Electronic spectrum (2pσ 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 1sσg ground state, and the upper states are the vibration-rotation levels of the 2pσu 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 magnetic analyser, and detected with an electron multiplier. The two microwave transitions were detected initially by microwave-infrared double resonance, but one of the transitions has also been observed by pure microwave spectroscopy, the phase of the signal showing that the 2pσu van der Waals levels are already populated in the ion beam before laser excitation. Theoretical calculations predict that the 2pσu long-range minimum, arising from the charge/induced-dipole interaction, supports seven vibration-rotation levels. We have detected transitions to all of them, determining the vibrational spacing and rotational constants which characterize the van der Waals state. The experimental results are compared with the predictions of Born-Oppenheimer and adiabatic calculations. The two theoretical methods give energies for the vibration-rotation levels of the 2pσu state which are in close agreement with each other, but for the ν = 21 level of the ground state the two methods differ in their predictions by almost 2 cm-1. The nonadiabatic correction to the energy of the ground state ν = 21 level is found to be close to -0·5 cm-1. The 2pσu-1sσg transition moments are also calculated for different values of the internuclear distance. All of the spectroscopic lines observed are unsplit, showing that the nuclear hyperfine interactions for the upper and lower states are closely similar in all cases studied. This result shows that the inversion symmetry of the electronic wavefunction is preserved even in the highest bound level of the 2pσu van der Waals state, which has a dissociation energy of 0·147 cm-1.