Iain R. McNab

Spectroscopy of the hydrogen molecular ion

The theory and spectroscopy of the hydrogen molecular ion in its isotopic forms H2+, HD+ and D2+ is reviewed. Theoretical treatments are directed towards the calculation of potential energy curves, vibration-rotation energies and nuclear hyperfine constants. In the Born-Oppenheimer approximation the Schrodinger equation for H2+ can be solved exactly, but further approximations must be developed to describe the coupling of electronic and nuclear motion. The lack of a centre of symmetry in HD+ creates difficulties in the theory. Radiofrequency hyperfine transitions have been measured for H2+ using quadrupole trapping and photoalignment. Ion beam methods have been used to measure vibration-rotation transitions in HD+, and attention has been paid to levels very close to the dissociation limit. Analysis of proton and deuteron nuclear hyperfine structure reveals extreme asymmetry of the electron distribution in these levels. The hyperfine interactions have been measured by radiofrequency/infrared and microwave/infrared double resonance experiments. An electronic spectrum of D2+ arising through excitation from the ground electronic state to the excited long-range state has been measured using both infrared and microwave radiation. Observation of a microwave electronic transition in H2+ has provided experimental identification of the related H;H long-range complex.

Photo double ionization spectra of CO: comparison of theory with experiment

High level ab initio calculations have been undertaken of potential energy curves of CO2+ (and for the CO neutral ground state). The accuracy of the potentials was tested by a synthesis of the available vibrationally resolved threshold photoelectrons in coincidence (TPEsCO) and time of flight, photo electron photo electron coincidence (TOF-PEPECO) spectra of CO2+. Good agreement was found between experimental and theoretical spectra once relative energies of the calculated double ionization energies were slightly adjusted (by approximately 1%) to match experiment. Vibrational separations within individual electronic states are very well reproduced (the worst error is 0.07%).

The infrared spectrum of HCl2+ and its isotopomers

Abstract We report ab initio calculations of potential energy and dipole functions for the lowest four electronic states of the doubly charged molecule (molecular dication) HCl 2+ . Spectroscopic constants are evaluated for the three states which support quasibound vibration-rotation levels, as are the lifetimes of these levels due to both spin-orbit mediated electronic predissociation and tunnelling. The prevalent model of bonding in doubly charged molecules is that of Coulomb repulsion plus a bonding attraction; for HCl 2+ this model is inappropriate because the charge is not centred equally on the two nuclei except a large bond lengths. We compare calculations with experimental data and discuss the feasibility of observing infrared spectra of HCl 2+ isotopomers in a fast ion beam/laser beam spectrometer.

AN INFRARED SPECTRUM OF THE MOLECULAR DICATION (DOUBLY POSITIVELY CHARGED MOLECULE), D35CL2+

We have observed an infrared spectrum within the X 3Σ− state of DCl2+ using a fast-ion-beam/laser-beam spectrometer. A preliminary analysis shows good agreement with the rotational constants and tunneling lifetimes calculated by Bennett and McNab for the ν=1–2 vibrational band, although the calculated band origin appear to be in error by 21.1 cm−1.

Interpreting vibrationally resolved spectra of molecular dications (doubly positively charged molecules): HCl2+

Vibrationally resolved spectra of HCl2+ appear to show five vibrational levels for the X3Σ− ground electronic state, whereas calculations of vibrational levels supported by ab initio potential energy curves have been able to locate only three vibrational levels below the barrier; this discrepancy is resolved by considering vibrational states that the potential function supports in the continuum above the barrier maximum. A low resolution spectrum produced from first principles is compared with a spectrum obtained with threshold photoelectrons in coincidence (TPEsCO) spectroscopy, with agreement sufficient to suggest that care must be taken in the inversion of vibrational spectroscopic data for molecular dications to avoid generating potential functions that are too strongly bound.

High resolution spectroscopy and structure of molecular dications

The structure and reactivity of molecular dications has been the subject of increasing attention from theoreticians and experimentalists. We consider vibrationally resolved spectra of molecular dications, with particular emphasis on the interpretation of the observed spectral lines, before reviewing in detail all known high resolution spectra of molecular dications. We give a progress report on the state of our own calculations and ion beam measurements of the molecular dications DCl2+ and NO2+.

TPEsCO spectra of HCl2+ and DCl2+: experiment and theory

Abstract Vibrationally resolved threshold photoelectrons in coincidence (TPEsCO) spectra of the X 3 Σ − and a 1 Δ states of HCl2+ and DCl2+ are compared with ab initio simulations. The four resonances observed in the TPEsCO spectra of both the X 3 Σ − and a 1 Δ states of HCl2+ are assigned as three quasi-bound vibrational levels and one continuum resonance with an energy greater than the potential barrier maximum. The four clearly identified resonances observed in the TPEsCO spectra of both the X 3 Σ − and a 1 Δ states of DCl2+ are assigned as quasi-bound vibrational levels.

Theoretical investigation of the SO2+ dication and the photo-double ionization spectrum of SO

Highly correlated ab initio methods were used in order to generate the potential energy curves of the electronic states of the SO(2+) dication and of the electronic ground state of the neutral SO molecule. These curves were used to predict the spectroscopic properties of this dication and to perform forward calculations of the double photoionization spectrum of SO. In light of spin-orbit calculations, the metastability of this doubly charged ion is discussed: for instance, the rovibrational levels of the X (1)Sigma(+) and A (3)Sigma(+) states are found to present relatively long lifetimes. In contrast, the other electronic excited states should predissociate to form S(+) and O(+) in their electronic ground states. The simulated spectrum shows structures due to transitions between the v=0 vibrational level of SO (X (3)Sigma(-)) and the vibrational levels below the barrier maximum of 11 of the calculated electronic states. The 2 (1)Sigma(+) electronic state of SO(2+) received further treatment: in addition to vibrational bands due to the below barrier energy levels of this electronic state, at least nine continuum resonances were predicted which are responsible for the special shape of the spectrum in this energy region. This work is predictive in nature and should stimulate future experimental investigations dealing with this dication.

Spectroscopy of HD+ in high angular momentum states

Abstract We present measurements and calculations of infrared transitions between highly rotationally excited states of HD+. Some of the states involved are rotationally quasibound levels lying up to 2300 cm−1 above the lowest dissociation limit, H++D(1s). The transitions to quasibound levels were located using new ab initio artificial channels scattering theory calculations, and recorded by monitoring the fragments produced by rotational predissociation through the centrifugal barrier. The bound to bound transitions were recorded by monitoring fragments produced by photodissociation. Experimental measurements of absolute transition wavenumbers are found to be in excellent agreement with the theoretical predictions. Predissociation linewidths were also measured and are compared with theoretical calculations.

Energy shifts and forbidden transitions in H+2 due to electronic g/u symmetry breaking

A full account is given of calculations and measurements of transition frequencies and intensities of the forbidden pure rotation transition (v = 19, N = 1)-(v = 19, N = 0) in the ground electronic state (1sσg) of H+ 2. The transition has measurable intensity because of ortho-paru mixing that arises from electronic g/u symmetry breaking caused by the Fermi contact hyper-fine interaction. Measurements of the transition were made in both single and double resonance using a fast ion beam/microwave spectrometer. The transition frequency was determined to be at 14961.7 ± 1.1 MHz (95% confidence, 5 measurements), in excellent agreement with the theoretical prediction of 14960 ± 3 MHz. The intensity of the transition relative to the allowed 1sσg (v = 19, N = 1)-2pσu,(v = 0, N = 2) transition was estimated from the available measurements to be 8000, in reasonable agreement with the theoretically predicted value of ≊3000.