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

We report the first spectroscopic observation of the HeN+ molecular ion. We use a technique in which a mass-selected ion beam is exposed to tunable microwave radiation, and transition involving the weakly bound long-range levels are detected by means of state-selective electric field dissociation, producing N+ fragment ions. Three of the observed microwave transitions exhibit 14N hyperfine structure, whilst a fourth transition consists of a single unsplit line. Preliminary observations of Zeeman effects are also described. The results are discussed in terms of molecular energy levels lying close to one or other of the three He(1S) + N+ (3P) dissociation asymptotes, which are split by spin-orbit coupling.