Microwave spectroscopy and interaction potential of the long-range He...Ar+ ion

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Abstract
We have measured and interpreted a microwave spectrum of the HeAr+ ion in which all of the observed energy levels lie within 8 cm−1 of the lowest dissociation limit, He(1S)+Ar+(2P3/2). We use an ion beam technique in which the HeAr+ ions are formed by electron impact, accelerated to kilovolt potentials, and mass‐analyzed. After passage through an appropriate section of waveguide, the ions enter an electric field lens in which state‐selective fragmentation occurs; the Ar+ ions produced in the lens are separated from all other ions by means of an electrostatic analyser and detected with an electron multiplier. Microwave transitions induced in the waveguide section result in population transfer which produces detected changes in the electric field‐induced Ar+ fragment current. Many transitions have also been observed by a microwave–microwave double resonance technique. We have observed 68 lines spanning the frequency range 6–170 GHz; no immediately recognizable pattern is apparent. We have measured the Zeeman splitting produced by a small axial magnetic field for almost every line, which enables us to determine the values of the total angular momentum J involved in each transition, and also effective g factors for the two levels involved. We are therefore able to construct a purely experimental pattern of 37 levels lying within 8 cm−1 of the dissociation limit. The data are treated first by means of a conventional effective Hamiltonian in a case (c) basis, which allows electronic and vibrational quantum numbers to be assigned to most of the levels; the assignments are approximate, however, because very strong rotational‐electronic coupling undermines the Born–Oppenheimer approximation. A more complete theoretical treatment is then presented, using the coupled‐channel method in a case (e) representation to calculate the energy levels without making the Born–Oppenheimer approximation. The microwave transition frequencies and g‐factors are fitted, together with earlier ultraviolet spectra, to provide a new interaction potential (designated MAL1) for He interacting with Ar+(2P3/2 and 2P1/2). The MAL1 potential is substantially more accurate than previous potentials, especially in the long‐range region and for the A1 2Π3/2 state, which had not been observed before. An important new feature of the MAL1 potential is that the long‐range C6 coefficient is strongly anisotropic, so that the different electronic curves have substantially different C6 coefficients.

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