We have observed and assigned seven microwave and millimeter wave lines arising from rovibronic components of the A 2 ⌺ g ϩ ←X 2 ⌺ u ϩ electronic spectrum of the He 2 ϩ ion; this is the first observation of any spectrum of the homonuclear 4 He 4 He ϩ species. The vibration-rotation levels involved in our observations all lie within 8 cm Ϫ1 of the lowest degenerate He͑ 1 S͒ϩHe ϩ ͑ 2 S͒ 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 ϩ 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 ϩ 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 ϩ 2 ⌺ g ϩ charge/induced-dipole state.

We have observed and measured seven microwave transitions in D2+, which are vibration–rotation components of the 2pσu–1sσg electronic band system. We also report two microwave rotational components involving the 1sσ v=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.

Articles you may be interested inOne and twocolor resonanceenhanced multiphoton ionization spectroscopy of the KrNO complex via the à 2Σ+ state New measurements of the a 3Σ+ u state of K2 and improved analysis of longrange dispersion and exchange interactions between two K atoms We have observed a microwave spectrum of the HeKr ϩ ion in which all of the observed levels lie within a few cm Ϫ1 of either the first or second dissociation limit. We use an ion beam technique in which HeKr ϩ ions, formed by electron impact, are mass analyzed. Passage of the ion beam through an electric field lens results in selective fragmentation of energy levels lying close to dissociation. Kr ϩ ions formed in the lens are separated from all other ions by means of an electrostatic analyzer, and are detected with an electron multiplier. Microwave radiation induces transitions which result in population transfer and produce detected changes in the electric field-induced Kr ϩ fragment ion current. Additional transitions have been detected by a microwave-microwave double resonance method, and we have also made extensive use of the Zeeman effects produced by small applied coaxial magnetic fields to identify the J quantum numbers of the levels involved. Coupled channel calculations of the bound states of the He•••Kr ϩ ion are carried out, fully including all the couplings between different electronic states correlating with HeϩKr ϩ ͑ 2 P 3/2 and 2 P 1/2 ͒. The calculations allow the spectra to be assigned to pure rotational transitions involving levels in the X, A 1 , and A 2 states that lie within 2.5 cm Ϫ1 of the dissociation limits. Because of a systematic near degeneracy between vibrational levels in the X and A 1 states, the long-range He•••Kr ϩ ion provides a very good example of Hund's case ͑e͒ in the form introduced by Mulliken, in which there are no projection quantum numbers onto the interatomic axis. Mulliken's case ͑e͒ is rather different from the Rydberg case ͑e͒ described by Lefebvre-Brion, and this is the first time that Mulliken's case ͑e͒ has been observed. The spectra allow the interaction potential for He•••Kr ϩ to be determined accurately, for the first time, by least-squares fitting of potential parameters to the experimental line frequencies and g factors. The resulting interaction potential ͑designated MAL1͒ is compared with that previously determined for He•••Ar ϩ : the He•••Kr ϩ potential is significantly shallower, because the long-range ion-induced dipole C 4 coefficient is the same for the two systems but the larger Kr ϩ ion prevents the He atom approaching as close.

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