach resonances. There is one exception, however, the 90.3em0exeV band in HCl has a large isotope effect.