ION-BEAM SPECTROSCOPY OF LONG-RANGE COMPLEXES

Ion-beam techniques provide opportunities for extremely sensitive spec troscopic 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 studi es of a range of molecular ions, mostly diatomic; we compare the diffe rent detection methods and the molecular information each provides. Th ese investigations have revealed the first electronic spectra of the H -2(+) and D-2(+) ions, leading to detailed characterisations of the lo ng-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 ena bled the nuclear hyperfine interactions to be measured; these revealed marked asymmetry in the electron distribution as the dissociation asy mptote is approached. H-3(+), the only polyatomic molecule to be inves tigated in our laboratory so far, exhibits an extraordinarily rich and complex if predissociation spectrum, which arises from metastable per iodic states embedded in a chaotic sea of levels lying above the lowes t dissociation limit. Other examples of predissociation spectra involv ing rotationally quasibound levels are described. In recent work the f irst 37 bound levels of the long-range He ... Ar+ complex, counting fr om the dissociation limit downwards, have been identified and describe d 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 ne w experiments using a combined neutral-ion beam source which has enabl ed the first state-to-state resonant spectra of the Ar-2(+) and Ne-2() molecular ions to be observed.