The structure and spectra of molecular ions

Abstract

I give a review of the theory of the hydrogen molecular ion H2+ and its isotopomers D2+ and HD+ including the direct analytical solution and the standard adiabatic approximation. I discuss dissociation limits for homonuclear and heteronuclear species; the effect of an external electric field; non adiabatic calculations, relativistic and radiative effects; and spectroscopic measurements of H2+ and D2+ with a comparison of theoretical to experimental values. I give a detailed description of the fast ion-beam spectrometer as used for both laser-beam and microwave spectroscopy and describe the challenges involved in making high resolution spectroscopic measurements. An account is given of theory, experimental details and measurements of transition frequencies and intensities of the forbidden rotational transition (v = 19, N = 1) - (v = 19, N = 0) in the ground electronic state (X2Σg+, also represented as 1sσg) of H2+. Theory has predicted that the transition has measurable intensity due to the Fermi contact hyperfine interaction causing a breaking of electronic g/u symmetry resulting in the mixing of ortho-para states. The measurements were made in both single and double resonance using a fast ion beam/microwave spectrometer at a transition frequency of 14961.7 ± 1.1 MHz, in agreement with the theoretical prediction of 14960 ± 3 MHz. An account is also given of a further search that was conducted for a second forbidden rotational transition (v = 0, N = 1) - (v = 0, N = 0) in the first excited electronic state (2pσu) of H2+. A discussion is given on the possibility of making further observations of forbidden rotational transitions and the experimental difficulties involved; and of adapting the experimental techniques used in order to observe the recently discovered (v = 1, N = 0) in the first excited electronic state (2pσu) of H2+.