The Faraday Lecture: spectroscopy of molecular ions at their dissociation limits

Abstract

Experimental and theoretical studies of the vibration–rotation spectra of HD⁺, HeH⁺ and H⁺ ₃ are described. The ion beam techniques used are especially sensitive for molecules in vibration–rotation levels lying close to dissociation limits, and in each case spectra are recorded by monitoring the fragment ions produced by infrared photodissociation. In HD⁺ vibration–rotation transitions between bound levels are measured, and the experimentally determined frequencies are compared with accurate ab initio calculations. The highest-lying vibrational level probed by the experiments is ν= 20, and the observed proton hyperfine splitting proves unambiguously that the electron distribution is polarised in the sense H⁺+ D, the preferred (lowest-energy) mode of dissociation. This observation shows that the Born–Oppenheimer approximation breaks down as the dissociation limit is approached. For HeH⁺ the observed spectrum arises from vibration–rotation transitions between bound and rotationally quasibound levels, the latter lying above the H⁺+ He dissociation limit. The widths of the observed lines yield values for the lifetimes of the quasibound states; experimental and theoretical values for the line frequencies and widths are in good agreement. H⁺ ₃ exhibits an extensive infrared spectrum in the region 870–1090 cm^–1 in which each line corresponds to predissociation to yield H₂+ H⁺. Measurements of the proton kinetic energy release prove that many resonance lines arise from transitions between pairs of levels, both of which lie at least 2000 cm^–1 above the lowest dissociation limit. The results are discussed in terms of the structure and dynamics of the H₂(ν, j) H⁺ complex, and the importance of rotational angular momentum of the complex is described.