Microwave spectroscopy of near-dissociation molecular ions

Abstract

Microwave spectroscopy has been used in conjunction with the technique of electric field dissociation to study the near-dissociation energy levels of two molecular ions: Ne2^ and Ara^. The analyses of these spectra have provided a significant challenge for the methods of theoretical chemistry, and furthered the understanding of long-range intermolecular forces. 278 microwave transitions have been recorded in Nea^ and 73 % of these have been assigned using a combination of double resonance experiments and Zeeman studies. Thus, an experimental energy level diagram of 102 levels has been formed, all of which lie within 10.3 cm ' of the first dissociation limit and have values of J < 21/2. From the measured g-factors, it may be concluded that the levels are best described by Hund's case (c) coupling at low J, but as J increases a transition to case (e) is observed. Coupled-channel calculations have been used to describe the experimentally observed energy levels and have also provided predictions for many of the remaining unassigned transitions. 69 transitions have been observed in Ari^, and whilst nine connectivities have been confirmed, it has not been possible to utilise the Zeeman effect and so none of the transitions have been assigned with certainty. It is shown that it is highly probable that the Ari"^ ions have been formed by the breakdown of larger clusters, leading to high rotational temperatures and thus preventing resolved Zeeman spectra to be recorded. One Zeeman spectrum has been simulated and is assigned as J = 27/2 to J =

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