Microwave electronic spectrum of the He+2 ion

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Abstract
We have observed and assigned seven microwave and millimeter wave lines arising from rovibronic components of the A 2Σ+g←X 2Σ+u electronic spectrum of the He+2 ion; this is the first observation of any spectrum of the homonuclear 4He4He+ species. The vibration‐rotation levels involved in our observations all lie within 8 cm−1 of the lowest degenerate He(1S)+He+(2S) dissociation limits for both electronic states. We use an ion beam technique in which weakly bound levels dissociate in an applied electric field to produce He+ fragments. These fragments are separated from all other ions with an electrostatic kinetic energy analyzer, and microwave transitions are detected as changes in the He+ fragment current arising from resonant population transfer. Four of the transitions are detected using a single microwave frequency; the remaining three are measured by means of a microwave–microwave double resonance method. The assignment of the spectrum is achieved by means of ab initio electronic structure calculations, made within the Born–Oppenheimer approximation. The agreement between experiment and theory is excellent and leads to an accurate characterization of the He...He+ 2Σ+g charge/induced‐dipole state.