Christine A. Montgomerie

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.

Microwave electronic spectrum of the H2+ ion

Abstract We report the observation of a microwave transition in the H 2 + ion, arising from an electronic transition between the highest bound vibration-rotation level of the 1 sσ g ground state, with v = 19, N = 1, and the v = 0, N = 2 level of the 2pσ u long-range van der Waals state.

Electronic spectrum (2pσ u -1sσ g ) of the D+ 2 ion

We describe the first measurements of an electronic spectrum of the hydrogen molecular ion, in its perdeutero form, D+ 2. The lower states involved in the spectrum are high-lying vibration-rotation levels of the 1sσ g ground state, and the upper states are the vibration-rotation levels of the 2pσ u long-range van der Waals state. Most of the observed spectral lines, which involve ν = 21 in the ground state, are in the infrared region spanned by the carbon dioxide laser, but two microwave electronic transitions, involving ν = 26 and 27, have also been observed. We use a D+ 2 ion beam, in which the high-lying vibrationrotation levels of the ground state are populated by the initial electron impact ionization process. The infrared transitions are induced by Doppler tuning the ion beam into resonance with an appropriate laser line, and excitation is detected by electric field dissociation of the upper levels, the resulting D+ fragments being separated from other fragment ions and the parent ions with a magnet...

Microwave spectra of the HD+ and D2+ ions at their dissociation limits

We have observed microwave transitions in the HD+ and D2 + ions involving energy levels very close to their dissociation limits. In HD+ the spectrum detected arises from a rotational transition between the two highest bound vibration-rotation levels in the 1sσ ground state, whilst in D2 + we have detected electronic transitions between high-lying vibration-rotation levels of the ground 1sσg electronic state and the 2pσu long-range van der Waals state.

Laser excitation and electric field dissociation spectroscopy of the HD+ ion

Abstract We describe a new technique for detecting the infrared spectra of molecular ions in levels close to dissociation. Laser excitation of an ion beam is followed by passage through a strong electric field, which selectively dissociates high-lying levels. For the 21,2-17,3 transition of HD + we obtain four orders-of-magnitude increase in sensitivity compared with our earlier method based upon photodissociation. This improvement arises from the efficiency of electric field dissociation, and the separation of protons produced by field dissociation from other fragment ions. The high sensitivity enables us to improve the spectral resolution, revealing further hyperfine interactions.

Spectroscopic observation of the highest vibrational level of the HD+(2Σ+) molecule

Spectroscopic observation of the highest vibrational level of HD+(v=21) in its ground electronic state is reported. Calculations of constants from the resolved hyperfine structure allow unambiguous determination of the Fermi contact parameters for the levels 17,1 and 21,0, showing that in the level 21,0 the electron distribution is asymmetric toward the lower dissociation limit H++D, with an average electron density of 0.094 at the proton and 0.902 at the deuteron.

Spectroscopic observation of HD+ in the ν = 22 level

We have observed three rotational components of the 22-17 band of the HD+ ion. The dissociation energies of 22, 0 and 22, 1 are found to be 0·438 and 0·123 cm-1. These levels are almost certainly the highest bound vibration-rotation levels of the HD+ ground state. Analysis of the nuclear hyperfine structure shows that the electron is virtually localized at the deuterium nucleus.

Spectroscopic observation of rotationally quasibound HD+ ions

Abstract The bound to rotationally quasibound 16,17-13,18 vibration-rotation transition of HD + is described, and the quasibound state, which correlates with the lowest dissociation limit, is shown to predissociate exclusively to protons. Saturation effects which lead to reversed phase lines are noted.

Radiofrequency/infrared double resonance spectroscopy of the HD+ ion

We describe a double resonance technique for obtaining radiofrequency spectra of the HD+ ion in vibration-rotation levels close to the dissociation limit. Infrared transitions are driven by Doppler tuning an HD+ ion beam into resonance with a carbon dioxide infrared laser, and are detected by measuring H+ fragment ions produced by electric field dissociation of the upper vibration-rotation level. Radiofrequency transitions between nuclear hyperfine components of the lower vibration-rotation level are then detected through resonant increases in the H+ fragment ion current. The high spectroscopic resolution obtained, and the ability to measure magnetic dipole hyperfine transitions, will enable the hyperfine constants to be determined accurately.

The Infrared Spectroscopy of the H3+ and HD+ Molecular Ions at their Dissociation Limits

Previous theoretical and experimental work on the ions H3 + and HD+ is reviewed. Earlier work using ion beam techniques to study the uppermost bound levels and quasibound levels of H3 + is discussed, and a suitable model for the molecule in these levels is proposed. Earlier work using ion beam techniques to study the vibration-rotation levels of HD+ is reviewed.