Ion-beam spectroscopy of long-range complexes

Abstract

Ion-beam techniques provide opportunities for extremely sensitive spectroscopic studies of molecular ions. Transitions involving the energy levels lying close to the dissociation limit may be detected through a range of indirect methods; electric field dissociation has proved to be particularly valuable. This review describes IR and microwave studies of a range of molecular ions, mostly diatomic; we compare the different detection methods and the molecular information each provides. These investigations have revealed the first electronic spectra of the H₂⁺ and D₂⁺ ions, leading to detailed characterisations of the long-range H⋯H⁺ and D⋯D⁺ charge–induced-dipole states, and also providing accurate data with which to test ab initio calculations. In the case of the HD⁺ ion the high spectroscopic resolution achieved enabled the nuclear hyperfine interactions to be measured; these revealed marked asymmetry in the electron distribution as the dissociation asymptote is approached. H₃⁺, the only polyatomic molecule to be investigated in our laboratory so far, exhibits an extraordinarily rich and complex IR predissociation spectrum, which arises from metastable periodic states embedded in a chaotic sea of levels lying above the lowest dissociation limit. Other examples of predissociation spectra involving rotationally quasibound levels are described. In recent work the first 37 bound levels of the long-range He⋯Ar⁺ complex, counting from the dissociation limit downwards, have been identified and described theoretically. In this very weakly bound region the Born–Oppenheimer approximation collapses because of extremely strong coupling between the electronic and nuclear rotational motions. Finally, we describe new experiments using a combined neutral–ion beam source which has enabled the first state-to-state resonant spectra of the Ar₂⁺ and Ne₂⁺ molecular ions to be observed.