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The collision-energy resolved rate coefficient for dissociative recombinationof HD+ ions in the vibrational ground state is measured using the photocathodeelectron target at the heavy-ion storage ring TSR. Rydberg resonancesassociated with ro-vibrational excitation of the HD+ core are scanned as afunction of the electron collision energy with an instrumental broadening below1 meV in the low-energy limit. The measurement is compared to calculationsusing multichannel quantum defect theory, accounting for rotational structureand interactions and considering the six lowest rotational energy levels asinitial ionic states. Using thermal equilibrium level populations at 300 K toapproximate the experimental conditions, close correspondence betweencalculated and measured structures is found up to the first vibrationalexcitation threshold of the cations near 0.24 eV. Detailed assignments,including naturally broadened and overlapping Rydberg resonances, are performedfor all structures up to 0.024 eV. Resonances from purely rotational excitationof the ion core are found to have similar strengths as those involvingvibrational excitation. A dominant low-energy resonance is assigned tocontributions from excited rotational states only. The results indicate strongmodifications in the energy dependence of the dissociative recombination ratecoefficient through the rotational excitation of the parent ions, and underlinethe need for studies with rotationally cold species to obtain resultsreflecting low-temperature ionized media.

Assignment of resonances in dissociative recombination of HD+ ions: high-resolution measurements compared with accurate computations