Radiofrequency/infrared double resonance spectroscopy of the HD+ ion

Abstract
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.