Microwave spectra of the D₂⁺ and HD⁺ ions near their dissociation limits

We have observed and measured seven microwave transitions in D ₂⁺, which are vibration-rotation components of the 2pσ_u-1sσ_g electronic band system. We also report two microwave rotational components involving the 1sσ u=21 level of HD⁺, which lies close to the first dissociation limit. Our experiments use ion beam techniques in which state selection is achieved by electric field dissociation. The techniques and theory of electric field dissociation are discussed, and fragment ion kinetic energy spectra described. Ab initio calculations of the transition frequencies for both D₂ ⁺ and HD⁺ are in excellent agreement with experiment, and the hyperfine structure of the rotational transitions in HD⁺ confirms previous demonstrations of the increasing asymmetry in the electron distribution as the dissociation limit is approached. The electronic transition in D₂⁺ which involves the highest bound level of the ground state shows an unexpected hyperfine splitting, which is interpreted in terms of g/u symmetry breaking by the deuteron Fermi contact interaction.