Origin of the low-energy component and isotope effect on dissociative recombinations ofHeH+andHeD+

Dissociative recombination of ${\mathrm{HeD}}^{+}$ was studied using a storage ring and compared with the results on ${\mathrm{HeH}}^{+}$. A strong peak at around 20-eV electron energy was observed in addition to a peak at nearly zero energy. Their shapes are very similar to corresponding ones on ${\mathrm{HeH}}^{+}$. The relative strengths of these peaks, however, are quite different between ${\mathrm{HeH}}^{+}$ and ${\mathrm{HeD}}^{+}$, which is clear evidence of isotope effect. By mixing ${\mathrm{O}}_2$ gas into the helium hydride ion source, it was confirmed that the peaks originate from the ground state of molecular ions. The dissociation energy for the low-energy peak was about 1 eV. This implies that the dissociation of the low-energy component occurs primarily to He+${\mathrm{H}}^{\mathrm{*}}$(n=2), although the possibility of some branching to n=1 cannot be excluded.