Dissociative recombination ofH2D+: Cross sections, branching fractions, and isotope effects

We have used the CRYRING heavy-ion storage ring at the Manne Siegbahn Laboratory in Stockholm to store and cool beams of ${\mathrm{H}}_2$ ${\mathrm{D}}^{+}$. These beams circulate with energies of MeV per amu and are merged with an electron beam traveling at nearly the same velocity. In this way, relative energies from a fraction of a meV to tens of an eV are achieved. When dissociative recombination (DR) takes place, the neutral fragments exit the ring and strike a detector, enabling the measurement of total DR cross sections. Using this technique, coupled with a ‘‘translucent barrier’’ in which some particles pass directly through holes and others pass through a thin solid and lose part of their energy before striking the detector, we are able to measure the total cross section for DR in ${\mathrm{H}}_3^{+}$ and the branching into the three-body (H+H+H) channel and the two-body (${\mathrm{H}}_2$+H) channel. Coupling this method with the use of energy absorbers, we are able to measure these quantities for ${\mathrm{H}}_2$ ${\mathrm{D}}^{+}$ and the separate two-body channels HD+H and ${\mathrm{H}}_2$+D. An isotope effect is observed favoring the formation of HD.