Observation of a microwave spectrum of the long-range He … H2+ complex

We report the first spectroscopic observations of the HeH2+ molecular ion. We use an ion beam technique in which mass-selected ions are exposed to microwave radiation, and transitions involving weakly bound long-range levels are detected by means of state-selective electric field dissociation. One of the observed transitions exhibits complex proton nuclear hyperfine structure; another shows a simple field-free spectrum but extensive Zeeman structure in the presence of an applied magnetic field. The results are discussed in terms of coupled-channel calculations using a recent ab initio potential surface.